Species

248 Species

They redefine monophyly in such a way as to be able to include species:

A monophyletic taxon is a group that contains all and only descendants of a common

ancestor, originating in a single event.^32

De Queiroz and Donoghue, on the other hand, treat species as systems that may
not be monophyletic, and indeed may be paraphyletic if a species has split from
it, in a parallel with cohesive and functional individuals who lose cells and repro-
duce.^33 They therefore exclude asexual organisms, and indistinct populations that
are not assignable, from being members of species. Mishler and Theriot, extending
the monophyletic (i.e., phylogenetic taxon) conception, include asexual organisms
largely on the grounds that asexual taxa do not markedly differ in overall phylo-
genetic nature from sexual taxa—the number of autapomorphies, for example, are
similar in both cases.^34
Synapomorphic species are usually based on the historical, actual, lines of ances-
try and descent that are represented in a cladogram. As a phylogenetic taxon, a
species is grouped by the synapomorphies shared by organisms that indicates mono-
phyly.^35 It is a phylogenetic or cladistic replacement for the evolutionary species con-
cept of Simpson. Mishler and Theriot give the following broad definition:

A species is the least inclusive taxon recognized in a formal phylogenetic classifi-

cation. As with all hierarchical levels of taxa in such a classification, organisms are

grouped into species because of evidence of monophyly. Taxa are ranked as species

because they are the smallest monophyletic groups deemed worthy of formal recogni-

tion, because of the amount of support for their monophyly and/or because of their

importance in biological processes operating on the lineage in question.^36

Here, the monophyletic conception is explicit. The dual nature of the epistemic

and the ontological aspects of species are clearly expressed, and the rank of species

is restricted to “biologically important” lineages. Their version of the synapomor-

phic concept allows for reticulation as a general problem in classification not merely

restricted to species taxa.

De Queiroz and Donoghue, on the other hand, do not think that species have to

be monophyletic, because monophyly of populations does not offer a way to specify

what the base rank is, and because species evolve from ancestral populations this

will leave the species from which the ancestral population derived as paraphyletic.^37

Of course, the monophyly spoken of here is somewhat different from the monophyly

of the Mishler et al. version—this one is based on populations as the base entities; the

Mishler et al. version is based on phylogenetic lineages. And both conceptions con-

verge on a similar solution—species are regarded as singular phylogenetic lineages,

(^32) Op. cit., 313 in Hull and Ruse 1998.
(^33) de Queiroz and Donoghue 1988, 1990.
(^34) Mishler and Theriot 2000, 52f. One does wonder, though, if this follows more from the logic of phy-
logenetic method than anything else.
(^35) Mishler and Theriot 2000, 47.
(^36) Op. cit., 4 6 – 47.
(^37) de Queiroz and Donoghue 1988, 1990.