Please note: This text is part of Species, Kinds, and Evolution, by John Wilkins, Reports of NCSE 26 (4), 2006.
Summary of 26 species concepts
There are numerous species "concepts" at the research and practical level 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. There have also been several additional concepts since Mayden's review, so I have added the views of Pleijel (1999) and Wu (2001a, 2001b), and several newer revisions presented in Wheeler and Meier (2000). I also add some "partial" species concepts — the compilospecies concept and the nothospecies concept. 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. Asterisks identify the "basic" conceptions, from which the others are formed.
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. See Cain (1954), Eigen (1993, for quasispecies). Synonyms: Microspecies, paraspecies, pseudospecies, semispecies, quasispecies, genomospecies (Euzéby 2006, for prokaryotes).
2. Autapomorphic species
A phylospecies conception. 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). Nelson and Platnick (1981); Rosen (1979).
Defined by John Ray, Buffon, Dobzhansky (1935); Mayr (1942). Inclusive Mendelian population of sexually reproducing organisms (Dobzhansky 1935, 1937, 1970); interbreeding natural population isolated from other such groups (Mayr 1942, 1963, 1970; Mayr and Ashlock 1991). Depends upon endogenous reproductive isolating mechanisms (RIMs). Synonyms: Syngen, speciationist species concept.
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. See Hennig (1950; 1966); Kornet (1993). Synonyms: Internodal species concept, Hennigian species concept, Hennigian convention.
5. Cohesion species
Evolutionary lineages bounded by cohesion mechanisms that cause reproductive communities. See Templeton (1989).
A species pair where one species "plunders" the genetic resources of another via introgressive interbreeding. See Harlan (1963); Aguilar and others (1999).
7. Composite Species
All organisms belonging to an internodon and its descendents until any subsequent internodon. An internodon is defined as a set of organisms whose parent–child relations are not split (have the INT relation). See Kornet and McAllister (1993).
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. See Simpson (1961); Sterelny (1999); Turesson (1922); Van Valen (1976). Synonyms: Ecotypes.
9. Evolutionary species*
A lineage (an ancestral–descendent sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies. See Simpson (1961); Wiley (1978, 1981). Synonyms: Unit of evolution, evolutionary group. Related concepts: Evolutionary significant unit.
10. Evolutionary significant unit
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. See Waples (1991).
11. Genealogical concordance species
Population subdivisions concordantly identified by multiple independent genetic traits constitute the population units worthy of recognition as phylogenetic taxa. See Avise and Ball (1990).
12. Genic species
A species formed by the fixation of all isolating genetic traits in the common genome of the entire population. See Wu (2001a; 2001b).
13. Genetic species*
A group of organisms that may inherit characters from each other, a common gene pool, a reproductive community that forms a genetic unit. See Dobzhansky (1950); Mayr (1969); Simpson (1943). Synonyms: Gentes (singular: gens).
14. Genotypic cluster
Clusters of monotypic or polytypic biological entities, identified using morphology or genetics, forming groups that have few or no intermediates when in contact. See Mallet (1995). Synonyms: Polythetic species.
15. Hennigian species
A phylospecies conception. A tokogenetic community that arises when a stem species is dissolved into two new species and ends when it goes extinct or speciates. See Hennig (1950, 1966); Meier and Willman (1997). Synonyms: Biospecies (in part), cladospecies (in part), phylospecies (in part), internodal species.
16. Internodal species
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. See Kornet (1993). Synonyms: Cladospecies and Hennigian species (in part), phylospecies.
17. Least Inclusive Taxonomic Unit (LITUs)
A taxonomic group that is diagnosable in terms of its autapomorphies, but has no fixed rank or binomial. See Pleijel (1999); Pleijel and Rouse (2000).
Defined by Aristotle and Linnaeus, and too many others to name, but including Owen, Agassiz, and recently, Cronquist. 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. See Cronquist (1978). Synonyms: Classical species, Linnaean species.
19. Non-dimensional species
Species delimitation in a non-dimensional system (a system without the dimensions of space and time). See Mayr (1942, 1963). Synonyms: Folk taxonomic kinds (Atran 1990).
Species formed from the hybridization of two distinct parental species, often by polyploidy. See Wagner (1983). Synonyms: hybrid species, reticulate species.
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. See Beckner (1959); Sokal and Sneath (1963). Synonyms: Phena (singular: phenon) (Smith 1994), operational taxonomic unit.
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. 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, and tends to be favored by process cladists. See Cracraft (1983); Eldredge and Cracraft (1980); Nelson and Platnick (1981); Rosen (1979). Synonyms: Autapomorphic phylospecies, monophyletic phylospecies, minimal monophyletic units, monophyletic species, lineages.
22. Phylogenetic Taxon species
A phylospecies conception. 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 organism 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 Nixon and Wheeler (1990); Mishler and Brandon (1987).
23. Recognition species
A species is that most inclusive population of individual, biparental organisms which share a common fertilization system. See Paterson (1985). Synonyms: Specific mate recognition system (SMRS).
24. Reproductive competition species
The most extensive units in the natural economy such that reproductive competition occurs among their parts. See Ghiselin (1974). Synonyms: Hypermodern species concept.
25. Successional species
Arbitrary anagenetic stages in morphological forms, mainly in the paleontological record. See George (1956); Simpson (1961). Synonyms: Paleospecies, evolutionary species (in part), chronospecies.
26. Taxonomic species*
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. Whatever a competent taxonomist chooses to call a species. See Blackwelder (1967), but see also Regan (1926); Strickland and others (1843). Synonyms: Cynical species concept (Kitcher 1984).