Species

Phylogenetic Species Concepts 247

Like Hennig and most proponents of the biospecies and other isolationist concep-

tions, they reject the use of a single taxonomic category such as species to apply to

asexual (“uniparental”) organisms. Instead, they call them “agamotaxa.”

A set-theoretic model of Hennig’s concept has been provided by Kornet (under

the title of “internodal species concept”) who formalizes the notion of internodal-

ity (INT) in a cladogram in terms of the tokogenetic relations being permanently

divided, so that any individual in the hologenetic group has a “gross dynastic rela-

tionship” (GDYN) with any other individual in that group.^28 Type specimens thus

fall out as an appropriate individual to start the GDYN analysis to establish the INT

relation. Kornet thus makes the relation of type specimens to the rest of the species

scrutable (see below) in set theoretic terms.

The Hennig account is fundamentally a biospecies concept. Hennig himself

accepted that species were reproductively isolated, and the criteria used for iden-

tifying the relevant phylogenetic edges of the cladogram are simply those of the

biospecies. We could therefore say that it is better considered to be considered under

that rubric, and that the issue of “extinction” of species at cladogenesis is one of the

reference of taxonomic names. In short, the “extinction” is a taxonomic extinction.^29

Phylogenetic Taxon (Synapomorphic) Species

There are many other conceptions that their authors refer to, or which are referred

to by others, as “phylogenetic” conceptions of species. It is not clear that these form

a natural class of conceptions. I believe there are two basic approaches that derive

from the cladistic terminology, but it is not therefore the case that all authors in

one or the other classes agree or that cladistic terminology resolves the differences

between them.

Synapomorphy-based species are largely due to the advocacy of Brent Mishler,

although earlier Joel Cracraft had defined the species taxon as

... the smallest diagnosable cluster of individual organisms within which there is a

parental pattern of ancestry and descent.^30

Mishler and Brandon summarize their Monophyletic version:

A species is the least inclusive taxon recognized in a classification, into which organ-

isms 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.^31

(^28) Kornet 1993, Kornet and McAllister 1993.
(^29) In the context of the debate between Mayr and Hennig over the “correct” way to classify [Mayr 1974,
Hennig 1975], Hennig criticizes Mayr for ambiguity in his terms, which lends support to this inter-
pretation. Above all, Hennig wanted clarity.
(^30) Cracraft 1983, 170.
(^31) Mishler and Brandon 1987, also in Hull and Ruse 1998, 310. See also Mishler and Donoghue 1982.