Phylogenetic Species Concepts 251
If the goal of distinguishing species is thereby to recognize the end-products of evolu-
tion, should we seek to suppress naming large numbers of species where large numbers
of differentiated end-products exist?^53Mishler considers a number of the diagnostic accounts to be phenetically based,
including Cracraft’s, Platnick’s, and Nixon and Wheeler’s.^54 Whether this is so (that
is, whether they make use of the Cartesian clustering of species in a state space of
traits typical of phenetic practice), it is clear that they assume that species are phy-
logenetically speaking the terminal taxa on a tree. Diagnosis assumes that the traits
are specified before the tree is constructed.
Where Is the Taxon Level, or Rank?
Operationally, phylogeneticists seem to have little operational difficulty in identify-
ing the level of species taxa in their cladograms, but there is a problem that arises if
the autapomorphic concept is taken too strictly. Consider cladograms of haplotype
groups. The authors have clearly already identified the species, and are consider-
ing whether or not the haplotype data taken from, say, mitochondrial DNA support
the claim that these populations form subspecies. But on a strictly autapomorphic
concept, each of the haplotype groups should be considered a separate species (in
exactly the way Whately described a logical notion of species would do for dog
breeds), unless other considerations, such as biogeography and interbreeding, are
taken into account, and if they are, then the species concept alone is insufficient to
delimit species. And so, it appears that some sort of prior knowledge is required to
specify at what level of a cladogram taxa begin and (for example) molecular lineages
cease to be subspecific diagnostic criteria.
The claim that species are defined by constant characters, made above by Wheeler
and Platnick, is problematic. Either we already know what characters count as
species-defining, or we are unable to find a level of species (for there are constant
characters for a great many higher-level groups, as well as lower-level groups, than
the usual level at which species are identified). Either way, this is not a full concept
of what makes a group a species.
Bibliography
Ashlock, Peter D. 1971. Monophyly and associated terms. Systematic Zoology 2 0 (1):63 – 69.
Cracraft, Joel. 1983. Species concepts and speciation analysis. In Current Ornithology, edited
by R. F. Johnston, 159–187. New York: Plenum Press.
Cronquist, A. 1978. Once again, what is a species? In BioSystematics in Agriculture, edited
by LV Knutson, 3–20. Montclair, NJ: Alleheld Osmun.
Davis, Jerrold I. 1997. Evolution, evidence, and the role of species concepts in phylogenetics.
Systematic Biology 22 (2):373 – 403.
(^53) Op. cit., 59.
(^54) Mishler, pers. comm.