Phylogenetic Species Concepts 243
The second phylospecies concept we might call the synapomorphic concept.
This further divides into the Monophyletic Concept of Mishler et al. and those who
merely apply synapomorphic criteria.^6 A species is, under this account, a taxon in
which there is no further division or taxonomic subdivisions—it is the unit of phylo-
genetic analysis. Mishler’s version adds the view that species must be monophyletic.
The third phylospecies concept, which I shall call the apomorphic species
concept,^7 is derived from the work of Donn Rosen.^8 In various versions, it tends
to rely upon the diagnosability of taxa,^9 or, as we may otherwise say, it is a largely
epistemological notion of species. All the diagnostic concepts rely upon a species
being the “terminal taxon” in a cladogram. Some proponents apply an evolutionary
exegesis to this, while others restrict it to a diagnostic relationship, a distinction often
referred to as the ontology–epistemology aspects of species.^10 Of course, a phylospe-
cies of one kind can also be a phylospecies of another. The division of the phylospe-
cies into two main kinds, ignoring Hennigian species for the moment, is a reflection
of the larger taxonomic debates. These raise the questions (i) should taxonomic clas-
sification proceed in terms of descent alone or on the basis of similarity (cladism ver-
sus gradism)? and (ii) if classification rests on clades, are homologies (apomorphies)
indicators of history, or are they patterns that are evidence in favor of a historical
reconstruction but not themselves a model of evolution? Briefly, this is the distinction
between ontological and epistemological notions of classification again.
Those who take the epistemological classification position (grouping in terms of syn-
apomorphic relationships only) often fall under the rubric of pattern cladists, although
this is not necessarily so. Mishler and Theriot, for example, take synapomorphies as
indicating relations at a time, synchronically, to avoid time paradoxes (in which a species
becomes paraphyletic after the speciation of daughter species, if it is treated as a sister
taxon). Only tokogenetic relations are treated as diachronic relations under this view.^11
Those who take the diagnosis of monophyly of a group to give a direct hypothesis
of evolutionary history are the so-called orthodox, or “traditional” cladists, called
“process cladists.”^12 Process cladism tends to treat taxa as relationships between
organisms, while pattern cladism tends to see taxa as composite entities of which
organisms are members. In this regard, the process orthodoxy is more closely allied
to some aspects of the evolutionary species concepts of recent times.^13
(^6) Mishler and Theriot 2000.
(^7) Meier and Willmann 2000, 36–37, call the Apomorphic Species Concept the Phylogenetic Species
Concept simpliciter, and Davis 1997 calls the process or Synapomorphic Species Concept the
Autapomorphic Species Concept, in direct contradiction to my usage in the first edition.
(^8) Rosen 1979.
(^9) Cronquist 1978.
(^10) Wheeler and Platnick 2000. The ontology–epistemology distinction is discussed in Chapter 14.
(^11) Mishler and Theriot 2000.
(^12) The term “process cladism” was introduced in print in Ereshefsky 2000. It is unclear to me that either
pattern cladism or process cladism form monolithic schools, and the differences of opinion on these
matters needs to be investigated. Thanks to David Williams for noting this, and catching my transpo-
sition of their core ideas.
(^13) It is not, in my opinion, true that pattern cladism commits its adherents to an antievolutionary view
of taxa. Neither is it true that it is an essentialistic view of taxonomy, as some—notably Mayr—have
claimed. It is, however, typological. But then, so is all systematics.