edited by Birger Hjørland and Claudio Gnoli




Table of contents:
1. Introduction
2. Species as taxon, species as category, and species concepts
3. Historical background: philosophy vs. natural history
    3.1. Aristotle
    3.2 Alphabetical order vs. classification
    3.3 Linnaeus
4. Species as taxon
    4.1 Species delimitation
    4.2 Cryptic species
    4.3 Linnaean nomenclature
    4.4 Open nomenclature and non-Linnaean nomenclatures
5. Species as category
    5.1. The species rank in biological classifications
    5.2. Infraspecific taxonomic categories and their names
6. Species concepts
    6.1 The debatable plurality of species concepts
    6.2 Species definitions: 6.2.1 Taxonomic species and morphospecies; 6.2.2 Species concepts based on reproduction and genetic exchange; 6.2.3 Agamospecies; 6.2.4 Evolutionary and ecological species concepts; 6.2.5 Cladistic and phylogenetic species concepts; 6.2.6 Phenetic and diagnosabile species, LITU and OTU; 6.2.7 Successional species
7. Species monism or pluralism?
    7.1. Ontological and epistemological aspects
    7.2 Soft (taxonomic) species pluralism
8. Conclusion

This article discusses the many usages and meanings of the term species in biology, as currently applied (1) to named taxa such as Homo sapiens and Panthera tigris, (2) to a rank (usually but not necessarily the lowest and/or the most fundamental one) of the biological classification, and (3) as a variegated set of notions, the most important among them being the morphospecies, the biospecies, the ecospecies, the evolutionary species, the agamospecies and the taxonomic species. The debatable relationship between the Aristotelian “definition by species and genus” and the historical roots of the usage of genus and species as basic ranks in biological classification are outlined, as are the historical roots of the scientific nomenclature applied to biological species.

[top of entry]

1. Introduction

Species is both (a) a specific kind of something and (b) a taxonomic group in biology, but also (c) a general concept in → knowledge organization (KO) about units being classified, often used synonymously with, for example, element, entity, individual and item.

As stated by Hull (1998), “The fundamental elements of any classification are its theoretical commitments, basic units and the criteria for ordering these basic units into a classification”. It is therefore of great interest to explore issues related to such units. The identification of the units is one of the most important issues in any classification and the present article will illuminate this in relation to biology. Surprisingly perhaps, the units in biological taxonomy (the species, the focus of this article) are subject of controversy, as are also the units in other scientific taxonomies, e.g., in chemistry (the chemical elements arranged in the Periodic Table; see, for example, Ruthenberg and Jaap 2008; Scerri and Ghibaudi 2020) and mineralogy [1], which are not treated here.

The focus of this article about species is biological, but there are important problems involving biological species that must be approached from the perspective of philosophy and other disciplines, e.g., questions about natural kinds (not treated here) and the contrast between monistic and pluralistic approaches, briefly treated in Section 7 below.

The species in biology is a complex topic and a controversial issue. This is due in part to the polysemic nature of the term species, which is used to denote (1) a taxon (a named aspect of biological diversity, such as Indian Elephant, or Baobab), (2) a category or rank (often regarded as the fundamental one in the biological classification) and (3) a unity of biological diversity recognized in a particular biological discipline or as defined according to a particular delimitation criterion. These different notions are currently mixed together in dictionary definitions, for example:

species (biology) taxonomic group [taxon] whose members can interbreed [biological species defined according to a criterion of reproductive compatibility, cf. section 6.2.2] (WordNet 3.1)
Zoology and Botany. A group or class of animals or plants [taxon] (usually constituting a subdivision of a genus [thus, referred to a precise taxonomic rank]) having certain common and permanent characteristics which clearly distinguish it from other groups [biological species defined according to criteria of objective diagnosability, cf. section 6.2.6] (Oxford English Dictionary)

The next section of this article is intended to characterize the three usages of the term species in biology (species as taxon, species as category, and species concepts), discussed in detail in Sections 4 to 6, while Section 3 offers some historical background.

[top of entry]

2. Species as taxon, species as category, and species concepts

Like any other set of objects, animals and plants could be classified according to a number of different criteria, nevertheless it is often taken for granted that there should be only one classification of living species. A frequently advocated reason for preferring a general-purpose classification is the expected possibility to get a stable and universal nomenclature to be used as common currency in the different biological disciplines and also in nonscientific contexts, e.g., agriculture, trade, legislation [2].

In its traditional formulation, the biological classification is → hierarchically structured by ranks (called the taxonomic categories). Some of these ranks are still those of Systema Naturae (Linnæus 1758 and other editions), i.e., classes, orders, genera, and species, in descending order; other ranks have been added later, some of them (family, between order and genus; phylum, above the class) used regularly, others (e.g., subclass, superfamily; or domain, at the top of the hierarchy) less frequently and to some extent optionally.

All named items in the classification are called taxa (sing., taxon). Species taxa are thus the taxa to which a taxonomist assigns species rank, i.e., the taxonomist’s choice between the rank of species and other ranks in classification, e.g., genus, and thus presupposes a classification system in which “species” is a defined rank [3]. For example, our species (Homo sapiens) is a species level taxon classified in the genus Homo within family Hominidae, order Primates, class Mammalia, phylum Chordata, kingdom Animalia.

Most biologists, and probably a number of philosophers of biology too [4], will accept that “species taxa, as all other taxa in biological classification [e.g., genera and families], serve as the foundation for all other biological analyses and hence should be as similar to one another as possible” (Bock 2004, 183). However, general agreement on this issue has emerged as highly controversial and possibly beyond hope of definitive solution.

Problems arise because of the conflict between the diversity of meanings the term species takes in disciplines other than taxonomy, such as evolutionary biology, ecology, etc., contrasting with the uniform nomenclature applied to the taxa of Linnaean tradition. Indeed, the units of biodiversity recognized in biology are mostly named by using the binomials of Linnaean taxonomy, although in some circumstances researchers use instead a diversity of non-Linnaean formulas (Minelli 2019): see subsections 4.4 and 6.3. However, the use of the same Linnaean binomials in taxonomy, evolutionary biology, ecology etc. is a consequence of the fact that we call species all the different biodiversity units worth recognition in all these disciplines, but does not attest that these units are, or can be, coextensive.

This is the origin of what is widely known as the species problem: “there are multiple, inconsistent ways to divide biodiversity into species (on the basis of multiple, conflicting species concepts), without any obvious way of resolving the conflict. No single species concept seems adequate” (Richards 2010, 5). This issue was first brought to the attention of naturalists by Bernard (1902), Bessey (1908) and Cowles (1908) but became matter of debate following the publication of Robson (1928), the first book on the species problem. Among the monographs on this subject are Hey (2001a), Stamos (2003), Richards (2010), Pavlinov (2013), Zachos (2016), Wilkins (2018), and Wilkins et al. (in press).

“The species problem is caused by two conflicting motivations; the drive to devise and deploy categories, and the more modern wish to recognize and understand evolutionary groups” (Hey 2001b, 329); further motivations might be added. The focus of the dispute is the biological nature of the biodiversity units that are represented as species taxa in the classification. These have been tentatively defined in ecological, evolutionary, genetic, phylogenetic terms, etc., giving rise to a plurality of alternative notions. These will be discussed in detail in Section 6.2, but three principal issues need be addressed here.

First, about the use of the term concept [5]. It can be argued that the notions traditionally known as the biological species concept, the evolutionary species concepts, etc., are not really different concepts, but merely alternative definitions. Wilkins (2011; 2018), after discussing in detail the nature and the mutual replationships of the many “species concepts” proposed to date, ends up with the reasonable conclusion that there is in fact only one species concept, upon which (even if often implicitly) rests the classification of the living species, i.e.: “Species are those groups of organisms that resemble their parents” (Wilkins 2018, 307). All the different “concepts” would thus be alternative (and often more or less extensively overlapping) conceptions, for which more or less precise definitions have been formulated, or simple operational criteria for species delimitation. However, the usage of “concepts” for the individual notions of species is so deeply entrenched in the literature, that in practice Wilkins himself uses concept and conception as synonyms, often in the same sentence, for example: “I distinguish between two phylospecies concepts that go by various names, […] to remedy this terminological inflation, I have christened them the autapomorphic species conception and the phylogenetic taxon species concept” (Wilkins 2018, 370, italics as in the original). Therefore, this critical issue (the legitimacy of the use of concept for the different species notions proposed thus far) needs be flagged, but, in practice, the access to the current literature (biological and of philosophy of biology alike) continues to go through the application of the term concept to the different notions of species, as presented in the subsection 6.2.

Second, it can be argued that species concepts are not subject matter of systematic biology, but a question of general biology, although not necessarily an integral part of the theory of evolution, as suggested by Szalay and Bock (1991) [6]. However, adopting one or the other of the many species notions proposed thus far (see subsection 6.2) can have dramatic consequences on classificatory practice, especially in so far as it may involve a choice between monism and pluralism (see subsection 6.3).

Third, species concepts do not necessarily translate into operational guides allowing the assignment of specimens to species. For example, despite the popularity of the so-called biological species concept, the instances in which biologists actually check if living specimens x and y actually interbreed, to decide whether they belong to the same species, are an extremely minor exception. Most identifications are made on preserved museum specimens; additionally, the name-bearing type specimens are nearly always preserved specimens and the exceptions to this rule are the living types of bacteria to which the biological species criterion would hardly apply. A great many researchers would probably accept the biological species concept in the abstract but, in their daily practice, to establish conspecificity of specimens they will use proxies, such as morphological or genetic similarity.

[top of entry]

3. Historical background: philosophy vs. natural history

3.1. Aristotle

Modern commentators have expressed contrasting and often unjustified interpretations regarding the meaning of the term species in zoological or botanical works published since classical antiquity up to Linnaeus (1707-1778), the author in whose works the modern classification and nomenclature of animals and plants has taken shape (cf. subsection 3.3).

A first point to be fixed is how far the term species, as used by biologists today, agrees with its meaning in the context of the so-called Aristotelian “definition by species and genus”. It is a widespread opinion that “formalization of individual kinds (species) and of collective groups (genera) [by Aristotle] was the point of departure for the more perceptive and elaborate classifications of the later period” (Mayr 1982, 153), but this assumption is arguably ill-founded.

Let’s move from Aristotle's Metaphysics, where this logical scheme is introduced, to subsequently see if and how, as often stated, this notion becomes the foundation of biological taxonomy, irrespective of the fact that Aristotle’s texts do not contain any explicit classification of animals or plants. Aristotle’s terms γένος and εἴδος will be translated here as “genus” and “species” respectively, but without any commitment to take these words as meaning the same as in modern classification.

In Aristotle, everything that differs from something else differs either in genus or in species: in genus, when both things have neither matter nor the way in which they are generated in common, whereas matter and way of generation are instead shared by the species of the same genus [7] (for example, a human is a species of the genus animal that differs from the other animals by being rational). Therefore, what a genus differs from every other genus (otherness, ἑτερότης) is other than what a species differs from other species of its genus (difference, διαφορά): to recognize a genus, it is not necessary to compare it with other genera, but a species is defined only based on its differences from the other species of the same genus [8]. A popular visualization of Aristotle’s logics based on the genus and species is Porphyry’s tree. This embodies the method of binary (dichotomous) → division upon which most modern keys to the identification of plant and animal species are built, but Aristotle did not commit himself strictly to dichotomy (Franklin 1986). Aristotle used eidos and genos at multiple levels (Lennox 1980; Pellegrin 1982; Balme 1987; Sloan 1987; Richards 2010), as a consequence, these were not taxonomic concepts. Specifically, “it is implausible that Aristotle was generally using the term eidos to refer to those groups of organisms that we identify as species taxa. Sparrow, for instance, does not refer to a species taxon in our usage. In modern classificatory terms, sparrow is the family Passeridae, which includes multiple genera, and many species” (Richards 2010, 33). Therefore, we cannot accept that “Linnaeus's system of static and discrete species was simply the result of filling in the abstract [Porphyry’s] Tree with the names of actual species” (Franklin 1986, 252).

As used in his works on animals, Aristotle’s genus was not a formal taxonomic category but just a group of fairly similar organisms on which attention was focussed and stable diagnostic characters (differentiæ specificæ) identified subgroups within each genus, whenever required (Reydon 2020).

[top of entry]

3.2 Alphabetical order vs. classification

Philosophers (from Neoplatonists to Scholastics) who took up the categories of genus and species from Aristotle used them as two terms in a logical relationship, without linking them to the natural world as levels of a taxonomy (Pavlinov in press). Reciprocally, Renaissance zoologists and botanists who used these terms do not seem to have been directly inspired by the Aristotelian-Scholastic tradition.

In many 16th and 17th century books on plants or animals, entries were ordered → alphabetically; examples are Leonhard Fuchs’ herbal De historia stirpium (1542) and John Ray's early botanical works, Catalogus plantarum circa Cantabrigiam nascentium (1660) and Catalogus plantarum Angliae (1677).

Deciding on the place of these works in the history of biological systematics depends on what we mean by → classification and, as a consequence, on whether we accept alphabetical ordering as a peculiar kind of classification. These issues are clearly discussed in Hjørland (2017), on which the following paragraph is based.

A dictionary, e.g., of animal or plant names could be described as a classification, where the ordering principle is the alphabetical sequence of the entries: this arrangement satisfies indeed → Bliss’ (1929, 143) definition: “A classification is a series or system of classes arranged in some order according to some principles or conception, purpose or interest, or some combination of such”. Specifically, the alphabetical ordering of entries fits into Suppe’s broad definition of conceptual classification, as “intrinsic to the use of language, hence to most if not all communication. Whenever we use nominative phrases we are classifying the designated subject as being importantly similar to other entities bearing the same designation; that is, we classify them together” (Suppe 1989, 292). However, to regard the simple alphabetical ordering of plant or animal names would not add any scientific content to the mere identification of phenomenological entities worth a name (phenomenological species sensu Sterelny 1999). A taxonomy, or systematic classification requires instead the arrangement of these units within a scheme in which at least two levels are recognized (Suppe 1989, 292).

A transition from a roughly alphabetical ordering to a distribution of items (“species”) into named groups (“orders”) is found in Conrad Gesner’s works (Enenkel 2014). While in his first zoological treatise, on quadrupeds (Gesner 1551), mammals are arranged alphabetically, in the second edition of the Icones Avium (Gesner 1560) birds are distributed in eight orders some of which, e.g., the nocturnal birds of prey, broadly correspond to those of modern ornithology.

Ordering by alphabet is not the only way to arrange items in a list, as shown for example by → the Great Chain of Being, once popular since classical antiquity into modern times, linking God, angels, humans, animals, plants, and minerals in the order (Lovejoy 1936).

In any case, ordering and classifying are two different operations and, to some extent, either of them can be present in the absence of the other. When Lamarck (1815-22) in the Histoire naturelle des animaux sans vertèbres arranged the main groups of invertebrates from the simplest to the most complex, thus reversing the traditional order, he demonstrated that the link between classification and ordering is not indissoluble.

[top of entry]

3.3 Linnaeus

Before finding a point of arrival in Linnaeus' encyclopedic work covering both animals and plants, zoological and botanical taxonomy evolved separately. Even Ray, the most prominent of taxonomists before Linnaeus and one of the few major scientists who published important works in both fields, never dealt with plants and animals in the same work and adopted very different treatments for plant and animal species. Plant taxonomy matured faster than animal taxonomy and this is well reflected in Linnaeus' scientific production (see below, 4.3).

Linnaeus (1751, aphorism 155) suggested an equivalence between the sequence of categories adopted in his botanical classification (classis, ordo, genus, species, varietas, in descending order from the most inclusive) and the philosophical categories genus summum, intermedium and proximum, species and individuum. However, this correspondence was offered by Linnaeus only by way of example, as evidenced by two further equivalents, a geographical sequence (Regnum, Provincia, Territorium, Parœcia, Pagus) and a military one (Legio, Cohors, Manipulus, Contubernium, Miles). In fact, Linnaeus derived from the scholastic tradition (through his predecessors in natural history) the names, rather than the meanings, of these categories (Minelli in press).

[top of entry]

4. Species as taxon

4.1 Species delimitation

A number of criteria and algorithms that would reduce subjectivity in species delimitation have been proposed (e.g., Wiens and Servedio 2000; Pons et al. 2006; Wiens 2007; Leliaert et al. 2009; Monaghan et al. 2009; Hausdorf and Hennig 2010; O’Meara 2010; Yang and Rannala 2010; 2014; Ence and Carstens 2011; Dupuis et al. 2012; Fujita et al. 2012; Puillandre et al. 2012a; 2012b; Carstens et al. 2013; Flot 2015; Rannala 2015; Luktanov 2019; Sukumaran et al. 2020).

Deciding whether the morphological or genetic differences between two sets, A and B, of individuals justify classifying these as distinct species is often uncontroversial when A and B are sympatric, i.e., occur in the same area: the absence of specimens with intermediate traits strongly suggests reproductive isolation.

Species delimitation based on apparently fixed differences was codified as a methodology called population aggregation analysis (Davis and Nixon 1992). Although satisfactory in principle, this criterion may fail when applied to limited population or character samples. On the one hand, if A and B have been compared for a few characters only, characters with alternative states fixed in A vs. B may not be observed; on the other hand, if sampling is limited to few specimens, an overlap between the ranges of variation of the putative species A and B may go unnoticed. As a consequence, the number of species will be easily underestimated in the first case, overestimated in the second.

The real problem, however, is delimiting species in conditions of allopatry, i.e., when comparing similar but distinguishable populations living in geographically isolated area, e.g., on different islands or mountains. In this case, some degree of morphological divergence is often accepted as a proxy for a real proof of reproductive isolation. However, what “some degree” may actually mean, remains undecidable. Thus, to avoid or at least to reduce this subjectivity, alternative approaches have been proposed, often advocating an “integrative taxonomy” (Dayrat 2005) resting on different sources of data, such as molecular, morphological, behavioral, and ecological, to delimit species.

Other approaches are based on dedicated bioinformatics tools newly developed at the interface between two traditional biological disciplines: phylogenetic analysis and population genetics. The General Mixed Yule Coalescent (GMYC) method (Pons et al. 2006; Monaghan et al. 2009; Leliaert et al. 2009) is used to analyze trees depicting the relationships of a sample of individuals representing an unknown number of related species, to discover the splits of related species from their common ancestor. The Automatic Barcode Gap Discovery (ABGD) method (Puillandre et al. 2012a) detects discontinuities (“barcode gaps”; Hebert et al. 2003) in the distribution of pairwise distances between DNA sequences. This is not easily distinguished from genetic differences internal to the populations. New mathematical models are currently being develop to address this difficulty (e.g., Sukumaran et al. 2020).

[top of entry]

4.2 Cryptic species

Experimental tests of the reproductive compatibility between individuals of different populations have frequently revealed the existence of reproductive barriers between groups of individuals not distinguishable from each other by diagnostic morphological characters. These cryptic species (Darlington 1940) have been regarded as a precious study object by evolutionary biologists, but as a source of difficult problems by systematists. In recent times, the increasingly widespread use of molecular characters in taxonomy has shown that the number of cryptic species and their occurrence in the most diverse groups have been strongly underestimated.

Many, only partly overlapping definitions of cryptic species have been given (for a list, see Struck et al. 2018), but most authors would agree with Bickford et al. (2007), that two or more species deserve to be defined “cryptic” if they are currently classified, or have been classified in the past, as a single species due to the apparent lack of diagnostic morphological characters. Thus, the status of cryptic species does not describe a natural phenomenon, but only a temporarily problematic formalization of the delineation of species (Korshunova et al. 2017).

Despite the fact that the term was introduced by a botanist (Darlington 1940), the literature would suggest that cryptic species are much more frequent in animals than in plants (Bickford et al. 2007). To some extent, this might be a consequence of the fact that quick, cheap and reliable diagnostic tools by which indications of the existence of cryptic species can be rapidly obtained are available for animals, but not yet for plants (Shneyer and Kotseruba 2015). In most animal groups, individual specimens can be assigned quite reliably to a species by extracting and sequencing a diagnostic DNA segment that evolves fast enough as to differentiate beween closely related species, but also shows very modest or no variation within each species. A nearly universally accepted “molecular barcode” for animals is a segment of the mitochondrial DNA, a part of a subunit of the cytochrome oxidase gene CO1 (Hebert et al. 2003). But in most plant lineages, mitochondrial DNA evolves too slowly; alternatives have been suggested (e.g., Chase and Fay 2009), including the sequence of the whole plastidial genome, i.e., the DNA associated with the chloroplast (Erickson et al. 2008; Parks et al. 2009; Nock et al. 2011), but no satisfactory solution has been obtained thus far (Li et al. 2015; Saddhe and Kumar 2018).

Eventually, cryptic species may turn out not be so rare among plants as recent reviews (e.g., Shneyer and Kotseruba 2015) suggest, thus fulfilling Grant’s (1957; 1981) prediction that they may be frequent among plants too. Cryptic species may also represent a significant part of the still undescribed species of algae (Guiry 2012) and perhaps also of fungi, judging from some report of cryptic diversity among plant pathogenic fungi (Pavlic et al. 2009; Bennett et al. 2011).

An important amount of cryptic diversity would be better described in term of MOTUs (molecular operational taxonomic units) (Floyd et al. 2002). These clusters of individuals or populations recognizable through molecular markers do not necessarily correspond to conventional taxonomic species, as pointed out for the first time by Blaxter et al. (2005) and confirmed by a large number of studies using a diversity of markers and different species delimitation criteria. More disturbing, there is no universal rule for translating MOTUs into named species with a place in the Linnaean classification. Not an easy job, considered the very large number of cryptic species that some studies have suggested need recognition within what was hitherto described as a single species. For example, a morphologically very uniform set of populations of subterranean crustaceans living in the desert springs of the Southern Great Basin of California and Nevada, USA, previously referred to one species (Hyalella azteca), was reported to correspond to 33 cryptic species (Witt et al. 2006). A still higher number (62 or 78, depending on the method used to delimit species) has been identified using ribosomal DNA sequences from 401 samples across the global distribution of the tiny flatworm Gyratrix hermaphroditus (Tessens et al. 2021).

In most instances, for a newly discovered cryptic species no formal description and naming is provided, for a while at least (Horton et al. 2017). Struck et al. (2018) analyzed 606 publications citing cryptic species, issued before June 2016; of these, less than one in five contains formal descriptions of the hypothesized species. However, despite the difficulties and size of the task, detailed study and formal description of this species diversity is seen by many researchers as a necessary effort (e.g., Trontelj and Fišer 2009; Pérez-Ponce de León and Nadler 2010; Minelli 2017).

[top of entry]

4.3 Linnaean nomenclature

The international scientific nomenclature of animal and plant species has its origin in the works of Carolus Linnaeus: more precisely, in the first edition of Species Plantarum (Linnæus 1753) for plant and the tenth edition of Systema Naturæ (Linnæus 1758) for animal names. We therefore speak of Linnaean nomenclature, where each species is indicated with an expression (the Linnaean binomial) consisting of two words: the generic name and the specific epithet. The generic name is identical for all the species that zoologists or botanists group in the same genus. For example, Solanum tuberosum (potato) and Solanum melongena (aubergine) are two species classified in the genus Solanum.

The specific epithet was introduced by Linnaeus (under the name of “nomen triviale”), as an advantageous mnemotechnical alternative to the polynomial formulas then in use, which summarized the main diagnostic traits on the basis of which a species was considered to differ from other species of the same genus. Some of these polynomial formulas were actually binomial, especially in the case of more common species, or of species attributed to genera with few species, where a single differential character, expressible with a single word, was sufficient to characterize a species (e.g., Asphodelus autumnalis and Asphodelus bulbosus, two plants listed under these names in Bauhin 1596). But in the most species-rich genera, the formulas tended to lengthen considerably, therefore the convenience that resulted from the introduction of the Linnaean binomials is evident. For example, the Old World swallowtail, a butterfly that Linnaeus himself had indicated (Linnæus 1746) as Papilio hexapus; aliis flavo nigroque variegatis: secundariis angulo subulato maculaque fulva, becomes Papilio Machaon (Linnæus 1758), the name by which it is still called, apart from the loss of the capitalization of the specific epithet. Linnaeus was the first author to adopt binomial nomenclature systematically, but the idea had been floated before him (Choate 1912) by some botanists, most explicitly by Rivinus (1690), who remarked that it would be easy to suggest the specific difference diagnostic for a plant species by simply attaching a second term to the genus name [9]. Before the deliberate systematic adoption of binomial nomenclature in Species Plantarum, an extensive although still unsystematic use of binomials is also found in the dissertation Pan suecicus defended in Uppsala on December 9, 1749, by Linnaeus’ pupil Nicolaus L. Hesselgren (Linnæus 1749). Of the 866 plant names listed there, 754 are binomials. A number of them, but not all, were retained by Linnaeus in Species Plantarum.

The rapidly growing popularity of Linnaeus’ works, including his encyclopaedic Systema naturae that provided a classification both for animal and plant species (and also minerals, classified at the time as the third kingdom of nature) contributed strongly to align zoological and botanical practices and to convince the scientific community of the enormous benefits that would derive from a general agreement, both in the delimitation of the phenomenological units recognized as species, and in the adoption of a unique name for each of them. On the latter issue, binominal nomenclature was an obviously advantageous choice; this caused a dramatic loss of interest in the pre-Linnaean literature, encumbered by its unmanageable nomenclature.

However, this revolution was neither easy nor complete. The main remaining issue was, that just adopting the Linnaean system did not guarantee uniqueness and universality of nomenclature. Two main problems showed up: synonymy and homonymy. If different names have been used for one and the same species, which of them should be accepted and used? If one and the same name has been used for two or more different species, for which of them should it be retained as valid? Common sense and legal practice may suggest to resolve these conflicts by applying a principle of temporal precedence (priority given to the oldest synonym or homonym), but this was just the tip of an iceberg that deserved a serious debate before a possible agreement on a precise set of rules.

Taxonomic traditions in botany and zoology began to diverge again soon after Linnaeus' death. This was annoying, especially in so far as this affected nomenclature.

In zoology, the British Association for the Advancement of Science made a first attempt to regulate these matters, by setting up a Committee “to consider of the rules by which the Nomenclature of Zoology may be established on a uniform and permament basis”. This Committee eventually produced a Report (Strickland et al. 1842) that is regarded as the first official set of rules for zoological nomenclature. However, this initiative was still at national level. Only towards the end of the 19th century did an international effort eventually produce a nomenclature code on which the entire zoological community could converge. A Commission appointed by the third International Congress of Zoology, held in Leiden in 1895, produced the document known as Règles internationales de la nomenclature zoologique (International Commission on Zoological Nomenclature 1905). This Commission has continued to work, both to resolve problematic cases that emerged from the application of the rules and to integrate or modify the latter. In the long run, the need for an overall rewriting of the rules was recognized; a new document was issued in 1961 as the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 1961). In the following years, this text was subjected to further revision, until the fourth edition (International Commission on Zoological Nomenclature 1999), in force since January 1, 2000. For a history of zoological nomenclature, 1895-1995, see Melville (1995).

In botany, a series of principles were more or less widely accepted as normative for the nomenclature of plant taxa, starting from a document prepared by Alphonse de Candolle for the International Congress of Botany held in Paris in 1867 (Candolle 1867), but the first official Code was voted on only at the Seventh International Congress of Botany, held in Stockholm in 1950 (Lanjouw et al. 1952). The current edition of the botanical code (Turland et al. 2018) was adopted by the Nineteenth International Botanical Congress, Shenzhen, China, in July 2017. For a history of botanical nomenclature, 1737-1989, see Nicolson (1991).

At the time the committee of the British Association for the Advancement of Science was about to publish a first set of rules aimed at ensuring stability and universality in the creation and use of names for animal species and higher taxa (Strickland et al. 1842), the Italian zoologist Luciano Bonaparte (Napoleon's nephew) initiated a cooperative effort towards the definition of common rules for the nomenclature of animals and plants, but this project was abandoned quite soon (Minelli 2008).

Coordination of animal and plant nomenclature would not be simply a formal exercise but would address two sets of problems that have been growing through time. First, the independence of zoological vs. botanical nomenclature has allowed the creation of names for animal genera identical to the names of plant or fungal genera. Second, the nomenclature of a certain number of genera of unicellular organisms, which over time have been classified sometimes as animals (“protozoans”) sometimes as plants (“algae”), is currently intractable. Some of them have two names, according to the zoological and the botanical nomenclature, respectively. There are also organisms recognized today as closely related to each other, one of which carries a zoological name, the other a botanical name. An additional problem is the lack of rules for fixing the name of an organism that has been originally described as an animal, but is now treated as a plant, or vice versa.

A new attempt at unifying the different traditions into a single set of rules intended to govern the nomenclature of all organisms (Hawksworth 1997) lead to the formulation of a BioCode (Greuter et al. 1996; 1998), but the Bionomenclature Committee which met in Naples in 2000 on the occasion of the General Assembly of the International Union of Biological Sciences (IUBS) decided to abandon the initiative (International Commission on Zoological Nomenclature 2001). A further attempt to relaunch the BioCode project (Greuter et al. 2011) was ephemeral. Therefore, the BioCode has remained at the level of a hypothesis to work on and has never been adopted by the scientific community.

Special codes of nomenclature have been created for prokaryotes (Eubacteria and Archaea), for viruses and for cultivated plants. The current editions of these three documents are Parker et al. (2019), International Committee on Taxonomy of viruses (ICTV) (2021) and Brickell et al. (2009), in the order. Binomial nomenclature is applied to prokaryotic species taxa, but not to viruses or to cultivated plants as such. However, ongoing discussion (e.g., Hull and Rima 2020; Siddell et al. 2020) suggests that virologists may also adopt binominal nomenclature before long. As for cultivated plants, nomenclature is centered on naming cultivars. A cultivar is defined (Brickell et al. 2009, Art. 2.3) as “an assemblage of plants that (a) has been selected for a particular character or combination of characters, (b) is distinct, uniform, and stable in these characters, and (c) when propagated by appropriate means, retains those characters”. Rules for naming cultivars are very flexible and do not presuppose a clear hierarchy from genus to species to cultivar, but names that include a Linnaean binomial, such as Chamaecyparis lawsoniana “Silver Queen” (Brickell et al. 2009, Example 5 to Recommendation 8A) are accepted.

Besides the principle of temporal precedence, accompanied by less general rules for determining which name must be used in specific cases of homonymy or synonymy, the codes are also based on additional principles that should help achieving the desirable aim of stability and universality of nomenclature. Among these principles, the most important is the permanent association between taxa and types, as explained in the following paragraph.

The association between species and types is double. On the one hand, through the specimen(s) selected as the species’ type(s); on the other, through the species, among those classified in the same genus, choosen as that genus’ type. Both type specimens and type species have a double significance: taxonomic and nomenclatural, respectively. This is better visible in the case of a type specimen. As taxonomic type, it is a voucher documenting the characters based on which a species taxon was first recognized, as well as a material store of additional information available for possible subsequent mining. As nomenclatural type, it fulfills the role spelled out in the following rule of the zoological code: “No matter how the boundaries of a taxonomic taxon may vary in the opinion of zoologists, the valid name of such a taxon is determined […] from the name-bearing type considered to belong within those boundaries” (International Commission on Zoological Nomenclature 1999, Art. 61.1.1). In other terms, the type specimen is an onomatophore, or name-bearer (Simpson 1940; Dubois 2011; Witteveen 2016). Similarly, taxonomists may disagree on the validity or delimitation of a genus, but so long as this is considered valid, the genus’ type species will remain associated forever with it.

[top of entry]

4.4 Open nomenclature and non-Linnaean nomenclatures

Nomenclature codes do not contain a definition of the species category, but rules for the adoption and use of the names by which species taxa must be designated.

The formal description and naming of species is not the same action as the identification of specimens (Collins and Cruickshank 2013), which often remains uncertain even in the hands of a specialist. To hide this uncertainty under an unflagged Linnaean name would carry a wrong message. To avoid this, some formats have been used, as in the following examples: “Harpalus sp.” (the specimen can be confidently identified as belonging to one of the species of the beetle genus Harpalus, but a more precise identification has not been possible); “Lannea cf. schimperi” (the specimen belongs to the wasp genus Lannea, possibly to L. schimperi, but it might instead belong to a different species, including a still undecribed one). The set of these formulae and related ones, that depart from pure Linnaean binomials, is sometimes called open nomenclature (nomenclatura aperta: Richter 1943; see also Matthews 1973). This includes cases where the identification at the species level is uncertain because of contingent difficulties, e.g., incomplete specimens, or the suggested identification is potentially but not certainly correct, or the identification at the species level is uncertain because no sound taxonomy is currently available for the group to which it belongs. Last but not least, a still undescribed species may be involved, but an exhaustive study and, if this is the case, the description and naming of the new species are reserved for a later time. Sigovini et al. (2016) have provided a detailed discussion of open nomenclature, with a detailed glossary of the terms in use and preliminary suggestions for their standardization (see also Minelli 2019).

Besides the necessary, frequent use of open nomenclature, current practice frequently deviates from Linnaean nomenclature because of the deliberate rejection of a specific rule of the Code, or even of its basic philosophy.

A biological nomenclature in which taxa and names are not fixed by reference to types has been proposed by a group of researchers who regard this departure from the traditional codes as a necessary implication of the adoption of the so-called phylogenetic systematics. This approach to biological systematics, which moves from the conceptual and methodological reform initiated by Hennig (1950; 1965; 1966), questions the maintenance of the ranks (e.g., genus, order, class) of the Linnaean classification (Griffiths 1976). More recently, some followers of phylogenetic systematics have launched a new “phylogenetic nomenclature” eventually formalized in a new Code (Cantino and de Queiroz 2020). This proposal, strongly defended by some authors (e.g., de Queiroz 1997; de Queiroz and Cantino 2001; Bryant and Cantino 2002; Pleijel and Rouse 2003; Cantino 2004; Pleijel and Härlin 2004; Laurin et al. 2006), has been strongly rejected by others (e.g., Lidén and Oxelman 1996; Dominguez and Wheeler 1997; Benton 2000; Forey 2002; Carpenter 2003; Keller et al. 2003; Nixon et al. 2003). According to some authors, in a consequent taxonomic system based on phylogenetic principles there would not even be room for species; the lowest unit recognized in the system should instead be an operationally defined LITU (least inclusive taxonomic unit; Pleijel and Rouse 1999) (see 6.2.6).

Another deviation from the official nomenclature is the proposed addition of a numerical code to the names formed according to the rules. This was suggested in the early 1990s to address a perceived shortage of scientific names as keywords for retrieving information from databases (Heppel 1991). Alphanumeric formulas have also been recommended as a useful complement to the unconventional names suggested by de Smet (1991) as a new biological nomenclature largely accepting the Linnaean hierarchy but represented by terms based on Esperanto rather than Latin.

[top of entry]

5. Species as category

5.1. The species rank in biological classifications

It is generally assumed that the species category is the fundamental level in the taxonomic hierarchy of biological classifications. However, it would be more correct to reverse the sentence: today, the basic taxonomic category in biological classifications is generally called species.

The fundamental unit in a classification is not necessarily the lowest one, especially if lower ranks are optional and not acknowledged to deserve a standardized nomenclature. This is the case of subspecies, varieties, etc. in a classification in which the species is explicitly chosen as the basic unit.

Moreover, caution is necessary in examining early taxonomies up to Linnaeus, where the author's choice between genus and species as the unit of the classification is not always easy to determine (Minelli in press).

Like the “species” of folk taxonomies (Berlin 1973), e.g., willow and oak among plants, sparrow and eagle among animals, those described by Renaissance herbalists are only groups of organisms that are easily recognizable and quite consistently identifiable, those that Sterelny (1999, 119) characterizes as phenomenological species: even where nomenclature might suggest otherwise, the “species” of 16th and early 17th century botanists and zoologists are not always the same as the species of Linnaeus and often correspond to taxa recognized today as genera (Arber [1912] 1986). For example, Bauhin (1596) described 29 species of Narcissus (daffodil), however, he added that of one of them, Narcissus albus, medio croceus vnico flore, there are three species, mainly differing in flowering time (“Huius tres species florendi tempore differentes”, p. 79).

[top of entry]

5.2. Infraspecific taxonomic categories and their names

That animals and plants of the same species can exhibit regional peculiarities, is something that could easily fit into the framework of pre-Linnaean zoology and botany. However, geographic variation was not considered an issue really worth of study: “varietates levissimas non curat botanicus” (Linnaeus 1751, aphorism 310). Eventually, geographic variation began to attract the attention of naturalists, at least since Pallas (1778) compared putatively conspecific populations of rodents from many localities and documented with great accuracy variation both within and between populations, highlighting the frequent difficulty in tracing with certainty the boundaries between one species and another.

In geographical terms, intraspecific variation is also quite diverse, sometimes continuous (clinal), sometimes discontinuous. The latter case makes it easier to identify distinct units, usually called subspecies.

The scientific nomenclature of infraspecific taxa is much more articulated and flexible for plants than for animals. The zoological code (International Commission on Zoological Nomenclature 1999) recognizes only taxa at one subordinate rank (subspecies; a term first defined in ornithological circles: Allen 1877), while the International Code of Botanical Nomenclature (Turland et al. 2018) allows the use of names for subspecies, varieties, sub-varieties, forms and subforms.

[top of entry]

6. Species concepts

6.1 The debatable plurality of species concepts

Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. (Darwin 1859, 44)

As mentioned in section 2, the controversial issue of species concepts belongs in principle to general biology rather than to taxonomy, because it involves concepts and theories that range over most of the biological disciplines, e.g., ecology, genetics, biology of reproduction, evolutionary biology. Nevertheless, it deserves to be treated here, because its many ramifications extend well into aspects of language and communication: in particular, when the contrasting positions are monism vs. pluralism (cf. subsection 6.3), with obvious implications for nomenclature. Philosophical debates that started in the late ’60s have even questioned the nature of the species as a kind of class (the “species-as-sets” conception), suggesting that biological species are instead historical individuals (the “species-as-individuals” thesis) [10].

As noted by Hey (2006, 448), “the current long list of [species] concepts spans a wide variety of inspirations, histories and purposes [Mayden 1997; Endler 1989; Hull 1997]. Indeed, it is now generally necessary when addressing general questions on species concepts to first attempt a classification of them on the basis of various properties [Mayden 1997; Endler 1989; Baum and Donoghue 1995; de Queiroz 1998; Pigliucci 2003]”.

Mayden’s (1997) classic overview listed 22 definitions of species; subsequent revisitations by Wilkins (2009a; 2011; 2018) have added five more items. These definitions, or at least a core selection from them, are traditionally discussed and contrasted as alternative (but also in part overlapping) species concepts; a list of works mentioning “species concept(s)’ in the title is found in the note 11 [11].

But there are several reasons for disagreement on the number of definitions of biological species thus far proposed, e.g., (i) when introducing a new term, some authors did not provide an explicit definition, thus leaving a degree of uncertainty as to the precise intended meaning; (ii) because of minor differences in the definition, it is questionable if some of the proposed terms should be better regarded as synonyms of others, or not; (iii) more intriguingly, several species definitions overlap to some extent with others, but this translates only in part into a hierarchy of more inclusive vs. less inclusive terms.

The best characterized and/or most popular among the many definitions of species thus far proposed are analyzed in the following subsection, largely following Wilkins (2011). This author identifies seven main notions of species, respectively defined (1) as classes of morphologically similar individuals (morphospecies), (2) as reproductively isolated sexual species (biospecies), (3) by their common gene pool (genetic species), (4) as occupants of distinct ecological niches (ecospecies) or (5) as evolving lineages (evolutionary species), plus (6) a notion introduced for organisms without sexual reproduction (agamospecies) and (7) a term to describe whatever a taxonomist calls a species (taxonomic species; incl. those recognized by palaeontologists, i.e. chronospecies).

However, as also noted by Wilkins (2011), some of these notions are more or less equivalent (e.g., biospecies and genetic species, treated together in 6.2.2 below; evolutionary species and ecospecies, cf. 6.2.3 below). Two additional groups of concepts are presented here in separate subsections. First, cladistic and phylogenetic species concepts (cf. 6.2.4), which are indeed a mix either of morphospecies, biospecies or evospecies or all of them (Wilkins 2009a), but are characterized by their specific background in phylogenetic systematics, either in its original formulation (Hennig 1950; 1965; 1966) or in its subsequent development commonly known as cladistics. Similarly, phenetic and diagnosabile species, LITU and OTU (treated in 6.2.5 below) are a variegated set of definitions that differ from the taxonomic species because of the specific theoretical contexts (phenetic or numerical taxonomy and phylogenetic systematics) in which they have been formulated.

Before moving to detail about the different definitions, let’s remark that Wilkins (2011) eventually recognizes only one species concept, in strict agreement with Ray’s (1686) approach to plant species [12]. As noted by Richards (2010, 69), “From Ray through Linnaeus and Buffon, there was a turn to a historical and genealogical conception of species. […] An organism was a member of a species taxon not because it was similar to other members of that species – not because they shared essential traits — but because its parents were members of that species taxon”. In Wilkins’ (2011, 59) explicit reformulation of this generative concept, “species are those groups of organisms that resemble their parents”.

[top of entry]

6.2 Species definitions

6.2.1 Taxonomic species and morphospecies

Before the development of methods to investigate the diversity of life at the molecular level (proteins, but especially DNA and RNA sequences), the diagnostic characters used by taxonomists were provided by morphology, with few exceptions such as the mating calls of crickets and grasshoppers. It has become customary to say that delimiting species based on morphology amounts to adopting a morphological species concept (morphospecies), but this is mostly accepted without a formal definion like those provided for other species notions (cf. the next subsections).

Eventually, in the absence of further specifications, the selection of morphological traits deemed to be diagnostic at the species level rests with the subjective choice of the taxonomist. Therefore, out of this tradition of morphology-based classificatory efforts, this vaguely circumscribed approach has been sometimes described as a taxonomic species concept, usually defined, with Regan (1926, 75), as “a community, or a number of related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them, to a specific name”.

[top of entry]

6.2.2 Species concepts based on reproduction and genetic exchange

In everyday practice, all species taxa are commonly perceived as equivalent, since they are all named by a Linnaean binomial. However, according to Bock (2004) they should be offered uniform taxonomic status and identical nomenclatural treatment only on condition that they are, as far as possible, “biologically equivalent”.

Efforts to define species in biological terms have been traced back to the work of John Ray (1627-1705) and Georges Louis Leclerc de Buffon (1707-1788).

According to Cain (1997), Ray published the first attempt to define living species based on their constancy throughout the generations. However, it is hard to construe Ray's actual words [13] as a definition of biological species, rather than as an empirical (experimental) criterion to check the conspecificity of similar but not identical kinds of plants, i.e., to verify how much variation can be accepted within the limits of genealogical continuity. Moreover, there is no evidence that Ray extended this view to animals too. On the contrary, some passages in his zoological books seem to exclude, on Ray's part, the adoption of a “true breeding” criterion to recognize animal species (Minelli in press).

There are also problems with Buffon, who is credited of introducing a concept of species as a reproductive community. In the pages of the Histoire naturelle, générale et particulière dealing with hybrids, Buffon (1749; 1766) made clear that the nearly complete reproductive isolation between horse and donkey cannot be generalized as providing a biological criterion revealing that two animals belong to different species. Like Ray, he was not interested in reproductive isolation per se, as a criterion to separate species, but in ascertaining the continuity of a lineage through the generations.

In the last century, the biospecies has been most frequently defined as a reproductive community of sexual individuals that do not cross with individuals of the other species, or as the most inclusive set of individuals sharing a common gene pool. In this latter form, the notion is also called genetic species (Simpson 1943; Dobzhansky 1950).

According to Dobzhansky (1970, 354), “a biological species is an inclusive Mendelian population; it is integrated by the bonds of sexual reproduction and parentage”. In Mayr’s (1963, 21) more detailed characterization, “species are reproductive communities. The individuals of a species of animals recognize each other as potential mates and seek each other for the purpose of reproduction […]. The species, finally, is a genetic unit consisting of a large, intercommunicating gene pool, whereas the individuai is merely a temporary vessel holding a small portion of the contents of the gene pool for a short time”.

In practice, however, it is not always easy, or possible, to recognize a clear boundary between interfertile and reproductively isolated populations. In extreme situations, e.g., among the hundreds of taxa of the cichlid fish family present in Lake Victoria, Africa, the reproductive communities seem to literally come and go in real time (Spinney 2010). New species are taking shape from hybridization between pairs of survivors from the severe crisis caused in the recent past by the introduction of an alien big predator, the Nile perch Lates niloticus: an example is the hybrid currently forming in the lake's Mwanza Gulf between the blue Pundamilia pundamilia and the red-back P. nyererei (Meier et al. 2018).

The biological species concept has been primarily applied to animals. Some botanists (e.g., Raven 1980) have been skeptic as to the possibility to apply it to plants; others, however, have openly sided in favor of its validity, e.g., Grant (1971; 1981; 1992), more recently Rieseberg et al. (2006).

According to an alternative view of the biological species concept, a species does not exist based on its relations with other species, but by virtue of a property that unites the individuals referable to it, thus forming a system that “defines itself” (Paterson 1985; Lambert et al. 1987; White et al. 1990). This was recognized long ago by Plate (1914) [14]. According to the recognition species concept developed by Paterson (1979; 1985; also Lambert and Paterson 1984), species are reproductive communities the members of which share a “specific mate recognition system”, i.e. an interpretative code for a series of information elements — for example, acoustic signals or pheromones — that are exchanged between potential partners.

A problem with species notions based on sexual reproduction, both in Mayr’s and in Paterson’s sense, is that these may work only for populations that are far from both of two opposite, extreme conditions: the absence of sex, in which case no species would exist (see subsection 6.2.3, Agamospecies), and the absence of barriers to sexual exchanges, as repeatedly observed in bacteria, where strongly different kinds of organisms would be classified as one species because transfer of genetic material between them is possible in natural conditions. To avoid these disturbing consequences, Templeton (1989, 12) proposed a cohesion species concept, according to which a species is “the most inclusive population of organisms having the potential for phenotypic cohesion through intrinsic cohesion mechanisms”. Mechanisms favouring cohesion are not necessarily the same in all instances, in particular both interfertility and the presence of shared mating recognition systems are included.

Hybrids, both natural and artificial, also continue to challenge modern definitions of species (Wagner 1983).

In many plant groups, hybrids are very common in nature and a large number of hybrid forms are described in the floras. Examples are abundant e.g., among the European orchids and willows. The well-defined, stable morphology of these hybrids suggests that they represent F1 (first generation) hybrids between the two parental species; a backcross with one of the parents would blur the distinction. But many hybrid individuals found in nature, especially among animals, are just elements of a chain, each of which shares to a different degree similarities with either parental species. Hewitt (1988) described this situation as a “hybrid zone”.

Over time, the need for a concept and a term to indicate the whole set of populations linked by hybridization has repeatedly emerged among systematists. The term syngameon was introduced by Lotsy (1925) and used mainly by botanists (e.g., for oaks, Quercus species; see Cannon and Petit 2020). In zoology, Dubois (1982) proposed an operational definition of genus that practically coincides with Lotsy’s syngameon. In recent times, the term has been borrowed by zoologists to describe cases where the boundaries between species are difficult to establish, due to reticulate evolution, e.g., among Darwin’s finches of the Galápagos islands (Grant and Grant 1996), the cichlid fishes of the large African lakes (Seehausen et al. 1997), the South American Heliconius butterflies (Beltrán et al. 2002) and the reef corals of the genus Acropora (Oppen et al. 2001; Mao et al. 2018; Mao 2020). Seehausen (2004, 198) has provided a new definition of syngameon as “a complex of selection-maintained, genetically weakly but ecologically highly distinctive species capable of exchanging genetic material”.

Within the biological species concept, additional taxonomic categories have been proposed to accommodate groups of closely related groups of populations in an evolutionary condition intermediate between still interbreeding subspecies, or races, and reproductively isolated species (Mayr 1931; Amadon 1966; Mallet 2013). The term species in statu nascendi, used for a while to describe them, has lost favour. A complex of imperfectly isolated, closely related taxa was called Artenkreis by Rensch (1928; 1929), a term largely replaced in later use by Mayr’s (1931) superspecies. Mayr's (1963, 672) eventually revised definition of a superspecies is: “a monophyletic group of entirely or essentially allopatric species that are too distinct to be included in a single species”, where “allopatric” means “with non-overlapping geographical distribution”.

The term semispecies was originally introduced to denote the component species of a superspecies, whatever the degree of reproductive isolation between them, as in the following definition: “Semispecies – In systematic zoology the species of which a superspecies is composed” (Mayr et al. 1953, 313). However, the term has been increasingly used in the sense of species in statu nascendi, to denote borderline cases between subspecies and full species: “populations that have acquired some, but not yet all, attributes of species' rank; borderline cases between species and subspecies” (Mayr 1963, 671).

A number of botanists realized that the different degrees of reproductive isolation between populations or groups of populations observed in nature could not be adequately expressed in terms of the categories of the Linnaean classification but required a special purpose classification (Stace 1989) for which Camp and Gilly (1943) coined the term biosystematics.

Three partly overlapping schemes of biosystematics categories were developped by Turesson (1922a; 1922b), Danser (1929) and Gilmour and Heslop-Harrison (1954), respectively. These categories are uniquely based on the extent of possible interbreeding between individuals or populations. In Turesson’s nomenclature (the first to be introduced and eventually the most popular), individuals capable of hybridizing with one another form a coenospecies (= comparium of Danser = syngamodeme of Gilmour and Heslop-Harrison). The terms commiscuum of Danser or coenogamodeme of Gilmour and Heslop-Harrison have been used if hybrids show some degree of fertility, whereas complete interfertility characterizes the ecospecies of Turesson (same as Danser’s convivium and Gilmour and Heslop-Harrison’s hologamodeme).

[top of entry]

6.2.3 Agamospecies

A strict, exclusive application of the biological species concept would imply that no species exist among those kinds of organisms that reproduce from a single parent. Mechanisms include asexual multiplication in the strict sense (i.e., through the detachment of body parts), self-fertilization (both male and female gamete being produced by the same individual, as in several tapeworms and other hermaphrodites) and thelitoky (development of a new individual from an unfertilized egg; see Fusco and Minelli (2019) for these and other, less common and often complex mechanisms).

Taxa recognized (and named) among uniparental organisms have been called agamospecies by Turesson (1929). The terms has been accepted by several authors, including Cain (1954), who regarded the agamospecies as a subset of the morphological species. A synonym is paraspecies (Mayr 1987), but this term had been used before by Ackery and Vane-Wright (1984) with a different meaning, for species lacking uniquely derived traits. With a beautiful metaphor, Ghiselin (1984) compared agamospecies to dead leaves forming a heap at the foot of the phylogenetic tree from which they originated but to which they no longer belong. Camp (1951) and Grant (1957) called them binoms, to stress the fact that those taxonomic entities deserve anyway a place in a classification using Linnaean names.

[top of entry]

6.2.4 Evolutionary and ecological species concepts

Simpson’s (1961, 153) evolutionary species concept identifies the species as “a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”, a definition slightly revised by Wiley (1978, 17) in the following terms: “An evolutionary species is a single lineage of ancestral-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”.

This notion applies equally to organisms with sexual or asexual reproduction, because the evolutionary processes give rise in either case to lineages subject to changes over time.

A related ecological species concept was introduced by Van Valen (1976, 333) as “a lineage or set of closely related lineages, occupying an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all other lineages outside its range”.

Conservation biologists have long disputed about the nature and circumscription of evolutionarily significant units (a term introduced by Ryder 1986) as the units of biodiversity individually worth of monitoring and eventually action to ensure their survival (Moritz 1994; Nielsen 1995; Waples 1995; 1998; Cracraft 1997; Pennock and Dimmick 1997; Dimmick et al. 1999; Crandall et al. 2000; Casacci et al. 2013; Reydon 2019). It is still an open question how far evolutionarily significant units may correspond to taxa of the Linnaean tradition.

[top of entry]

6.2.5 Cladistic and phylogenetic species concepts

Evolution is not just matter of adaptation [15], but also one of → genealogical relationships (phylogeny), and biological systematics has been revolutioned since the relationships expressed in phylogenetic trees have been targeted as the first (or only) criterion on which to base the classification (Hennig 1950; 1965; 1966). In this context, there have been repeated efforts to define the species in relation to phylogeny. Wilkins (2009b) classifies the resulting definitions under six terms: Hennigian species, cladospecies, internodal species, phylogenetic taxon species, autapomorphic species and composite species. Here we recognize two main notions, a cladistic species concept and a phylogenetic species concept.

With reference to an explicit (albeit hypothetical) reconstruction of the phylogenetic relationships among a given set of extant and/or extinct organisms, Ridley (1989, 3) defines a cladistic species as the “set of organisms between two speciation events, or between one speciation event and one extinction event, or that are descended from a speciation event”.

Again, with reference to hypothesized phylogenetic relationships, a phylogenetic species identifies, according to Rosen (1978) and de Queiroz and Donoghue (1988), the smallest recognizable biological entity that is monophyletic, i.e., includes all descendants from a given ancestor, to the exclusion of any other organism. The proof of monophyly is the presence of one or more autapomorphies (derived characters present only within the group considered).

In a different version (Mckitrick and Zink 1988), the phylogenetic concept of species requires both monophyly and unambiguous diagnosability.

In addition to these two formulations, Mayden (1997) grouped also a third one (as the diagnosable version) under the umbrella term of phylogenetic species concept, but it seems more reasonable to classify it in the group of phenetic species concepts, as described in the next subsection.

[top of entry]

6.2.6 Phenetic and diagnosabile species, LITU and OTU

Definitions of Mayden’s (1997) diagnosable version of the phylogenetic species concept, mentioned in the previous subsection, were proposed by Eldredge and Cracraft (1980), Nelson and Platnick (1981), Cracraft (1983; 1987) and Nixon and Wheeler (1990). According to Cracraft (1983, 170), “A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent”. Adopting this notion of species may easily cause a phenomenon described as species inflation (Zachos et al. 2013). An example was provided by Cracraft (1992) himself with his taxonomic revision of the birds of Paradise. Within this group, the taxonomy of previous authors, based on the biological species concept, recognized 40-42 species, whereas Cracraft, under his “minimalist” species concept, raises this number to about 90. More recently, species inflation has repeatedly occurred in mammal taxonomy, two examples being the three species recognized by Cracraft et al. (1998) among tigers instead of the only species (Panthera tigris) accepted by traditional taxonomy, and the three species recognized by Groves and Grubb (2011) among the European red deer hitherto classified as one species (Cervus elaphus).

According to Nelson (1989) and Lidén (1992), in nature there are no objects such as species or processes such as speciation (formation of two or more species by splitting of a previously existing one), therefore no aspect of biological diversity can be unambiguously referred to a hypothetical species rank. Due to the common ancestry of all living beings, it is possible to recognize relations of relative inclusion of a lower taxon within a higher taxon, without one or the other being assigned an absolute (and named) rank, eg, species. In the same vein, a least inclusive taxonomic unit (LITU) was proposed by Pleijel and Rouse (1999; see also Pleijel 2000). The word species does not appear in this denomination, therefore this operational unit should perhaps not be included in a list of species concepts. In a nutshell, LITUs are the smallest taxonomic groups that can be diagnosed based on derived traits (apomorphies). They do not refer to any hierarchical level, nor are they necessarily labeled with Linnaean binomials. The prospect of abandoning the species category was discussed by LaPorte (2007), who concluded that no alternative to the use of species among those proposed for a rankless taxonomy has provided an adequate substitute for it. The current edition of the PhyloCode (Cantino and de Queiroz. 2020) does not include rules for “species” (or LITUs) names, contrary to previous plans (cf. Dayrat et al. 2008).

Strictly operational, thus free from this kind of ontological implications, is the phenetic species concept developed by Sokal and Sneath (1963) and Sneath and Sokal (1973) in the context of their numerical approach to taxonomy. Setting an objective and reproducible classification as the only target of their effort, and deliberately ignoring phylogenetic relationships, these authors classified organisms on the basis of overall similarity (a phenetic approach). Rather than species defined by biological properties such as descendance or interfertility, the units of their classification are thus operational taxonomic units (OTUs). Eventually, OTUs may overlap with Eldredge and Cracraft’s (1980) diagnosable species mentioned in the first lines of this subsection.

[top of entry]

6.2.7 Successional species

Extinct animals and plants did not find a place in the Linnaean classification of living organisms. In the 12th edition of Systema Naturae, Linnaeus (1768) listed eight genera of fossils, but classified them with minerals. Eventually, however, extinct species found their places in the classification in close proximity of the living forms to which they are most similar. As in collection drawers the shells of both living and fossil molluscs were often arranged together, i.e. sorted out according to their similarities rather than to their age, so Lamarck (1815-22) arranged living and extinct species according to their presumed affinities.

The species recognized by paleontologists are morphospecies (e.g., Hallam 1988) that are difficult to reconcile with current concepts of biological species. Different terms have been suggested to stress their distinctness from the latter, such as chronospecies (George 1956) or successional species, a more suggestive alternative proposed by Imbrie (1957, 151) to designate a not better delineated segment of evolutionary lineage and accepted by Simpson (1961, 166), who regarded this entity as “a segment of an evolutionary species delimited in a certain span of time”, as opposed to a species recognizable among contemporary organisms as “a cross section of an evolutionary species at any one time”. This clearly indicates an effort to reconcile within one definition (evolutionary species concept, see subsection 6.2.4) the taxonomic units recognized both in extant and extinct organisms, but also denounces the unavoidable arbitrariness, explicitly acknowledged by George (1956), in the temporal delimitation of successional species.

[top of entry]

7. Species monism or pluralism?

7.1. Ontological and epistemological aspects

As mentioned in subsection 6.1, Wilkins (2011) reduces the dozens of species notions thus far proposed to seven distinct definitions — all referable to one species concept — with 27 variants and mixtures. Most of these different definitions (or variations) refer to the different aspects of biological diversity relevant in the context of different biological disciplines; however, the corresponding taxa are almost always called “species” and receive Linnaean names, similar to the phenomenological species of the Linnaean taxonomy (Sterelny 1999). Nevertheless, if Linnaean binomials are to be useful in evolutionary biology, ecology, etc., they should refer to units overlapping as much as possible with the different units in which this or that discipline is interested. Ideally, for all of them, hence the so-called species problem mentioned in section 2. This problem has been mostly addressed in one of two ways (Brigandt 2020).

A first way is to decide that the units of taxonomy must be, for example, those of evolutionary biology, or segments of the phylogenetic tree, to the exclusion of any other criterion.

This strategy has been adopted by Mayden (1997), based on Mayr’s (1957) distinction between “primary” and “secondary” species concepts. A primary species concept would embody the most important properties shared by all entities assigned to the species category, whereas secondary species concepts are intended as operational tools to be used to discover species in practice. Mayden’s primary species concept is the evolutionary species concept.

Subjectively restricting the choice to one primary species concept may bring to the rejection of the species category as a privileged rank in the Linnean hierarchy, or even to denying that it constitutes a rank at all (species eliminativism; e.g., Mishler 1999; Cellinese, Baum and Mishler 2012).

A second way to solve the species problem is to look for a general concept of species within which the greatest number of independent and sometimes conflicting criteria can be accommodated. According to de Queiroz (1998; 1999; 2005), recognizing the common features of a general lineage (or metapopulation) species concept would represent a simple solution to the species problem. Indeed, “all modern species definitions either explicitly or implicitly equate species with segments of population level evolutionary lineages” (de Queiroz 1998, 60). Similarly, Van Valen (1988) advanced a polythetic species concept, according to which we accept as species all populations or set of populations that fulfill a majority of a set of stated criteria, but none of them is expected to fullfill all of them [16].

However, there is also a third way out of the “species problem”, that is to accept a certain degree of taxonomic and nomenclatural pluralism, to account for the different units of representation of biological diversity required in the different disciplinary or operational contexts, most of which were not foreseeable in the times of Linnaeus or Ray (Pavlinov 2020).

Some authors (e.g., Mishler and Donoghue 1982; Kitcher 1984b; Stanford 1995; Dupré 2001; Ereshefsky 1998; 2001; Slater 2013 [17]; Conix 2019) support taxonomic pluralism, others (e.g., Ghiselin 1987; Hull 1987; Hey 2006) reject it. At any rate, despite the intentions and efforts of many taxonomists, current taxonomic practice is already, to some extent, pluralistic (Minelli 2019).

Nathan (2019) distinguises two independent versions of pluralism. According to the first, various kinds of species are present in nature. A plurality of species concepts may thus be adopted. This correspond to Davis and Heywood’s (1963) remark that among living forms there are many kinds of discontinuities, all of which may be of interest. As a consequence, “a variety of species concepts are necessary to adequately capture the complexity of variation patterns in nature. To subsume this variation under the rubric of any one concept leads to confusion and tends to obscure important evolutionary questions” (Mishler and Donoghue 1982, 500). The species category is thus heterogeneous, in the sense that it encompasses multiple types of species taxa, likely produced by a number of phenomena (Nathan and Cracraft 2020).

The other version of species pluralism claims that the assignment of species-level taxa is always relative to a theory, explanatory aim, or classificatory purpose (Dupré 1981; 1999; Kitcher 1984a; 1984b). Accepting this form of pluralism seems to require that the different species concepts adopted in circumscribing different sets of taxa are always evident and possibly stated in explicit terms (Conix 2019). This form of pluralism has been often regarded (e.g., by Ereshefsky 1992; 1998; Hull 1999) as a reason for rejecting species realism, i.e., species realism would necessarily accompany species monism. But it should be noted that other authors have merged pluralism with realism (Kitcher 1984b; Dupré 1993; 1999; Boyd 1999; Wilkins 2003; Slater 2013).

[top of entry]

7.2 Soft (taxonomic) species pluralism

As noted quite long ago by Kitcher (1984b), description and naming of the different kinds of units relevant to the different biological disciplines require pluralism of species notions. This is not without consequences, however, because the resulting classifications are not the same: number and circumscription of the units recognized following the adoption of different criteria may differ in an unpredictable way and to an unpredictable extent (Conix 2018; Brigandt 2020). Moreover, pluralistic taxonomy necessarily requires some degree of nomenclatural pluralism (Minelli 2019; 2020).

A somehow cryptic form of nomenclatural pluralism is due to the instability of taxonomy. The following is an example of ambiguity in the meaning of species names caused by differences in taxonomic treatment (Minelli 2019). According to Zachos et al. (2013) the red deer Cervus elaphus Linnaeus (1758) includes the Mediterranean subspecies C. e. corsicanus (Erxleben, 1777), which is insteasd treated by Groves and Grubb (2011) as a distinct species (Cervus corsicanus Erxleben, 1777). As a consequence, in the absence of further qualifications, Linnaean names may become semantically instable. The problem can be solved by specifying taxonomic concepts (Berendsohn 1995), i.e. the precise meanings of names in the different sources and the semantic relationships among them. In the above example, “Cervus elaphus Linnaeus (1758) sensu Groves and Gubb (2011)” is part of “Cervus elaphus Linnaeus (1758) sensu Zachos et al. (2013)”. Taxonomic groups for which two or more, conflicting taxonomies coexist are all but rare. For example, ca. 10,000 species and 22,000 subspecies of birds are currently recognized (Lepage 2019), but over 1.5 million distinct taxonomic concepts are available for them in the literature (Lepage et al. 2014).

To contrast the standing instability of taxonomy and, as a consequence, of species names, an international effort towards establishing a global list of accepted species has been launched. Conceptual, political, and technical aspects of this initiative have been discussed in a set of articles in the December 2021 issue of the journal Organisms Diversity and Evolution (Conix et al. 2021; Hobern et al. 2021; Lien et al. 2021; Pyle et al. 2021; Thiele et al. 2021; Thomson et al. 2021).

[top of entry]

8. Conclusion

“The diversity of life is not seamless but comes in relatively discrete packages, species” (Sterelny 1998, 78). However, much of the difficulty in classifying living organisms and defining a unit for such a classification is, that the more closely the biological world have been studied, the more exceptions have been found for what formerly was considered a relatively well-defined system of species. Taxonomy’s unceasing developments remind of Thomas Kuhn’s view: “the history of developed science shows that nature will not indefinitely be confined in any set which scientists have constructed so far” (Kuhn 1970, 263).

This article has also demonstrated that “the species problem” has being growing in recent years. If somebody has expected that the unit of classification is given and that just the criteria for classifying them has to be decided, we have now learned another lesson: conceptions of species, and thereby the way to specify them, is closely related to the general approach to classification such that, for example, cladistics tends to develop a cladistic species concept, while phenetics and numerical taxonomy provide alternatives.

The question is whether science can ever reach consensus about one classification of animals and plants reflecting evolutionary history (the “tree of life”) [18]? Richards (2016, 282-285) thinks not. He speaks about an innate tension between our psychological biases and the theory that governs biological classification, which has played out in the history of classification from Aristotle to the present, for example in the disputes between evolutionary taxonomists and pheneticists and (284-285):

Until human psychology changes, and there is little reason to believe it will soon, and as long as we learn and apply general terms, this tension will remain. We can learn the theoretical framework of modern evolutionary biological classification, but we cannot avoid our psychological tendencies in learning the general terms that we apply to biological taxa. We cannot avoid thinking about all the kinds of organisms we see in the world as timeless sets of things, where set inclusion is determined by a set of properties, even though we also believe these kinds to be historical and branches on an evolutionary tree. If so, then the practice of biological classification will continue to be fraught with the problems that arise from this fundamental tension.

Other kinds of tensions may also be relevant, perhaps even more so. There may be different social interests associated with classification including different demands by different biological subdisciplines and different goals with producing classifications such as demands to simply communication versus demands to reflect the evolutionary history. When all comes to all, the lesson we have learned is that further empirical discoveries themselves cannot solve this problem, which involves deep conceptual, theoretical and philosophical issues.

[top of entry]


I am grateful to Claudio Gnoli and Birger Hjørland for inviting me to contribute this entry to IEKO. To Birger Hjørland and three referees for their stimulating comments on a previous version.


1. Interestingly, the term species is also used in mineralogy. The mineralogical species was first critically discussed by Dolomieu (1801); its current notion is comparable to the diagnosable biological species discussed in subsection 6.2.6 of this article, as shown by the following quote: “The concept of a mineral species. A mineral species is defined mainly on the basis of its chemical composition and crystallographic properties, and these must therefore be the key factors in determining whether the creation of a new mineral species and a new mineral name is justified. If a mineral is found whose composition and/or crystallographic properties are substantially different from those of any existing mineral species, there is a possibility that it may be a new species. A general guideline for compositional criteria is that at least one structural site in the potential new mineral should be predominantly occupied by a different chemical component than that which occurs in the equivalent site in an existing mineral species” (Nickel and Grice 1998, 238-239).

2. However, “One might think that there is a single best way of grouping entities: the grouping that exactly represents the various kinds of organisms that exist “out there”. This is not the case: “To see this, consider a fundamental dichotomy that is made in philosophical discussions of scientific classification between natural systems of classification and artificial systems of classification. One could say that a natural system of classification represents “the natural order of things” […] while an artificial system of classification groups things together in a way that suits whatever purposes we might have but does not represent the natural order. […] While artificial systems of classification may be useful for some purposes, in science the focus is usually on natural systems of classification [note 2 here omitted]. […] An important aim of science is to produce knowledge that, once obtained by studying a sample of a kind, can be extrapolated to all other members of the same kind” (Reydon 2020, 219-220, italics in original).

3. This definition is thus not about what Wilkins (2018, 376) wrote: “27 Taxonomic Species […] Also: Whatever a competent taxonomist chooses to call a species […] Synonyms: Cynical species concept (Kitcher 1984b)”. Wilkins and Kitcher are concerned with the notion of taxonomic species, as discussed in subsection 6.2.1.

4. Dupré (2001) describes his own view as “contrary to the prevailing orthodoxy” and it is an exception to the view that species taxa should be as similar to one another as possible. He wrote (204): “The fact that classification cannot, at least, be closely tied to the central theory of biology leaves room for the thoroughly pragmatic and pluralistic approach to biological taxonomy that I shall advocate”.

5. Hjørland (2009, 1522-3) defined: “Concepts are dynamically constructed and collectively negotiated meanings that classify the world according to interests and theories. Concepts and their development cannot be understood in isolation from the interests and theories that motivated their construction, and, in general, we should expect competing conceptions and concepts to be at play in all domains at all times”.
Brigandt (2020) discussed the nature, use and transformation of “concept” in biology. On p. 80, he wrote “Philosophers construe a concept as the mental content associated with a term, and because of its content, the concept plays a distinctive role in reasoning, from theorizing to practical action”.

6. Dupré (2001) argues the other way round: species has always been a well established term in biological taxonomy and should remain so, but should not be considered a unit of evolution, although often confused with this.

7. “πᾶν γὰρ τὸ διαφέρον διαφέρει ἢ γένει ἢ εἴδει, γένει μὲν ὧν μὴ ἔστι κοινὴ ἡ ὕλη μηδὲ γένεσις εἰς ἄλληλα, οἷον ὅσων ἄλλο σχῆμα τῆς κατηγορίας, εἴδει δὲ ὧν τὸ αὐτὸ γένος” (Aristotle, Metaphysics I, 3, 1054b27) (For everything which is different differs either in genus or in species—in genus, such things as have not common matter and cannot be generated into or out of each other, e.g., things which belong to different categories; and in species, such things as are of the same genus; transl. after Aristotle, Mtaphysics (English), Perseus Digital Library; http://data.perseus.org/citations/..., accessed January 14, 2022).

8. “διαφορὰ δὲ καὶ ἑτερότης ἄλλο. τὸ μὲν γὰρ ἕτερον καὶ οὗ ἕτερον οὐκ ἀνάγκη εἶναι τινὶ ἕτερον: πᾶν γὰρ ἢ ἕτερον ἢ ταὐτὸ ὅ τι ἂν ᾖ ὄν: τὸ δὲ διάφορον τινὸς τινὶ διάφορον, ὥστε ἀνάγκη ταὐτό τι εἶναι ᾧ διαφέρουσιν” (Aristotle, Metaphysics I, 3, 1054b23) (but "difference" is distinct from "otherness". For that which is "other" than something need not be other in a particular respect, since everything which is existent is either "other" or "the same". But that which is different from something is different in some particular respect, so that that in which they differ must be the same sort of thing; transl. after Aristotle, Metaphysics (English), Perseus Digital Library; http://data.perseus.org/citations/..., accessed January 14, 2022).

9. “levissimo artificio superaddi potest nota specificae differentiae per modum cognominis” (Rivinus 1690, 11) (by a very simple trick, through the added name [Linnaeus’ nomen triviale, i.e. the specific epithet in modern parlance] it is possible to add a remark pointing to the specific difference).

10. Following a first publication (Ghiselin 1966), where he first rejected the traditional view of species as classes of individuals, in an article titled “A radical solution to the species problem” Ghiselin (1974, 538) defined species as “the most extensive units in the natural economy such that reproductive competition occurs among their parts”. Thus, treating species as separate reproductive communities, Ghiselin regarded them as individuals endowed with unity, space-time continuity, cohesion and integration, their parts being linked together by historical-genealogical links. This means that species have a birth (speciation) and a death (extinction); between these events the species may undergo significant phenotypic changes. On this contentious issue, see also Hull (1976; 1978); Ghiselin (1987; 1988; 1997); Stamos (2003); Wilkins (2009a; 2009b; 2018); Richard (2010) and Zachos (2016). Richard’s (2010, 176, italics in original) found: “The bottom line is that this metaphysical framework based on individuality promises to be a fertile way to think about species within the evolutionary context that coheres with what evolutionary theory tells us about species and that is confirmed by empirical investigations”. Richards (ibid.) also wrote: “[G]iven that species taxa have various characteristics and function within evolutionary theory in specific ways, what is the best general, fundamental way to think about them – as sets or individuals? The species-as-individuals answer seems most promising in terms of coherence with evolutionary theory, fertility in thinking about species taxa, and in the development of evolutionary theory. If the species-as-individuals conception is so superior for its value to biological thinking, why is the species-as-sets conception so attractive to philosophers? There is, I believe, a clash of two disciplines here – philosophy and biology – with different goals, methods, tools and traditions. Natural kind and set thinking has been an important part of philosophy for a very long time – at least since Plato. Because philosophers have been trained in this tradition, it is natural for them to turn to it to make sense of the world”.
While agreeing that biological species are not classes, Mayr (1987) was inclined to call them populations, rather than individuals, because the term population conveys the impression of the multiplicity and composite nature of species, but his view has never gained much favor.

11. Dobzhansky 1935; Ramsbottom 1938; Burma and Mayr 1949; George 1956; Sylvester-Bradley 1956; Mayr 1957; 1992; 2000a; 2000b; Sokal and Crovello 1970; Ghiselin 1977; 1987; Kottler 1978; Wiley 1978; Mishler and Donoghue 1982; Cracraft 1983; 1987; 1992; 1997; 2000; Lambert and Paterson 1984; Paterson 1985; 1993; Atran et al. 1987; Mishler and Brandon 1987; Mckitrick and Zink 1988; Avise and Ball 1990; Nixon and Wheeler 1990; Mayr and Ashlock 1991; Grant 1992; Eldredge 1993; Kornet 1993; Baum and Donoghue 1995; Lambert and Spencer 1995; Mayden 1997; Davis 1997; Hull 1997; de Queiroz 1998; 1999; 2007; Pleijel and Rouse 1999; Wheeler 1999; Meier and Willmann 2000a; 2000b; Mishler and Theriot 2000; Wheeler and Meier 2000; Wheeler and Platnick 2000; Wiley and Mayden 2000; Hey 2001a; 2006; Wilkins 2003; 2011; Bock 2004; Vos 2011; Hausdorf 2011; Zachos 2015; 2016.

12. “Quæ specie differunt speciem suam perpetuo servant, neque hæc ab illius semine oritur, aut vice versa” (Ray 1686, 40) (those [plants] which differ in species keep their own species forever, and one does not arise from the seed of the other and vice versa; transl. after Lazenby 1995, 1157).

13. “Quæcunque ergo Differentiæ ex eiusdem seu in individuo, seu specie plantæ semine oriuntur, accidentales sunt, non specificæ. Hæ enim speciem suam satione iterum non propagant […] aut si inter duas aliquas comparatio instituatur, quæ plantæ ex alterutrius semine non proveniunt, nec unquam semine satæ transmutantur in se invicem, eæ demum specie distinctæ sunt” (Ray 1686, 40). (Therefore whatever differences arise from a seed of a particular kind of plant either in an individual or in a species, they are accidental and not specific. For they do not propagate their species again from seed; […] if a comparison is made between two kinds of plant, those plants which do not arise from the seed of one or the other, nor when sown from seed are ever changed one into the other, these finally are distinct in species; transl. after Lazenby 1995, 1157).

14. “Ein [reales] Band […] welches ganz unabhängig ist von der menschlichen Betrachtung […] ist vorhanden zwischen den Gliedern einer Art, insofern sie sich als Zusammengehörige erkennen und miteinander fortpflanzen” (Plate 1914, 117) (Totally independent from human perspective, among the members of a species there is a real bond, in so far as they perceive themselves as relatives and reproduce among themselves).

15. If evolution was just a matter of adaptation, similarity due to similar adaptation would not be distinguished from similarity due to common ancestry, therefore we would probably continue classifying whales with fishes, bats with birds etc. — For a while, especially between 1965 and 1974, Ernst Mayr and others defendend their approach to biological systematics as the one that took into account both aspects of evolution, i.e. adaptation and phylogenetic relationships; this contrasted Hennig’s approach, strictly focused on phylogeny. Only the phenetic school, championed by Sokal and Sneath, deliberately took distance from phylogeny — eventually, however, the numerical methods developed in order to implement this approach were extensively “translated” to the service of phylogenetic reconstruction.

16. “a population is a species to the extent that it has most of the members of some such list of criteria as […]
(1) A single origin and final extinction, with reproductive (informational) continuity between these events.
(2) Origin taking many, rather than several or one, generations.
(3) Limited but real extension in both time and space.
(4) Origin by transformation of a population of individuals, not from one or two parents.
(5) Capacity to evolve.
(6) Capacity to act as a unit in evolution.
(7) Occurrence in spatially disjunct populations.
(8) Potential reproductive continuity among all included populations; compatibility for development and fitness of offspring as well as for mating and fertilization.
(9) A mechanism for recognizing other individuals or gametes of the same species.
(10) Reproductive isolation from other species.
(11) Being composed of individuals.
(12) Capacity to speciate, either with or without phyletic branching.
(13) Capacity to remain the same species while, and after, part becomes another species after phyletic branching.
(14) Possession of phenotypic, genic, and genotypic characters jointly distinct from those of any other species.
(15) Occupancy of a perhaps broad and flexible niche (in the sense of a perhaps arbitrarily bounded part, or even disjoint parts, of the [biotic and abiotic] environmental hyperspace) different from that of any sympatric species.
(16) Ultimate regulation of population or metapopulation density being causally different from that of any sympatric species” (Van Valen 1988, 51; 53).

17. In his review of Slater (2013) Richards (2015) wrote: “Slater […] distinguishes between taxic pluralism, a plurality of partitions of organisms into species taxa, and category pluralism, a plurality of ways to conceive the species category. According to Slater, the former, but not the latter, challenges the idea that species are real. That different species are different kinds of things does not by itself imply that species are unreal. But taxic pluralism, the partitioning of organisms in multiple inconsistent ways, is a challenge to realism in that if there is no single way to divide biodiversity into species taxa, it is not clear that there really are species things. Rather there would just be different ways to divide biodiversity, based on our interests”. Richards also found that Slater underplays the role that the species concept plays in biological theory.

18. The metaphor ”tree of life” is itself subject to criticism. See Makarenkov et al. (2004), Rivera (2004), Gontier (2017), Quammen (2018).

[top of entry]


Ackery, P. R. and R. I. Vane-Wright. 1984. Milkweed Butterflies: Their Cladistics and Biology. London: Department of Entomology, British Museum Natural History.

Allen, J. A. 1877. ”The Influence of Physical Conditions in the Genesis of Species”. Radical Review 1: 108-140.

Amadon, Dean. 1966. “The Superspecies Concept”. Systematic Zoology 15, no. 3: 245-249.

Arber, Agnes. 1912. Herbals: Their Origin and Evolution: A Chapter in the History of Botany, 1470-1670. Cambridge: Cambridge University Press; 1986, 3rd ed., ibid.

Aristotle. 1924. Metaphysics, ed. W. D. Ross. Oxford: Clarendon Press. http://data.perseus.org/texts/urn:cts:greekLit:tlg0086.tlg025.

Atran, Scott, Richard W. Burkhardt, Pietro Corsi, Anne Diara, Bernardino Fantini, Jean-Louis Fischer, M. J. S. Hodge, Goulven Laurent, Antonello La Vergata, Anto Leikola, Pierre Louis, Ernst Mayr, Gerhard H. Müller, Roselyne Rey, Jacques Roger and Philipp R. Sloan. 1987. Histoire du concept d’espèce dans les sciences de la vie. Paris: Fondation Singer-Polignac.

Avise, John C. and R. Martin Ball Jr. 1990. ”Principles of Genealogical Concordance in Species Concepts and Biological Taxonomy”. In Oxford Surveys in Evolutionary Biology, eds. Douglas J. Futuyma and Janis Antonovics. Oxford: Oxford University Press, 45-67.

Balme, D. M. 1987. “Aristotle’s Use of Division and Differentiae”. In Philosophical Issues in Aristotle’s Biology, eds. Allan Gotthelf and James G. Lennox. Cambridge: Cambridge University Press, 69-89.

Bauhin, Caspar. 1596. Phytopinax, seu, Enumeratio plantarum ab herbariis nostro seculo descriptarum: cum earum differentiis. Basileae: S. Henricpetri.

Baum, David A. and Michael J. Donoghue. 1995. “Choosing Among Alternative ‘Phylogenetic’ Species Concepts”. Systematic Botany 20, no. 4: 560-573.

Beltrán, Margarita, Chris D. Jiggins, Vanessa Bull, Mauricio Linares, James Mallet, W. Owen McMillan and Eldredge Bermingham. 2002. “Phylogenetic Discordance at the Species Boundary: Comparative Gene Genealogies Among Rapidly Radiating Heliconius Rutterflies“. Molecular Biology and Evolution 19, no. 12: 2176-2190.

Bennett, Chandalin, M. Catherine Aime and George Newcombe. 2011. ”Molecular and Pathogenic Variation Within Melampsora on Salix in Western North America Reveals Numerous Cryptic Species”. Mycologia 103, no. 5: 1004-1018.

Benton, Michael J. 2000. “Stems, Nodes, Crown Clades, and Rank-Free Lists: Is Linnaeus Dead?” BioIogical Reviews 75, no. 4: 633-648.

Berendsohn, Walter G. 1995. “The Concept of “Potential Taxa” in Databases”. Taxon 44, no. 2: 207-212.

Berlin, Brent. 1973. “Folk Systematics in Relation to Biological Classification and Nomenclature”. Annual Review of Ecology and Systematics 4: 259-271.

Bernard, Henry M. 1902. “The Species Problem in Corals”. Nature 65, no. 1694: 560.

Bessey, Charles E. 1908. “The Taxonomic Aspect of the Species Question”. American Naturalist 42, n. 496: 218-222.

Bickford, David, David J. Lohman, Navjot S. Sodhi, Peter K. L. Ng, Rudolf Meier, Kevin Winker, Krista K. Ingram and Indraneil Das. 2007. “Cryptic Species as a Window on Diversity and Conservation”. Trends in Ecology & Evolution 22, no. 3: 148-155.

Blaxter, Mark, Jenna Mann, Tom Chapman, Fran Thomas, Claire Whitton, Robin Floyd and Eyualem Abebe. 2005. “Defining Operational Taxonomic Units Using DNA Barcode Data”. Philosophical Transactions of the Royal Society of London B Biological Sciences 360: 1935-1943.

Bliss, Henry Evelyn. 1929. The Organization of Knowledge and the System of the Sciences. New York: Henry Holt and Company.

Bock, Walter J. 2004. “Species: The Concept, Category and Taxon”. Journal of Zoological Systematics and Evolutionary Research 42, no. 3: 178-190.

Boyd, Richard. 1999. “Homeostasis, Species and Higher Taxa”. In Species: New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, MA: MIT Press, 141-185.

Brickell, C. D., C. Alexander, J. C. David, W. L.A. Hetterschied, A. C. Leslie, V. Malécot, Xiaobai Jin and J. J. Cubey. 2009. International Code of Nomenclature for Cultivated Plants (Scripta Horticulturae 10), 8th edn. Leuven: International Society of Horticultural Science.

Brigandt, Ingo. 2020. “How are Biology Concepts Used and Transformed?” In Philosophy of science for biologists, eds. Kostas Kampourakis and Tobias Uller. Cambridge: University Press Cambridge, 79-101.

Bryant, Harold N. and Philip D. Cantino. 2002. “A Review of Criticisms of Phylogenetic Nomenclature: Is Taxonomic Freedom the Fundamental Issue?” BioIogical Reviews 77, no. 1: 39-55.

Buffon, Georges Louis Leclerc comte de. 1749-89. Histoire naturelle, générale et particulière: avec la description du cabinet du Roy. 36 vols. Paris: Imprimerie royale. (Tome I, 1749; Tome II, 1749; Tome XIV, 1766).

Burma, Benjamin H. and Ernst Mayr. 1949. ”The Species Concept: A Discussion”. Evolution 3, no. 4: 369-373.

Cain, Arthur J. 1954. Animal Species and Their Evolution. London: Hutchinson University Library.

Cain, A. J. 1997. “John Locke on Species”. Archives of Natural History 24, no. 3: 337-360.

Camp, W. H. 1951. “Biosystematy“. Brittonia 7, no. 3: 113-127.

Camp, W. H. and C. L. Gilly. 1943. “The Structure and Origin of Species with a Discussion of Intraspecific Variability and Related Nomenclatural Problems”. Brittonia 4, no. 3: 323-385.

Candolle, Alphonse de. 1867. Lois de la nomenclature botanique adoptées par le Congrès International de Botanique tenu à Paris en Août, 1867, suivies d’une deuxième édition de l’introduction historique et du commentaire qui accompagnaient la rédaction préparatoire presentée au Congrès. Genève-Bâle: Georg; Paris: Ballière.

Cannon, Charles H. and Rémy J. Petit. 2020. “The Oak Syngameon: More Than the Sum of Its Parts”. New Phytologist 226, no. 4: 978-983.

Cantino, Philip D. 2004. “Classifying Species versus Naming Clades”. Taxon 53, no. 3: 795–798.

Cantino, Philip D. and Kevin de Queiroz. 2020. International Code of Phylogenetic Nomenclature (PhyloCode). Boca Raton, London & New York: CRC Press.

Carpenter, James M. 2003. “Critique of Pure Folly”. Botanical Review 69, no. 1: 79-92.

Carstens, Bryan C., Tara A. Pelletier, Noah M. Reid and Jordan D. Satler. 2013. “How to Fail at Species Delimitation?” Molecular Ecology 22, no. 17: 4369-4383.

Casacci, L. P., Francesca Barbero and Emilio Balletto. 2013. “The “Evolutionarily Significant Unit” Concept and Its Applicability in Biological Conservation”. Italian Journal of Zoology 81, no. 2: 182-193.

Cellinese, Nico, David A. Baum and Brent D. Mishler. 2012. “Species and Phylogenetic Nomenclature”. Systematic Biology 61, no. 5: 885-891.

Chase, Mark W. and Michael F. Fay. 2009. “Barcoding of Plants and Fungi“. Science 325, no. 5941: 682-683.

Choate, Helena. 1912. “The Origin and Development of the Binomial System”. Plant World 15, no. 11: 257-263.

Collins, R. A. and R. H. Cruickshank. 2013. “The Seven Deadly Sins of DNA Barcoding”. Molecular Ecology Resources 13, no. 6: 969-975.

Conix, Stijn. 2018. “Integrative Taxonomy and the Operationalization of Evolutionary Independence”. European Journal of Philosophy of Science 8, no. 3: 587-603.

Conix, Stijn. 2019. “Radical Pluralism, Classificatory Norms and the Legitimacy of Species Classifications”. Studies in History and Philosophy of Biological and Biomedical Sciences 73: 27-34.

Conix, Stijn, Stephen T. Garnett, Kevin R. Thiele, Les Christidis, Peter Paul van Dijk, Olaf S. Bánki, Saroj K. Barik, John S. Buckeridge, Mark J. Costello, Donald Hobern, Paul M. Kirk, Aaron Lien, Svetlana Nikolaeva, Richard L. Pyle, Scott A. Thomson, Zhi-Qiang Zhang and Frank E. Zachos. 2021. “Towards a Global List of Accepted Species III. Independence and Stakeholder Inclusion”. Organisms Diversity & Evolution 21, no. 4: 631-643.

Cowles, H. C. 1908. “An Ecological Aspect of the Conception of Species”. American Naturalist 42, no. 496: 265-271.

Cracraft, Joel. 1983. “Species Concepts and Speciation Analysis”. Current Ornithology 1: 159-187.

Cracraft, Joel. 1987. “Species Concepts and the Ontology of Evolution”. Biology and Philosophy 2, no. 3: 329-346.

Cracraft, Joel. 1992. “The Species of the Birds-of-Paradise (Paradisaeidae): Applying the Phylogenetic Species Concept to a Complex Pattern of Diversification”. Cladistics 8, no. 1: 1-43.

Cracraft, J. 1997. “Species Concepts in Systematics and Conservation Biology—An Ornithological Viewpoint”. In Species: The Units of Biodiversity, eds. M. F. Claridge, H. A. Dawah and M. R. Wilson. New York: Chapman and Hall, 325-339.

Cracraft, Joel. 2000. ”Species Concepts in Theoretical and Applied Biology: A Systematic Debate with Consequences”. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 3-14.

Cracraft, Joel, Julie Feinstein, Jeffrey Vaughn and Kathleen Helm-Bychowski. 1998. “Sorting Out Tigers (Panthera tigris): Mitochondrial Sequences, Nuclear Inserts, Systematics, and Conservation Genetics”. Animal Conservation 1, no. 2: 139–150.

Crandall, Keith A., Olaf R. P. Bininda-Emonds, Georgina M. Mace and Robert K. Wayne. 2000. “Considering Evolutionary Processes in Conservation Biology”. Trends in Ecology & Evolution 15, no. 7: 290-295.

Danser, B. H. 1929. “Ueber die Begriffe Komparium, Kommiskuum und Konvivium und die Entstehungsweise der Konvivien“. Genetica 11, no. 5: 399-450.

Darlington, C. D. 1940. “Taxonomic Species and Genetic Systems”. In The New Systematics, ed. Julien S. Huxley. Oxford: University Press, 137-160.

Darwin, Charles. 1859. On the Origin of Species by Means of Natural Selection. London: Murray.

Davis, Jerrold I. 1997. ”Evolution, Evidence, and the Role of Species Concepts in Phylogenetics”. Systematic Biology 22, no. 2: 373-403.

Davis, Jerrol I. and Kevin C. Nixon. 1992. “Populations, Genetic Variation, and the Delimitation of Phylogenetic Species”. Systematic Biology 41, no. 4: 421-435.

Davis, Peter H. and Vernon Heywood. 1963. Principles of Angiosperm Taxonomy. Edinburgh: Oliver and Boyd.

Dayrat, Benoît. 2005. “Towards Integrative Taxonomy”. Biological Journal of the Linnean Society 85, no. 3: 407-415.

Dayrat, Benoît, Philip D. Cantino, Julia A. Clarke and Kevin De Queiroz. 2008. “Species Names in the PhyloCode: The Approach Adopted by the International Society for Phylogenetic Nomenclature”. Systematic Biology 57, no. 3: 507-514.

de Queiroz, Kevin. 1997. “Misunderstandings About the Phylogenetic Approach to Biological Nomenclature: A reply to Lidén and Oxelman”. Zoologica Scripta 26, no. 1: 67-70.

de Queiroz, Kevin. 1998. “The General Lineage Concept of Species, Species Criteria, and the Process of Speciation: A Conceptual Unification and Terminological Recommendations”. In Endless Forms: Species and Speciation, eds. Daniel J. Howard and Steward H. Berlocher. Oxford: Oxford University Press, 57-75.

de Queiroz, Kevin. 1999. “The General Lineage Concept of Species and the Defining Properties of the Species Category”. In Species: New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, Ma.: MIT Press, 49-89.

De Queiroz, Kevin. 2005. “Different Species Problems and Their Resolution”. BioEssays 27, no. 12: 1263-1269.

de Queiroz, Kevin. 2007. ”Species Concepts and Species Delimitation”. Systematic Biology 56, no. 6: 879-886.

de Queiroz, Kevin and Philip D. Cantino. 2001. “Phylogenetic Nomenclature and the PhyloCode”. Bulletin of Zoological Nomenclature 58, no. 4: 254-271.

de Queiroz, Kevin and Michael J. Donoghue. 1988. “Phylogenetic Systematics and the Species Problem”. Cladistics 4, no. 4: 317-338.

Dimmick, Walter W., Michael J. Ghedotti, Michael J. Grose, Anne M. Maglia, Daniel J. Meinhardt and David S. Pennock. 1999. “The Importance of Systematic Biology in Defining Units of Conservation”. Conservation Biology 13, no. 3: 653-660.

Dobzhansky, Theodosius. 1935. ”A Critique of the Species Concept in Biology”. Philosophy of Science 2, no. 3: 344-355.

Dobzhansky, Theodosius. 1950. “Mendelian Populations and their Evolution”. American Naturalist, 84, no. 819: 401-418.

Dobzhansky, Theodosius. 1970. Genetics of the Evolutionary Process. New York: Columbia University Press.

Dolomieu, D.. An IX [= 1801]. Sur la philosophie minéralogique, et sur l’espèce minéralogique. Paris: Bossange, Masson & Besson.

Dominguez, Eduardo and Quentin D. Wheeler. 1997. “Taxonomic Stability is Ignorance”. Cladistics 13, no. 4: 367-372.

Dubois, Alain. 1982. “Les notions de genre, sous-genre et group d’espèces en zoologie à la lumière de la systématique evolutive”. Monitore Zoologico Italiano 16, no. 1: 9-63.

Dubois, Alain. 2011. “The Rich but Confusing Terminology of Biological Nomenclature: A First Step Towards a Comprehensive Glossary”. Bionomina 3: 71-76.

Dupré, John. 1981. “Natural Kinds and Biological Taxa”. The Philosophical Review 90, no. 1: 66-90.

Dupré, John. 1993. The Disorder of Things: Metaphysical Foundations of the Disunity of Science. Cambridge, MA: Harvard University Press.

Dupré, John. 1999. “On the Impossibility of a Monistic Account of Species”. In Species: New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, MA: MIT Press, 3-22.

Dupré, John. 2001. “In Defence of Classification”. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences 32, no. 2: 203–219,

Dupuis, Julian R., Amanda D. Roe and Felix A.H. Sperling. 2012. “Multi-Locus Species Delimitation in Closely Related Animals and Fungi: One Marker Is Not Enough”. Molecular Ecology 21, no. 18: 4422-4436.

Eldredge, Niles. 1993. ”What, If Anything, Is a Species?” In Species, Species Concepts, and Primate Evolution, eds. William H. Kimbel and Lawrence B. Martin. New York: Plenum Press, 3-20.

Eldredge, Niles and Joel Cracraft. 1980. Phylogenetic Patterns and the Evolutionary Process. New York: Columbia University Press.

Ence, Daniel D. and Bryan C. Carstens. 2011. “SpedeSTEM: A Rapid and Accurate Method for Species Delimitation”. Molecular Ecology Resources 11, no. 3: 473-480.

Endler, John A. 1989. “Conceptual and Other Problems in Speciation”. In Daniel Otte and John A. Endler eds., Speciation and its Consequences. Sunderland: Sinauer Associates, 625-648.

Enenkel, Karl A. E. 2014. “The Species and Beyond: Classification and the Place of Hybrids in Early Modern Zoology”. In Zoology in Early Modern Culture: Intersections of Science, Theology, Philology, and Political and Religious Education, eds. Karl A. E. Enenkel and Paul J. Smith. Leiden and Boston: Brill, 57-148.

Ereshefsky, Marc. 1992. “Eliminative Pluralism”. Philosophy of Science 59, no. 4: 671-690.

Ereshefsky, Marc. 1998. “Species Pluralism and Anti-Realism”. Philosophy of Science, 65, no. 1: 103-120.

Ereshefsky, Marc. 2001. The Poverty of the Linnaean Hierarchy: A Philosophical Study of Biological Taxonomy. Cambridge: Cambridge University Press.

Erickson, David L., John Spouge, Alissa Resch, Lee A. Weigt and W. John. 2008. “DNA Barcoding in Land Plants: Developing Standards to Quantify and Maximize Success“. Taxon 57, no. 4: 1304-1316.

Flot, Jean-François. 2015. “Species Delimitation’s Coming of Age”. Systematic Biology 64, no. 6: 897-899.

Floyd, Robin, Abebe Eyualem, Artemis Papert and Mark Blaxter. 2002. “Molecular Barcodes for Soil Nematode Identification”. Molecular Ecology 11, no. 4: 839-850.

Forey, Peter L. 2002. “PhyloCode – Pain, No gain”. Taxon 51, no. 1: 43-54.

Franklin, James. 1986. Aristotle on Species Variation. Philosophy 61, no. 236, 245-252.

Fuchs, Leonhart. 1542. De historia stirpium commentarii insignes. Basel: Officina Isingriniana.

Fujita, Matthew K., Adam D. Leaché, Frank T. Burbrink, Jimmy A. McGuire and Craig Moritz. 2012. “Coalescent-Based Species Delimitation in an Integrative Taxonomy”. Trends in Ecology and Evolution 27, no. 9: 480-488.

Fusco, Giuseppe and Alessandro Minelli. 2019. The Biology of Reproduction. Cambridge: Cambridge University Press.

George, T. Neville. 1956. “Biospecies, Chronospecies and Morphospecies”. In The Species Concept in Paleontology, ed. P. C. Sylvester-Bradley. London: The Systematics Association, 123-137.

Gesner, Conrad. 1551. Historiae animalium liber primus de quadripedibus viviparis. Tiguri [Zurich]: C. Froschoverus.

Gesner, Conrad. 1560. Icones avium omnium. Editio secunda. Tiguri [Zurich]: C. Froschoverus.

Ghiselin, Michael T. 1966. “On Psychologism in the Logic of Taxonomic Controversies”. Systematic Zoology 15, no. 3: 207-215.

Ghiselin, Michael T. 1974. “A Radical Solution to the Species Problem”. Systematic Zoology 23, no. 4: 536-544.

Ghiselin, Michael T. 1977. ”On Paradigms and the Hypermodern Species Concept”. Systematic Biology 26, no. 4: 437-438.

Ghiselin, Michael T. 1984. “Narrow Approaches to Phylogeny: A Review of Nine Books on Cladism”. In Oxford Surveys in Evolutionary Biology, eds. Richard Dawkins and Mark Ridley, 1. Oxford: Oxford University Press, 209-222.

Ghiselin, Michael T. 1987. “Species Concepts, Individuality, and Objectivity”. Biology & Philosophy 2, no. 2: 127-143.

Ghiselin, Michael T. 1988. “The Individuality Thesis, Essences, and Laws of Nature”. Biology & Philosophy 3, no. 4: 467-471.

Ghiselin, Michael T. 1997. Metaphysics and the Origin of Species. Albany, NY: State University of New York Press.

Gilmour, J. L. and Heslop-Harrison, J. 1954. “The Deme Terminology and the Units of Microevolutionary Change”. Genetica 27: 147-161.

Gontier, Nathalie (ed.). 2015 Reticulate Evolution: Symbiogenesis, Lateral Gene Transfer, Hybridization and Infectious Heredity. Heidelberg, Germany: Springer International Publishing AG Switzerland.

Grant, B. Rosemary and Peter R. Grant. 1996. “High Survival of Darwin’s Finch Hybrids: Effects of Beak Morphology and Diets“. Ecology 77, no. 2: 500-509.

Grant, Verne. 1957. “The Plant Species in Theory and Practice“. In The Species Problem, ed. Ernst Mayr. American Association for the Advancement of Science Publication no. 50, 39-80.

Grant, Verne. 1971. Plant Speciation. New York: Columbia University Press.

Grant, Verne. 1981. Plant Speciation, 2nd edition. New York: Columbia University Press.

Grant, Verne. 1992. “Comments on the Ecological Species Concept”. Taxon 41, no. 2: 300-312.

Greuter, Werner, George Garrity, David L. Hawksworth, Regine Jahn, Paul M. Kirk, Sandra Knapp, John McNeill, Ellinor Michel, David J. Patterson, Richard Pyle and Brian J. Tindall. 2011. “Draft BioCode (2011): Principles and Rules Regulating the Naming of Organisms”. Bionomina 3: 26-44; Taxon 60, no. 1: 201–212; Bulletin of Zoological Nomenclature 68, no. 1: 10-28.

Greuter, W., D. L. Hawksworth, J. McNeill, M. A. Mayo, A. Minelli, P. H. A. Sneath, B. J. Tindall, P. Trehane and P. Tubbs. 1996. “Draft BioCode: The Prospective International Rules for the Scientific Names of Organisms”. Taxon 45, no. 2: 349-372.

Greuter, W., D. L. Hawksworth, J. McNeill, M. A. Mayo, A. Minelli, P. H. A. Sneath, B. J. Tindall, P. Trehane and P. Tubbs. 1998. “Draft BioCode (1997): The Prospective International Rules for the Scientific Names of Organisms”. Taxon 47, no. 1: 127-150.

Griffiths, Graham C. D. 1976. “The Future of Linnaean Nomenclature”. Systematic Zoology 25, no. 2: 168-173.

Groves, Colin and Peter Grubb. 2011. Ungulate Taxonomy. Baltimore: Johns Hopkins University Press.

Guiry, Michael D. 2012. ”How Many Species of Algae Are There?”. Journal of Phycology 48, no. 5: 1057-1063.

Hallam, Anthony. 1988. “The Contribution of Paleontology to Systematics and Evolution”. In Prospects in Systematics, ed. David L. Hawksworth. Oxford: Clarendon Press, 128-147.

Hausdorf, Bernhard. 2011. ”Progress Toward a General Species Concept”. Evolution and Development 65, no. 4: 923-931.

Hausdorf, Bernhard and Christian Hennig. 2010. “Species Delimitation Using Dominant and Codominant Multilocus Markers”. Systematic Biology 59, no. 5: 491-503.

Hawksworth, David L. (ed.) 1997. The New Bionomenclature: The BioCode Debate (Biology International, Special Issue 34). Paris: International Union of Biological Sciences.

Hebert, Paul D. N. , Alina Cywinska, Shelley L. Ball and Jeremy R. deWaard. 2003. “Biological Identifications Through DNA Barcodes”. Proceedings of the Royal Society of London, Series B: Biological Sciences 270, no. 1512: 313-321.

Hennig, Willi. 1950. Grundzüge einer Theorie der phylogenetischen Systematik. Berlin: Deutscher Zentralverlag.

Hennig, Willi. 1965. “Phylogenetic Systematics”. Annual Review of Entomology 10: 97-116.

Hennig, Willi. 1966. Phylogenetic Systematics. Urbana, IL: University of Illinois Press.

Heppell, David. 1991. “Names Without Number?” In Improving the Stability of Names: Needs and Options, ed. David L. Hawksworth. Königstein: Koeltz Scientific Books, 191-196.

Hewitt, Godfrey M. 1988. “Hybrid Zones - Natural Laboratories for Evolutionary Studies”. Trends in Ecology and Evolution 3, no. 7: 158-167.

Hey, Jody. 2001a. Genes, Categories, and Species: The Evolution and Cognitive Causes of the Species Problem. New York: Oxford University Press.

Hey, Jody. 2001b. “The Mind of the Species Problem”. Trends in Ecology & Evolution 16, no. 7: 326-329.

Hey, Jody. 2006. “On the Failure of Modern Species Concepts”. Trends in Ecology & Evolution 21, no. 8): 447-450.

Hjørland, Birger. 2009. “Concept Theory”. Journal of the American Society for Information Science and Technology 60, no. 8: 1519-1536.

Hjørland, Birger. 2017. Classification. Knowledge Organization 44, no. 2: 97–128. Also in ISKO Encyclopedia of Knowledge Organization, eds. Birger Hjørland and Claudio Gnoli, http://www.isko.org/cyclo/classification.

Hobern, Donald, Saroj K. Barik, Les Christidis, Stephen T. Garnett, Paul Kirk, Thomas M. Orrell, Thomas Pape, Richard L. Pyle, Kevin R. Thiele, Frank E. Zachos and· Olaf Bánk. 2021. “Towards a Global List of Accepted Species VI: The Catalogue of Life Checklist”. Organisms Diversity & Evolution 21, no. 4: 677-690.

Horton, Tammy, Serge Gofas, Andreas Kroh, Gary C.B. Poore, Geoffrey Read, Gary Rosenberg, Sabine Stöhr, Nicolas Bailly, Nicole Boury-Esnault, Simone N. Brandão, Mark J. Costello, Wim Decock, Stefanie Dekeyzer, Francisco Hernandez, Jan Mees, Gustav Paulay, Leen Vandepitte, Bart Vanhoorne and Sofie Vranken. 2017. “Improving Nomenclatural Consistency: A Decade of Experience in the World Register of Marine Species”. European Journal of Taxonomy 389: 1-24.

Hull, David L. 1976. “Are Species Really Individuals?” Systematic Zoology 25, no. 2: 174-191.

Hull, David L. 1978. “A Matter of Individuality”. Philosophy of Science 45, no. 3: 335-360.

Hull, David L. 1987. “Genealogical Actors in Ecological Roles”. Biology and Philosophy 2, no. 2: 168-184.

Hull, David L. 1997. “The Ideal Species Concept – And Why We Cannot Get It”. In Species: The Units of Diversity, eds. Michael F. Claridge, Hassan A. Dawah and Michael R. Wilson. London: Chapman and Hall, 357-380.

Hull, David L. 1998. “Taxonomy”. In Routledge Encyclopedia of Philosophy. Version 1.0, ed. Edward Craig. London: Routledge. doi:10.4324/9780415249126-Q102-1

Hull, David L. 1999. “On the Plurality of Species: Questioning the Party Line”. In Species: New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, MA: MIT Press, 23-48.

Hull, Roger and Bert Rima. 2020. “Virus Taxonomy and Classification: Naming of Virus Species”. Archives of Virology. 165, no. 11: 2733-2736.

Imbrie, John. 1957. “The Species Problem with Fossil Animals”. In The Species Problem, ed. Ernst Mayr. American Association for the Advancement of Science Publication no. 50, 125-153.

International Commission on Zoological Nomenclature. 1905. Règles internationales de la nomenclature zoologique. International Rules of Zoological Nomenclature. Internationale Regeln der zoologischen Nomenklatur. Paris: Rudeval.

International Commission on Zoological Nomenclature. 1961. International Code of Zoological Nomenclature, Adopted by the XV International Congress of Zoology. London: The International Trust for Zoological Nomenclature.

International Commission on Zoological Nomenclature. 1999. International Code of Zoological Nomenclature (4th ed.). London: The International Trust for Zoological Nomenclature.

International Commission on Zoological Nomenclature. 2001. “International Committee on Bionomenclature, Report”. Bulletin of Zoological Nomenclature 58, no. 1: 6-7.

International Committee on Taxonomy of Viruses (ICTV). 2021. The International Code of Virus Classification and Nomenclature (ICVCN). March 2021 https://talk.ictvonline.org/information/w/ictv-information/383/ictv-code.

Keller, Roberto A., Richard N. Boyd and Quentin D. Wheeler. 2003. “The Illogical Basis of Phylogenetic Nomenclature”. Botanical Review 69, no. 1: 93-110.

Kitcher, Philip. 1984a. “Against the Monism of the Moment: A Reply to Elliott Sober”. Philosophy of Science 51, no. 4: 616-630.

Kitcher, Philip. 1984b. “Species”. Philosophy of Science 51, no. 2: 308-333.

Kornet, Dina Joanna. 1993. ”Permanent Splits as Speciation Events: A Formal Reconstruction of the Internodal Species Concept”. Journal of Theoretical Biology 164, no. 4: 407-435.

Korshunova, Tatiana, Alexander Martynov, Torkild Bakken and Bernard Picton. 2017. “External Diversity Is Restrained by Internal Conservatism: New Nudibranch Mollusc Contributes to the Cryptic Species Problem”. Zoologica Scripta 46, no. 6: 683-692.

Kottler, Malcolm Jay. 1978. ”Charles Darwin’s Biological Species Concept and Theory of Geographic Speciation: The Transmutation Notebooks”. Annals of Science 35, no. 3: 275-297.

Kuhn, Thomas S. 1970. “Reflections on My Critics”. In Criticism and the Growth of Knowledge, eds. Imre Lakatos and Alan Musgrave. Cambridge, MA: Cambridge University Press, 231-278.

Lamarck, chevalier de. 1815-22. Histoire naturelle des animaux sans vertèbres. Paris: Verdière (vols 1 (1815), 2 (1816), 3 (1816); Deterville & Verdière (vols. 4 /1817), 5 (1818); chez l’Auteur (vols. 6 (1819, 1822), 7 (1822)).

Lambert, David M., Bernard Michaux and Chris S. White. 1987. “Are Species Self-defining?” Systematic Zoology 36, no. 2: 196-205.

Lambert, David M. and Hugh E. H. Paterson. 1984. “On 'Bridging the Gap Between Race and Species: The Isolation Concept and an Alternative”. Proceedings of the Linnean Society of New South Wales 107 (1983), no. 4: 501-514.

Lambert, David M. and Hamish G. Spencer, eds. 1995. Speciation and the Recognition Concept: Theory and Application. Baltimore, MD: Johns Hopkins University Press.

Lanjouw, J., Baehni C., Merrill E. D., Rickett H. W., Robyns W., Sprague T. A. and Stafleu F. A. 1952. International Code of Botanical Nomenclature, Adopted by the Seventh International Botanical Congress, Stockholm, July 1950. Utrecht: International Bureau for Plant Taxonomy and Nomenclature of the International Association for Plant Taxonomy.

LaPorte, Joseph. 2007. “In Defense of Species”. Studies in History and Philosophy of Biological and Biomedical Sciences 38, no. 1: 255-269.

Laurin, Michael, Kevin de Queiroz and Philip D. Cantino. 2006. “Sense and Stability of Taxon Names“. Zoologica Scripta 35, no. 1: 113-114.

Lazenby, Elizabeth Mary. 1995. The Historia Plantarum Generalis of John Ray: Book I – A Translation and Commentary. Thesis submitted for the degree of Doctor of Philosophy in the University of Newcastle upon Tyne. Available at: http://hdl.handle.net/10443/327.

Leliaert, Frederik, Heroen Verbruggen, Brian Wysor and Olivier De Clerck 2009. “DNA Taxonomy in Morphologically Plastic Taxa: Algorithmic Species Delimitation in the Boodlea complex (Chlorophyta: Cladophorales)”. Molecular Phylogenetics and Evolution 53, no. 1: 122-133.

Lennox, James. 1980. “Aristotle on Genera, Species and the More and the Less”. Journal of the History of Biology, 13, no. 2: 321-346.

Lepage, Denis. 2019. Avibase-The World Bird Database. http://aviba se.bsc-eoc.org (Accessed July 30, 2019).

Lepage, Denis, Gaurav Vaidya, and Robert Guralnick, R. 2014. “Avibase - A Database System for Managing and Organizing Taxonomic Concepts“. ZooKeys 420: 117-135.

Li, Xiwen, Yang Yang, Robert J. Henry, Maurizio Rossetto, Yitao Wang and Shilin Chen, 2015. “Plant DNA Barcoding: From Gene to Genome“. Biological Reviews 90, no. 1: 157-166.

Lidén, Magnus. 1992. “Species – Where’s the Problem?” Taxon 41, no. 2: 315-317.

Lidén, Magnus and Bengt Oxelman. 1996. “Do We Need Phylogenetic Taxonomy?” Zoologica Scripta 25, no. 2: 183-185.

Lien, Aaron M., Stijn Conix, Frank E. Zachos, Les Christidis, Peter Paul van Dijk, Olaf S. Bánki, Saroj K. Barik, John S. Buckeridge, Mark John Costello, Donald Hobern, Narelle Montgomery, Svetlana Nikolaeva, Richard L. Pyle, Kevin Thiele, Scott A. Thomson, Zhi-Qiang Zhang and Stephen T. Garnett. 2021. “Towards a Global List of Accepted Species IV: Overcoming Fragmentation in the Governance of Taxonomic Lists”. Organisms Diversity & Evolution 21, no. 4: 645-655.

Linnæus, Carolus, 1746. Fauna Svecica, sistens animalia Sveciae regni: quadrupedia, aves, amphibia, pisces, insecta, vermes, distributa per classes & ordines, genera & species, cum differentiis specierum, synonymis autorum, nominibus incolarum, locis habitationum, descriptionibus insectorum. Stockholmiae: Laurentius Salvius.

Linnæus, Carolus. 1749. Pan svecicus, quem … præside Carolo Linnæo … publico examini … submittit Nicolaus L. Hesselgren, Wermelandus. Upsaliæ.

Linnæus, Carolus. 1751. Philosophia botanica in qua explicantur fundamenta botanica cum definitionibus partium, exemplis terminorum, observationibus rariorum. Stockholm: G. Kiesewetter.

Linnæus, Carolus. 1753. Species plantarum: exhibentes plantas rite cognitas, ad genera relatas, cum differentiis specificis, nominibus trivialibus, synonymis selectis, locis natalibus, secundum systema sexuale digestas. Holmiae: Laurentius Salvius.

Linnæus, Carolus. 1758. Systema naturae per regna tria naturae secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Editio decima, reformata. Tomus I. . Holmiae: Laurentius Salvius.

Linnæus, Carolus. 1768. Systema naturae: per regna tria natura, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Editio duodecima, reformata. Tomus III. Regnum Lapideum. Holmiae: Laurentius Salvius.

Lotsy, J. P. 1925. “Species or Linneon?” Genetica 7, no. 5-6: 487-506.

Lovejoy, Arthur Oncken, 1936. The Great Chain of Being. Cambridge, MA: Harvard University Press.

Lukhtanov, V. A. 2019. “Species Delimitation and Analysis of Cryptic Species Diversity in the XXI Century”. Entomological Review 99, no. 4: 463-472.

Makarenkov, Vladimir, Pierre Legendre and Yves Desdevises. 2004. “Modelling Phylogenetic Relationships Using Reticulated Networks”. Zoologica Scripta 33, no. 1: 89-96.

Mallet, James. 2013. “Subspecies, Semispecies, Superspecies”. In Encyclopedia of Biodiversity, ed. Simon A. Levin. Second edition. Amsterdam: Elsevier, Academic Press, 7: 45-48.

Mao, Yafei. 2020. “Genomic Insights into Hybridization of Reef Corals”. Coral Reefs 39, no. 1: 61-67.

Mao, Yafei, Evan P. Economo and Noriyuki Satoh. 2018. “The Roles of Introgression and Climate Change in the Rise to Dominance of Acropora Corals“. Current Biology 28, no. 21: 3373-3382.

Matthews, S. C. 1973. “Notes on Open Nomenclature and on Synonymy Lists”. Palaeontology 16, no. 4: 713-719.

Mayden, Richard L. 1997. “A Hierarchy of Species Concepts: The Denouement in the Saga of the Species Problem”. In Species: The Units of Diversity, eds. Michael F. Claridge, Hassan A. Dawah and Michael R. Wilson eds. London: Chapman and Hall, 381-423.

Mayr, Ernst. 1931. “Birds Collected During the Whitney South Sea Expedition. XII. Notes on Halcyon chloris and Some of Its Subspecies”. American Museum Novitates 469: 1-10.

Mayr, Ernst. 1957. ”Species Concepts and Definitions”. In The Species Problem, ed. Ernst Mayr. Washington, DC: American Association for the Advancement of Science, 1-22.

Mayr, Ernst. 1963. Animal Species and Evolution. Cambridge, Ma.: Belknap Press of Harvard University Press.

Mayr, Ernst. 1982. The Growth of Biological Thought. Cambridge, Ma.: The Belknap Press of Harvard University Press.

Mayr, Ernst. 1987. “The Ontological Status of Species: Scientific Progress and Philosophical Terminology”. Biology & Philosophy 2, no. 2: 145-166.

Mayr, Ernst. 1992. ”Species Concepts and Their Application”. In The Units of Evolution: Essays on the Nature of Species, ed. Marc Ereshevsky. Cambridge, MA: MIT Press, 15-26.

Mayr, Ernst. 2000a. ”The Biological Species Concept”. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 17-29.

Mayr, Ernst. 2000b. ”A Critique from the Biological Species Concept: What Is a Species, and What Is Not? ” In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 93-100.

Mayr, Ernst and Peter D. Ashlock. 1991. Principles of Systematic Zoology. 2nd ed. New York: McGraw-Hill.

Mayr, Ernst, E. Gorton Linsley and Robert L. Usinger. 1953. Methods and Principles of Systematic Zoology. New York: McGraw-Hill.

Mckitrick, Mary C. and Robert M. Zink. 1988. “Species Concepts in Ornithology”. Condor 90, no. 1: 1-14.

Meier, Joana I., David A. Marques, Catherine E. Wagner, L. Excoffier and O. Seehausen 2018. “Genomics of Parallel Ecological Speciation in Lake Victoria Cichlids”. Molecular Biology and Evolution 35, no. 6: 1489-1506.

Meier, Rudolf and Rainer Willmann. 2000a. ”The Hennigian Species Concept”. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 30-43.

Meier, Rudolf, and Rainer Willmann. 2000b. ”A Defence of the Hennigian Species Concept”. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 167-178.

Melville, R. V. 1995. Towards Stability in the Names of Animals. London: International Trust for Zoological Nomenclature.

Minelli, Alessandro. 2008. “Zoological vs. Botanical Nomenclature: A Forgotten ‘BioCode’ Experiment From the Times of the Strickland Code”. In Updating the Linnaean Heritage: Names as Tools for Thinking About Animals and Plants, eds. Alessandro Minelli, Lucio Bonato and Giuseppe Fusco. Zootaxa 1950: 21-38.

Minelli, Alessandro. 2017. “Updating Taxonomic Practice to Cope with Challenges from Within and Without the Discipline”. Biodiversity Journal 8, no. 2: 671-674.

Minelli, Alessandro. 2019. “The Galaxy of the Non-Linnaean Nomenclature”. History and Philosophy of the Life Sciences 41 no. 3, article 31.

Minelli, Alessandro. 2020. “Taxonomy Needs Pluralism, But a Controlled and Manageable One”. Megataxa 1, no. 1: 9-18.

Minelli, Alessandro, in Press. “The Species Before and After Linnaeus – Tension Between Disciplinary Nomadism and Conservative Nomenclature”. In Species and Beyond, eds. John S. Wilkins, Igor Pavlinov and Frank Zachos. Boca Raton: CRC Press.

Mishler, Brent D. 1999. “Getting Rid of Species”. In Species: New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, MA: MIT Press, 307–315.

Mishler, Brent D. and Robert N. Brandon. 1987. “Individuality, Pluralism, and the Phylogenetic Species Concept“. Biology and Philosophy 2, no. 4: 397-414.

Mishler, Brent D. and Michael J. Donoghue. 1982. “Species Concepts: A Case for Pluralism”. Systematic Zoology 31, no. 4: 491-503.

Mishler, Brent D. and Edward C. Theriot. 2000. “The Phylogenetic Species Concept (Sensu Mishler and Theriot): Monophyly, Apomorphy, and Phylogenetic Species Concepts“. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 44-54.

Monaghan, Michael T., Ruth Wild, Miranda Elliot, Tomochika Fujisawa, Michael Balke, Daegan J.G. Inward, David C. Lees, Ravo Ranaivosolo, Paul Eggleton, Timothy G. Barraclough and Alfried P. Vogler. 2009. “Accelerated Species Inventory on Madagascar Using Coalescent-Based Models of Species Delineation”. Systematic Biology 58, no. 3: 298-311.

Moritz, Craig. 1994. “Defining “Evolutionarily Significant Units” for Conservation”. Trends in Ecology and Evolution 9, no. 10: 373-375.

Nathan, Marco J. 2019. “Pluralism is the Answer! What is the Question?” Philosophy, Theory, and Practice in Biology 11: 15.

Nathan, Marco J. and Joel Cracraft. 2020 “The Nature of Species in Evolution”. In The Theory of Evolution, eds. Samuel M. Scheiner and David P. Mindell. Chicago: University of Chicago Press, 102-122.

Nelson, Gareth. 1989. “Species and Taxa. Systematics and Evolution”. In Speciation and its Consequences, eds. Daniel Otte and John A. Endler. Sunderland: Sinauer Associates, 60-81.

Nelson, Gareth J. and Norman I. Platnick. 1981. Systematics and Biogeography: Cladistics and Vicariance. New York: Columbia University Press.

Nickel, E. H. and J. D. Grice. 1998. “The IMA Commission on New Minerals and Mineral Names: Procedures and Guidelines on Mineral Nomenclature”. Mineralogy and Petrology 64, no. 1-4: 237-263.

Nicolson, Dan H. 1991. “A History of Botanical Nomenclature“. Annals of the Missouri Botanical Garden 78, no. 1: 33-56.

Nielsen, Jennifer L. (ed.) 1995. Evolution and the Aquatic Ecosystem: Defining Unique Units in Population Conservation. Symposium 17: Proceedings from “Evolution and the Aquatic Ecosystem Symposium,” held in Monterey, California, May 1994. Bethesda, MD: American Fisheries Society.

Nixon, Kevin C., James M. Carpenter and Dennis W. Stevenson. 2003. “The PhyloCode Is Fatally Flawed, and the "Linnaean" System Can Easily Be Fixed”. Botanical Review 69, no. 1: 111-120.

Nixon, Kevin C. and Quentin D. Wheeler. 1990. “An Amplification of the Phylogenetic Species Concept”. Cladistics 6, no. 3: 211-223.

Nock, Catherine J., Daniel L. E. Waters, Mark A. Edwards, Stirling G. Bowen, Nicole Rice, Giovanni M. Cordeiro and Robert J Henry, 2011. “Chloroplast Genome Sequences from Total DNA for Plant Identification“. Plant Biotechnology Journal 9, no. 3: 328-333.

O’Meara, Brian C. 2010. “New Heuristic Methods for Joint Species Delimitation and Species Tree Inference”. Systematic Biology 59, no. 1: 59-73.

Oppen, Madeleine J. H. van, Brenda J. McDonald, Bette Willi and David J. Miller. 2001. “The Evolutionary History of the Coral Genus Acropora (Scleractinia, Cnidaria) Based on a Mitochondrial and a Nuclear Marker: Reticulation, Incomplete Lineage Sorting, or Morphological Convergence?“ Molecular Biology and Evolution 18, no.7: 1315-1329.

Oxford English dictionary: The Definitive Record of the English Language (OED). “Species” Oxford University Press. Accessed from https://www-oed-com.ep.fjernadgang.kb.dk/view/Entry/185995 (toll access).

Pallas, Peter S. 1778. Novae species quadrupedum e glirium ordine cum illustrationibus variis complurium ex hoc ordine animalium. Erlangen: Walther.

Parker, Charles T., Brian J. Tindall and George M. Garrity, eds. 2019. “International Code of Nomenclature of Prokaryotes. Prokaryotic Code (2008 Revision)”. International Journal of Systematic and Evolutionary Microbiology 69, no. 1A: S1–S111

Parks, Matthew, Richard Cronn and Aaron Liston. 2009. “Increasing Phylogenetic Resolution at Low Taxonomic Levels Using Massively Parallel Sequencing of Chloroplast Genomes“. BMC Biology 7, 84.

Paterson, Hugh E. 1979. “A Comment on ‘Mate Recognition Systems’”. Evolution 34, no. 2: 330-331.

Paterson, Hugh E. 1985. “The Recognition Concept of Species”. In Species and Speciation, ed. Elisabeth S. Vrba. Pretoria: Transvaal Museum, 21-29.

Paterson, Hugh E. H. 1993. Evolution and the Recognition Concept of Species. Collected Writings. Baltimore, MA: John Hopkins University Press.

Pavlic, Draginja, Bernard Slippers, Teresa A. Coutinho and Michael J. Wingfield. 2009. ”Multiple Gene Genealogies and Phenotypic Data Reveal Cryptic Species of the Botryosphaeriaceae: A Case Study on the Neofusicoccum parvum/N. ribis Complex”. Molecular Phylogenetics and Evolution 51, no. 2: 259-268.

Pavlinov, Igor Y., ed. 2013. The Species Problem – Ongoing Issues. Rijeka: InTech.

Pavlinov, Igor Y. 2020. “Multiplicity of Research Programs in the Biological Systematics: A Case for Scientific Pluralism”. Philosophies 5, 7.

Pavlinov, I. Y. in press. “The Species Problem from a Conceptualist’s Viewpoint”. In Species and Beyond, eds. John S. Wilkins, Igor Pavlinov and Frank Zachos. Boca Raton: CRC Press.

Pellegrin, Pierre. 1982. La classification des animaux chez Aristote. Statut de la biologie et unité de l’Aristotelisme. Paris : Les belles lettres.

Pennock, David S. and Walter W. Dimmick. 1997. “Critique of the Evolutionarily Significant Unit as a Definition for “Distinct Population Segments” under the U.S. Endangered Species Act”. Conservation Biology 11, no. 3: 611-619.

Pérez-Ponce de León, Gerardo and Steven A. Nadler. 2010. “What We Don’t Recognize Can Hurt Us: A Plea for Awareness about Cryptic Species”. Journal of Parasitology 96, no. 2: 453-464.

Pigliucci, Massimo. 2003. “Species as Family Resemblance Concepts: The (Dis-)Solution of the Species Problem?” BioEssays 25, no. 6: 596-602.

Plate, L. 1914. Prinzipien der Systematik mit besonderer Berücksichtigung des Systems der Tiere. In Paul Hinneberg ed., Die Kultur der Gegenwart 3(4)4. Leipzig-Berlin: Teubner, 92-164.

Pleijel, Fredrick. 2000. “Phylogenetic Taxonomy, a Farewell to Species, and a Revision of Heteropodarke (Hesionidae, Polychaeta, Annelida)”. Systematic Biology 48, no. 4: 755-789.

Pleijel, Fredrick and Mikael Härlin. 2004. “Phylogenetic Nomenclature Is Compatible with Diverse Philosophical Perspectives”. Zoologica Scripta 33, no. 6: 587-591.

Pleijel, Fredrick and Greg W. Rouse. 1999. “Least-Inclusive Taxonomic Unit: A New Taxonomic Concept for Biology”. Proceedings of the Royal Society of London B 267, no. 1443: 627-630.

Pleijel, F. and G. W. Rouse. 2003. “Ceci n'est pas une pipe: Names, Clades and Phylogenetic Nomenclature”. Journal of Zoological Systematics and Evolutionary Research 41, no. 3: 162-174.

Pons, Joan, Timothy G. Barraclough, Jesus Gomez-Zurita, Anabela Cardoso, Daniel P. Duran, Steaphan Hazell, Sophien Kamoun, William D. Sumlin and Alfried P. Vogler 2006. “Sequence-Based Species Delimitation for the DNA Taxonomy of Undescribed Insects”. Systematic Biology 55, no. 4: 595-609.

Puillandre, N., A. Lambert, S. Brouillet and G. Achaz. 2012a. “ABGD, Automatic Barcode Gap Discovery for Primary Species Delimitation”. Molecular Ecology 21, no. 8: 1864-1877.

Puillandre, Nicolas, M. V. Modica, Y. Zhang, L. Sirovich, M.-C. Boisselier, C. Cruaud, M. Holford and S. Samadi 2012b. “Large-scale Species Delimitation Method for Hyperdiverse Groups”. Molecular Ecology 21 no. 11: 2671-2691.

Pyle, Richard L., Saroj K. Barik, Les Christidis, Stijn Conix, Mark John Costello, Peter Paul van Dijk, Stephen T. Garnett, Donald Hobern, Paul M. Kirk, Aaron M. Lien, Thomas M. Orrell, David Remsen, Scott A. Thomson, Nina Wambiji, Frank E. Zachos, Zhi-Qiang Zhang and Kevin R. Thiele. 2021. “Towards a Global List of Accepted Species V. The Devil is in the Detail”. Organisms Diversity & Evolution 21, no. 4: 657-675.

Quammen, David. 2018. The Tangled Tree: A Radical New History of Life. London: William Collins.

Ramsbottom, John. 1938. “Linnaeus and the Species Concept“. Proceedings of the Linnean Society of London 150, no. 4: 192-220.

Rannala, Bruce. 2015. “The Art and Science of Species Delimitation”. Current Zoology 61, no. 5: 846-853.

Raven, P. H. 1980. “Hybridization and the Nature of Plant Species”. Bulletin of the Canadian Botanical Association 13, Supplement: 3-10.

Ray, John. 1660. Catalogus plantarum circa Cantabrigiam nascentium. Cambridge: John Field.

Ray, John. 1677. Catalogus plantarum Angliae, et insularum adjacentium: tum indigenas, tum in agris passim cultas complectens. Editio secunda. Londini: A. Clark, impensis J. Martyn.

Ray, John. 1686. Historia plantarum. Tomus Primus. Londini: Typis Mariæ Clark.

Regan, C. Tate. 1926. “Organic Evolution”. Report of the British Association for the Advancement of Science 1925: 75-86.

Rensch, Bernhard. 1928. “Grenzfälle von Rasse und Art”. Journal für Ornithologie 76, no. 1: 222-231.

Rensch, Bernhard. 1929. Das Prinzip geographischer Rassenkreise und das Problem der Artbildung. Berlin: Bornträger.

Reydon, Thomas A. C. 2019. “Are Species Good Units for Biodiversity Studies and Conservation Efforts?” In From Assessing to Conserving Biodiversity: Conceptual and Practical Challenges, eds. Elena Casetta, Jorge Marquez da Silva and Davide Vecchi. Cham: Springer, 167-193.

Reydon, Thomas A. C. 2020. “What Is the Basis of Biological Classification?” In Philosophy of Science for Biologists, eds. Kostas Kampourakis and Tobias Uller. Cambridge: Cambridge University Press, 216-234.

Richards, Richard A. 2010. The Species Problem – A Philosophical Analysis. Cambridge: Cambridge University Press.

Richards, Richard A. 2015. Book review of Slater (2013). Mind 124, no. 496: 1388-1393.

Richards, Richard A. 2016. Biological Classification: A Philosophical Introduction. Cambridge, UK: Cambridge University Press.

Richter, Rudolf. 1943. Einführung in die Zoologische Nomenklatur durch Erläuterung der Internationalen Regel. Frankfurt: Waldemar Kramer.

Ridley, Mark. 1989. “The Cladistic Solution to the Species Problem”. Biology & Philosophy 4, no. 1: 1-16.

Rieseberg, Loren H., Troy E. Wood and Eric J. Baack. 2006. “The Nature of Plant Species”. Nature 440, no. 7083: 524-527.

Rivera, Maria C, and James A. Lake. 2004. “The Ring of Life Provides Evidence for a Genome Fusion Origin of Eukaryotes”. Nature 431, no. 7005: 152-155.

Rivinus, Augustus Quirinus. 1690. Introductio generalis in rem herbariam. Lipsiae: Güntherl.

Robson, G. C. 1928. The Species Problem. An Introduction to the Study of Evolutionary Divergence in Natural Populations. Edinburgh: Oliver and Boyd.

Rosen, Donn E. 1978. “Vicariant Patterns and Historical Explanation in Biogeography”. Systematic Zoology 27, no. 2: 159-188.

Ruthenberg, Klaus and Jaap van Brakel (eds.) 2008. Stuff: The Nature of Chemical Substances. Würzburg: Königshauen & Neumann.

Ryder, Oliver A. (1986). “Species Conservation and Systematics: The Dilemma of Subspecies”. Trends in Ecology & Evolution 1, no. 1: 9-10.

Saddhe, Ankush Ashok and Kundan Kumar. 2018. “DNA Barcoding of Plants: Selection of Core Markers for Taxonomic Groups“. Plant Science Today 5, no. 1: 9-13.

Scerri, Eric R. and Elena Ghibaudi (eds.). 2020. What Is a Chemical Element? A Collection of Essays by Chemists, Philosophers, Historians, and Educators. New York, NY: Oxford University Press.

Seehausen, Ole. 2004. “Hybridization and Adaptive Radiation”. Trends in Ecology & Evolution 19, no. 4: 198-207.

Seehausen, Ole, Jacques J. M. van Alphen and Frans Witte 1997. “Cichlid Fish Diversity Threatened by Eutrophication That Curbs Sexual Selection“. Science 277, no. 5333: 1808-1811.

Shneyer, V. S. and V. V. Kotseruba. 2015. “Cryptic Species in Plants and Their Detection by Genetic Differentiation Between Populations”. Russian Journal of Genetics: Applied Research 5, no. 5: 528-541.

Siddell Stuart G., Peter J. Walker, Elliot J. Lefkowitz, Arcady R. Mushegian, Bas E. Dutilh, Balázs Harrach, Robert L. Harrison, Sandra Junglen, Nick J. Knowles, Andrew M. Kropinski,·Mart Krupovic,·Jens H. Kuhn, Max L. Nibert,· Luisa Rubino, Sead Sabanadzovic, Peter Simmonds, Arvind Varsani, Francisco Murilo Zerbini and Andrew J. Davison. 2020. “Binomial Nomenclature for Virus Species: A Consultation”. Archives of Virology 165, no. 2: 519-525.

Sigovini, Marco, Erica Keppel and Davide Tagliapietra. 2016. “Open Nomenclature in the Biodiversity Era”. Methods in Ecology and Evolution 7, no. 10: 1217-1225.

Simpson, George Gaylord. 1940. “Types in Modern Taxonomy”. American Journal of Science 238, no. 6: 413-431.

Simpson, George Gaylord. 1943. “Criteria for Genera, Species and Subspecies in Zoology and Paleozoology”. Annals of the New York Academy of Science 44, no. 2: 145-178.

Simpson, George Gaylord. 1961. Principles of Animal Taxonomy. New York: Columbia University Press.

Slater, Matthew H. 2013. Are Species Real? An Essay on the Metaphysics of Species. London: Palgrave Macmillan.

Sloan, Philip R. 1987. “From Logical Universals to Historical Individuals,” In Histoire du concept d’espèce dans les sciences de la vie, eds. Scott Atran, Richard W. Burkhardt, Pietro Corsi, Anne Diara, Bernardino Fantini, Jean-Louis Fischer, M. J. S. Hodge, Goulven Laurent, Antonello La Vergata, Anto Leikola, Pierre Louis, Ernst Mayr, Gerhard H. Müller, Roselyne Rey, Jacques Roger and Philipp R. Sloan. Paris: Fondation Singer-Polignac, 101-140.

Smet, Wim M. A. de. 1991. “Meeting User Needs by an Alternative Nomenclature”. In Improving the Stability of Names: Needs and Options, ed. David L. Hawksworth. Königstein: Koeltz Scientific Books, 179-181.

Sneath, Peter H. A. and Robert R. Sokal. 1973. Numerical Taxonomy. San Francisco: W.H. Freeman.

Sokal, Robert R. and T. Crovello. 1970. “The Biological Species Concept: A Critical Evaluation“. American Naturalist 104, no. 936: 127-153.

Sokal, Robert R. and Peter H. A. Sneath. 1963. Principles of Numerical Taxonomy. San Francisco: W.H. Freeman.

Spinney, Laura 2010. “Evolution Dreampond Revisited”. Nature 466, no. 7303: 174-175.

Stace, Clive A. 1989. Plant Taxonomy and Biosystematics. 2nd ed. Cambridge: Cambridge University Press.

Stamos, David N. 2003. The Species Problem. Biological Species, Ontology, and the Metaphysics of Biology. Lanham, Md.: Lexington Books.

Stanford, P. Kyle. 1995. “For Pluralism and against Realism about Species”. Philosophy of Science 62, no. 1: 70-91.

Sterelny, Kim. 1998. “Species”. In Routledge Encyclopedia of Philosophy, Version 1.0, ed. Edward Craig. London: Routledge, 78-81.

Sterelny, Kim. 1999. “Species as Ecological Mosaics”. In Species, New Interdisciplinary Essays, ed. Robert A. Wilson. Cambridge, MA: MIT Press, 119-138.

Strickland, H. E., J. S. Henslow, John Phillips, W. E. Shuckard, John Richardson, G. R. Waterhouse, Richard Owen, W. Yarrell, Leonard Jenyns, C. Darwin, W. J. Broderip and J.O. Westwood. 1842. Report of a Committee Appointed "to Consider the Rules by Which the Nomenclature of Zoology May be Established on a Uniform and Permanent Basis”. London: John Murray, for the British Association for the Advancement of Science.

Struck, Thorsten H., Jeffrey L. Feder, Mika Bendiksby, Siri Birkeland, José Cerca, Vladimir I. Gusarov, Sonja Kistenich, Karl-Henrik Larsson, Lee Hsiang Liow, Michael D. Nowak, Brita Stedje, Lutz Bachmann, and Dimitar Dimitrov. 2018. “Finding Evolutionary Processes Hidden in Cryptic Species”. Trends in Ecology & Evolution 33, no. 3: 153-163.

Sukumaran, Jeet, Mark T. Holder and L. Lacey Knowles. 2020. “Incorporating the Speciation Process into Species Delimitation”. PLoS Computational Biology 17, no. 5: e1008924.

Suppe, Frederick. 1989. “Classification”. In International Encyclopedia of Communications, ed. Erik Barnouw ed., vol. 1. Oxford: Oxford University Press, 292–296.

Sylvester-Bradley, Peter Colley, ed. 1956. The Species Concept in Palaeontology. London: Systematics Association.

Szalay, Frederick S. and Walter J. Bock. 1991. “Evolutionary Theory and Systematics: Relationships Between Process and Patterns”. Zeitschrift für Zoologische Systematik und Evolutionsforschung 29, no. 1: 1-39.

Templeton, Alan R. 1989. “The Meaning of Species and Speciation: A Genetic Perspective”. In Speciation and Its Consequences, eds. Daniel Otte and John A. Endler. Sunderland: Sinauer, 3-27.

Tessens, Bart, Marlies Monnens, Thierry Backeljau, Kurt Jordaens, Niels Van Steenkiste, Floris C. Breman, Karen Smeets and Tom Artois. 2021. “Is ‘Everything Everywhere’? Unprecedented Cryptic Diversity in the Cosmopolitan Flatworm Gyratrix hermaphroditus”. Zoologica Scripta 50, no. 6: 837-851.

Thiele, Kevin R., Stijn Conix, Richard L. Pyle, Saroj K. Barik, Les Christidis, Mark John Costello, Peter Paul van Dijk, Paul Kirk, Aaron Lien, Scott A. Thomson, Frank E. Zachos, Zhi-Qiang Zhang and Stephen T. Garnett. 2021. “Towards a Global List of Accepted Species I. Why Taxonomists Sometimes Disagree, and Why This Matters”. Organisms Diversity & Evolution 21, no. 4: 615-622.

Thomson, Scott A., Kevin Thiele, Stijn Conix, Les Christidis, Mark John Costello, Donald Hobern, Svetlana Nikolaeva, Richard L. Pyle, Peter Paul van Dijk, Haylee Weaver, Frank E. Zachos, Zhi-Qiang Zhang and Stephen T. Garnett. 2021. “Towards a Global List of Accepted Species II. Consequences of Inadequate Taxonomic List Governance”. Organisms Diversity & Evolution 21, no. 4: 623-630.

Trontelj, Peter and Cene Fišer. 2009. “Cryptic Species Diversity Should Not Be Trivialized”. Systematics and Biodiversity 7, no. 1: 1-3.

Turesson, Göte. 1922a. “The Species and the Variety as Ecological Units”. Hereditas 3, no. 1: 100-113.

Turesson, Göte. 1922b. “The Genotypical Response of the Plant Species to the Habitat”. Hereditas 3, no. 3: 211-350.

Turesson, Göte. 1929. “Zur Natur und Begrenzung der Arteinheiten“. Hereditas 12, no. 3: 323-334.

Turland, Nicholas J., John H. Wiersema, Fred R. Barrie, Werner Greuter, David L. Hawksworth, Patrick S. Herendeen, Sandra Knapp, Wolf-Henning Kusber, De-Zhu Li, Karol Marhold, Tom W. May, John McNeill, Anna M. Monro, Jefferson Prado, Michelle J. Price and Gideon F. Smith, eds. 2018. International Code of Nomenclature for Algae, Fungi, and Plants (Shenzhen Code) Adopted by the Nineteenth International Botanical Congress Shenzhen, China. Glashütten: Koeltz Botanical Books.

Van Valen, Leigh. 1976. “Ecological Species, Multispecies, and Oaks”. Taxon 25, no. 2-3: 233-239.

Van Valen, Leigh M. 1988. “Species, Sets and the Derivative Nature of Philosophy”. Biology & Philosophy 3, no 1: 49-66.

Vos, Michiel. 2011. “A Species Concept for Bacteria Based on Adaptive Divergence“. Trends in Microbiology 19, no. 1: 1-7.

Wagner, Warren H. 1983. “Reticulistics: The Recognition of Hybrids and Their Role in Cladistics and Classification”. In Advances in Cladistics, eds. Norman I. Platnick and V. A. Funk eds. New York: Columbia University Press, 63-79.

Waples, Robin S. 1995. “Evolutionarily Significant Units and the Conservation of Biological Diversity Under the Endangered Species Act”. American Fisheries Society Symposium 17: 8-27.

Waples, Robin S. 1998. “Evolutionarily Significant Units, Distinct Population Segments, and the Endangered Species Act: Reply to Pennock and Dimmick”. Conservation Biology 12, no. 3: 718-721.

Wheeler, Quentin D. 1999. “Why the Phylogenetic Species Concept?—Elementary“. Journal of Nematology 31, no. 2: 134-141.

Wheeler, Quentin D. and Rudolf Meier, eds. 2000. Species Concepts and Phylogenetic Theory: A Debate. New York: Columbia University Press.

Wheeler, Quentin D. and Norman I. Platnick. 2000. The Phylogenetic Species Concept (Sensu Wheeler and Platnick). In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 55–69.

White, Chris S., Bernard Michaux and David M. Lambert. 1990. “Species and Neo-Darwinism”. Systematic Zoology 39, no. 4: 399-413.

Wiens, John J. 2007. “Species Delimitation: New Approaches for Discovering Diversity”. Systematic Biology 56, no. 6: 875-878.

Wiens, John J. and Maria R. Servedio. 2000. “Species Delimitation in Systematics: Inferring Diagnostic Differences Between Species”. Proceedings of the Royal Society of London B 267, no. 1444: 631-636.

Wiley, E. O. 1978. “The Evolutionary Species Concept Reconsidered”. Systematic Zoology 27, no. 1: 17-26.

Wiley, Edward Orlando and Richard L. Mayden. 2000. “The Evolutionary Species Concept“. In Species Concepts and Phylogenetic Theory: A Debate, eds. Quentin D. Wheeler and Rudolf Meier. New York: Columbia University Press, 70-89.

Wilkins, John S. 2003. “How to be a Chaste Species Pluralist-Realist: The Origins of Species Modes and the Synapomorphic Species Concept“. Biology and Philosophy 18, no. 5: 621-638.

Wilkins, John S. 2009a. Species. A History of the Idea. Berkeley: University of California Press.

Wilkins, John S. 2009b. Defining Species. A Sourcebook from Antiquity to Today. New York: Peter Lang.

Wilkins, John S. 2011. “Philosophically Speaking, How Many Species Concepts are There?” Zootaxa 2765, no. 1: 58-60.

Wilkins, John S. 2018. Species: The Evolution of the Idea. 2nd. ed. of 2009a. Boca Raton, FL: CRC Press.

Wilkins, John S., Igor Pavlinov and Frank Zachos, eds. In Press. Species and Beyond. Boca Raton: CRC Press.

Witt, Jonathan D. S., Doug L. Threloff and Paul D. N. Hebert. 2006. “DNA Barcoding Reveals Extraordinary Cryptic Diversity in an Amphipod Genus: Implications for Desert Spring Conservation”. Molecular Ecology 15, no. 10: 3073-3082.

Witteveen, Joeri. 2016. “Suppressing Synonymy with a Homonym: The Emergence of the Nomenclatural Type Concept in Nineteenth Century Natural History”. Journal of the History of Biology 49, no. 1: 135-189.

WordNet 3.1. “Species”. Retrieved from http://wordnetweb.princeton.edu/perl/webwn?s=species.

Yang, Ziheng and Bruce Rannala. 2010. “Bayesian Species Delimitation Using Multilocus Sequence Data”. Proceedings of the National Academy of Sciences of the United States of America 107, no. 20: 9264-9269.

Yang, Ziheng and Bruce Rannala. 2014. “Unguided Species Delimitation Using DNA Sequence Data from Multiple Loci”. Molecular Biology and Evolution 31, no. 12: 3125-3135.

Zachos, Frank E. 2015. “Taxonomic Inflation, the Phylogenetic Species Concept and Lineages in the Tree of Life—A Cautionary Comment on Species Splitting“. Journal of Zoological Systematics and Evolutionary Research 53, no. 2: 180-184.

Zachos, Frank E. 2016. Species Concepts in Biology. Historical Development, Theoretical Foundations and Practical Relevance. Basel: Springer.

Zachos, Frank E., Marco Apollonio, Eva V. Bärmann, Marco Festa-Bianchet, Ursula Göhlich, Jan Christian Habel, Elisabeth Haring, Luise Kruckenhauser, Sandro Lovari, Allan D. McDevitt, Cino Pertoldi, Gertrud E. Rössner, Marcelo R. Sánchez-Villagra, Massimo Scandura and Franz Suchentrunk. 2013. “Species Inflation and Taxonomic Artefacts. A Critical Comment on Recent Trends in Mammalian Classification”. Mammalian Biology 78, no. 1: 1-6.

[top of entry]


Visited Hit Counter by Digits times.

Version 1.0 published 2022-01-24

Article category: KO in specific domains

©2022 ISKO. All rights reserved.