What is a species?

From Appendix B in Species: A History of the Idea (2nd ed.) by John Wilkins:

Groups and categories are distinctly different and there is no real connection between them. The tradition in codes of nomenclature artificially forces the use of taxonomic categories in a hierarchy for groups. A biological classification is a contrived system of categories used for the storage and retrieval of information about biological diversity, taxa, or groups. The concept 'category' is a class and has no separate existence from its use in organizing objects or thoughts; categories have no reality. Unlike groups, categories have no attributes; things or objects are not members of categories, but are parts of groups; and organisms are not members of any taxonomic category.

R. L. Mayden¹

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There are numerous species "concepts" at the research and practical levels in the scientific literature. Mayden's (1997) list of 22 distinct species concepts along with synonyms is a useful starting point for a review. I have added authors where I can locate them in addition to Mayden's references, and I have tried to give the concepts names, such as biospecies for "biological species," and so on, following George 1956, except where nothing natural suggests itself. I have used Mayden's abbreviations, except as noted, and added new ones for novel conceptions. A closely similar annotated list is found in Zachos 2016. In some cases I have adopted his terminology. There have also been several additional concepts since Mayden's review, so I have added the views of Pleijel 1999 and Wu 2001a, b, and several newer revisions presented in Wheeler and Meier 2000 as well as Hausdorf 2011. I also add some "partial" species concepts-the compilospecies concept and the nothospecies concept. Conceptions with asterisks are what I consider to be "foundational" conceptions, from which the others are constructed in whole or part.

REPRODUCTIVE ISOLATION CONCEPTIONS (RISC)

Basing species on the ability of organisms to reproduce successfully is as old as natural history; as we have seen, Darwin considered (and rejected) it, as did Linnaeus and many others. It is not even the sole biological conception. As Van Valen noted, "The term 'biological species concept' is a propagandist ploy."³ Many have noted that conceptions based on living things are always biological, and that this is better called something like a reproductive species concept; hence my use of RISC.

PHYLOSPECIES (PSC)

In addition to Hennig's conception (1950, 1966), I distinguish between two phylospecies concepts that go by various names, mostly the names of the authors presenting at the time, as in (Wheeler and Meier 2000). To remedy this terminological inflation, I have christened them the autapomorphic species conception and the phylogenetic taxon species concept.⁴ Asterisks identify the "primary" conceptions (Mayden 1997), from which the others are formed. Mayden has criticized several of these as being insufficient for that purpose, but I have tagged those which seem to function as primary conceptions in scientific practice, if not in philosophical terms. Definition: The smallest unit appropriate for phylogenetic analysis, the smallest biological entities that are diagnosable and monophyletic, unit product of natural selection and descent. A geographically constrained group with one or more unique apomorphies (autapomorphies). There are two versions of this and they are not identical. One derives from Rosen and is what I call the autapomorphic species conception [conception 2]. It is primarily a concept of diagnosis and tends to be favored by the tradition known as pattern cladism. The other is what I call the phylogenetic taxon species conception [conception 23], and tends to be favored by process cladists. Principal Authors: Cracraft 1983, Eldredge and Cracraft 1980, Nelson and Platnick 1981, Rosen 1979. Synonyms: Autapomorphic phylospecies, monophyletic phylospecies, minimal monophyletic units, monophyletic species, lineages. Related Concepts: Similar to: internodal species cladospecies, composite species, least inclusive taxonomic units.

INDIVIDUAL CONCEPTIONS

1. AGAMOSPECIES [ASC]
Definition: Asexual lineages, uniparental organisms (parthenogens and apomicts), that cluster together in terms of their genome. May be secondarily uniparental from biparental ancestors. Quasispecies are asexual viruses or organisms that cluster about a "wild-type" due to selection. Principal Authors: Cain 1954; Eigen 1993 for quasispecies. Synonyms: Microspecies, paraspecies, pseudospecies, semispecies, quasispecies, genomospecies (Euzéby 2006), for prokaryotes, bacterial species (Cohan 2002).

2. AUTAPOMORPHIC SPECIES [APSC]
A phylospecies conception. Definition: A geographically constrained group of individuals with some unique apomorphous characters, the unit of evolutionary significance (Rosen 1979); simply the smallest detected samples of self-perpetuating organisms that have unique sets of characters (Nelson and Platnick 1981); the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character traits (Wheeler and Platnick 2000). Principal Authors: Eldredge and Cracraft 1980, Nelson and Platnick 1981, Rosen 1979, Wheeler and Platnick 2000. Synonyms: Diagnosable version, diagnosable and monophyly version (Zachos 2016).

3. BIOSPECIES [BSC]*
Definition: Inclusive Mendelian population of sexually reproducing organisms (Dobzhansky 1935, 1937, 1970) interbreeding natural population isolated from other such groups (Mayr 1940, 1942, 1963, 1991). Depends upon endogenous reproductive isolating mechanisms (RIMs). Principal Authors: Defined by John Ray; Buffon, Dobzhansky 1935, Mayr 1942. Synonyms: Syngen, speciationist species concept.

4. CLADOSPECIES [CISC]
A phylospecies conception. Definition: Set of organisms between speciation events or between speciation event and extinction (Ridley 1989), a segment of a phylogenetic lineage between nodes. Upon speciation the ancestral species is extinguished and two new species are named. Principal Authors: Hennig 1950, 1966, Kornet and McAllister 1993. Synonyms: Internodal species concept, Hennigian species concept, Hennigian convention.

5. COHESION SPECIES [CSC]
Definition: Evolutionary lineages bounded by cohesion mechanisms that cause reproductive communities of demographically and/or genetically exchangeable individuals. The most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. Principal Author: Templeton 1989.

6. COMPILOSPECIES [COSC]
Definition: A species pair where one species "plunders" (Latin: compīlo) the genetic resources of another via introgressive interbreeding. Principal Authors: Aguilar et al. 1999, Harlan and De Wet 1963.

7. COMPOSITE SPECIES [CPSC]
Definition: All organisms belonging to an internodon and its descendants until any subsequent internodon. An internodon is defined as a set of organisms whose parent–child relations are not split (have the INT relation). Principal Authors: Kornet and McAllister 1993. Synonyms: Phylospecies (in part; Hennigian species), internodal species (in part), cladospecies (in part).

8. DIFFERENTIAL FITNESS SPECIES [DFSC]
Definition: Groups of individuals that are reciprocally characterized by features that would have negative fitness effects in other groups and that cannot be regularly exchanged between groups upon contact. Principal Author: Hausdorf 2011. Related Concepts: Genealogical concordance species, genic species, genotypic cluster species, reproductive competition species.

9. ECOSPECIES* [ECSC]
Definition: A lineage (or closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range. Principal Authors: Simpson 1961, Turesson 1922, Van Valen 1976. Synonyms: Ecotypes, ecological mosaics.

10. EVOLUTIONARY SPECIES [ESC]*
Definition: A lineage (an ancestral–descendent sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies. Principal Authors: Simpson 1961, Wiley 1978, 1981. Synonyms: Unit of evolution, evolutionary group. Related Concepts: Evolutionary significant unit.

11. EVOLUTIONARY SIGNIFICANT UNIT [ESU]
Definition: A population (or group of populations) that (1) is substantially reproductively isolated from other conspecific population units, and (2) represents an important component in the evolutionary legacy of the species. Principal Author: Waples 1991. Related Concepts: Biospecies (in part) and evolutionary species (in part).

12. GENEALOGICAL CONCORDANCE SPECIES [GCC]
Definition: Population subdivisions concordantly identified by multiple independent genetic traits constitute the population units worthy of recognition as phylogenetic taxa. Principal Authors: Avise and Ball Jr 1990. Related Concepts: Biospecies (in part), cladospecies (in part), phylospecies (in part).

13. GENERAL LINEAGE CONCEPT [GLC]
Definition: A series of entities forming a single line of ancestry and descent. Principal Author: de Queiroz 1998, 1999. Synonyms: Unified species conception; universal species conception.

14. GENIC SPECIES [GESC]
Definition: A species formed by the fixation of all isolating genetic traits ("speciation genes") in the common genome of the entire population. Related Concepts: Genealogical concordance species, genetic species (in part), biospecies (in part), autapomorphic species (in part). Principal Author: Wu 2001a, b.

15. GENETIC SPECIES [GSC]*
Definition: A group of organisms that may inherit characters from each other, a common gene pool, a reproductive community that forms a genetic unit. Principal Authors: Dobzhansky 1950, Mayr 1969, Simpson 1943. Synonyms: Gentes (singular: gens), Mendelian population. Related Concepts: Biospecies, phenospecies, morphospecies.

16. GENOTYPIC CLUSTER [GCD]
Definition: Clusters of monotypic or polytypic biological entities, identified using morphology or genetics, forming groups that have few or no intermediates when in contact. Principal Author: Mallet 1995. Synonyms: Polythetic species.

17. HENNIGIAN SPECIES [HSC]
A phylospecies conception. Definition: A tokogenetic community that arises when a stem species is dissolved into two new species and ends when it goes extinct or speciates. Principal Authors: Hennig 1950, 1966, Meier and Willmann 2000. Synonyms: Biospecies (in part), cladospecies (in part), phylospecies (in part), internodal species. Related Concepts: Agamospecies, biospecies, genetic species, Hennigian species, morphospecies, non-dimensional species, phenospecies, autapomorphic phylospecies, successional species, taxonomic species.

18. INTERNODAL SPECIES [ISC]
A phylospecies conception. Definition: Organisms are conspecific in virtue of their common membership of a part of a genealogical network between two permanent splitting events or a splitting event and extinction. Principal Author: Kornet 1993. Synonyms: Cladospecies and Hennigian species (in part), phylospecies.

19. MORPHOSPECIES [MSC]*
Definition: Species are the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means. Contrary to the received view, this was never anything more than a diagnostic account of species. Principal Authors: Aristotle and Linnaeus, and too many others to name, but including Owen, Agassiz, and recently, Cronquist 1978. Synonyms: Classical species, Linnaean species, Linneons.

20. NON-DIMENSIONAL SPECIES [NDSC]
Definition: Species delimitation in a non-dimensional system (a system without the dimensions of space and time). Principal Author: Mayr 1963. Synonyms: Folk taxonomic kinds (speciemes) (Atran 1990). Related Concepts: Biospecies, genetic species, morphospecies, paleospecies, successional species, taxonomic species.

21. NOTHOSPECIES [NSC]
Definition: Species formed from the hybridization of two distinct parental species, often by polyploidy. Principal Author: Wagner 1983. Synonyms: Hybrid species, reticulate species.

22. PHENOSPECIES [PHSC]
Definition: A cluster of characters that statistically covary; a family resemblance concept in which possession of most characters is required for inclusion in a species, but not all. A class of organisms that share most of a set of characters. Principal Authors: Beckner 1959, Sokal and Sneath 1963. Synonyms: Phena (singular: phenon) (Smith 1994), operational taxonomic unit.

23. PHYLOGENETIC TAXON SPECIES [PTSC]⁵
A phylospecies conception. Definition: A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983, Eldredge and Cracraft 1980); the least inclusive taxon recognized in a classification, into which organisms are grouped because of evidence of monophyly (usually, but not restricted to, the presence of synapomorphies), that is ranked as a species because it is the smallest important lineage deemed worthy of formal recognition, where "important" refers to the action of those processes that are dominant in producing and maintaining lineages in a particular case (Mishler and Brandon 1987, Nixon and Wheeler 1990). Principal Authors: Cracraft 1983, Eldredge and Cracraft 1980, Mishler and Brandon 1987, Nixon and Wheeler 1990. Synonyms: Monophyletic phylospecies, minimal monophyletic units, monophyletic species.

24. RECOGNITION SPECIES [RSC]
Definition: A species is that most inclusive population of individual, biparental organisms which share a common fertilization system. Principal Author: Paterson 1985. Synonyms: Specific mate recognition system (SMRS). 5 Here I part ways with Mayden's list, dropping his subdivisions of the PSC.

25. REPRODUCTIVE COMPETITION SPECIES [RCC]
Definition: The most extensive units in the natural economy such that reproductive competition occurs among their parts. Principal Author: Ghiselin 1974, 1977. Synonyms: Hypermodern species concept, Biospecies (in part).

26. SYNAPOMORPHIC SPECIES [SYSC]
A phylospecies conception. Definition: The least inclusive taxon recognized in a classification, into which organisms are grouped because of evidence of monophyly (usually, but not restricted to, the presence of synapomorphies), that is ranked as a species because it is the smallest "important" lineage deemed worthy of formal recognition, where "important" refers to the action of those processes that are dominant in producing and maintaining lineages in a particular case (Mishler et al.). A species is a lineage separated from other lineages by causal differences in synapomorphies (Wilkins). Principal Authors: Mishler and Brandon 1987, Mishler and Donoghue 1982, Wilkins 2003.

27. SUCCESSIONAL SPECIES [SSC]
Definition: Arbitrary anagenetic stages in morphological forms, mainly in the paleontological record. Principal Authors: George 1956, Simpson 1961. Synonyms: Paleospecies, evolutionary species (in part), chronospecies.

28. TAXONOMIC SPECIES [TSC]*
Definition: Specimens considered by a taxonomist to be members of a kind on the evidence or on the assumption they are as alike as their offspring of hereditary relatives within a few generations. Also: Whatever a competent taxonomist chooses to call a species. Principal Authors: Blackwelder 1967, but see also Regan 1926, Strickland et al. 1843. Synonyms: Cynical species concept (Kitcher 1984). Related Concepts: Agamospecies, genealogical concordance species, morphospecies, phenospecies, phylospecies.

REPLACEMENT CONCEPTIONS
Several so-called species conceptions are in fact replacement concepts that displace species. Some (e.g., LITUs) are intended to replace species entirely; others are parallel concepts that are neutral regarding species.

1. OPERATIONAL TAXONOMIC UNIT [OTU]
Definition: Taxonomic units of different categorical ranks to be grouped into more inclusive aggregates during classification (Sneath and Sokal). In microbiology, a phylotype is an environmental DNA sequence or group of sequences sharing more than an arbitrarily chosen level of similarity of a particular gene marker (Moreira and López-García 2011). Principal Authors: Sneath and Sokal 1973, Sokal and Sneath 1963. Related Concepts: Microbial species, bacterial species, phenospecies, taxo- nomic species, phylotype.⁶

2. LEAST INCLUSIVE TAXONOMIC UNIT [LITU]
A phylospecies conception. Definition: A taxonomic group that is diagnosable in terms of its autapomor- phies, but has no fixed rank or binomial. Principal Authors: Pleijel 1999, Pleijel and Rouse 2000, 2003.

3. METAPOPULATION
Definition: A population of populations that go extinct locally and recolonize (Levins 1970). A set of local populations of a single species that are linked by dispersal (Hanski and Gilpin 1991). Principal Authors: Hanski and Gilpin 1991, Levins 1970. Related Concepts: Ecospecies, Mendelian populations.

4. SMALLEST NAMED AND REGISTERED CLADE [SNARC]
A phylospecies concept. Definition: The smallest named and registered clade [in the Phylocode database]. Principal Authors: Mishler and Wilkins (2018). Related Concept: Least inclusive taxonomic units.