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9

Phylogenetic Species

Concepts

Phylogenetic conceptions of species are very similar in some ways to the classifica-
tions of the older logic of division discussed in Section I. Hennig’s classification is
done by division, from synapomorphies to autapomorphies, or in other words, top-
down. Hennig distinguished between derived and underived states in taxonomy.
Plesiomorphic characters are underived in a monophyletic group (a clade) from which
the transformations begin, and apomorphies are those derived from them in evolu-
tion. An apomorphy of a group can be a plesiomorphy of a clade contained within that
group. It follows that in Hennig’s classificatory scheme that what makes a plesiomor-
phy and an apomorphy is not absolute rank, but its relative position in a cladogram. In
the older terminology, we discussed in Section 1, they are relative to the differentiae,
the predicates and the properties those predicates denote which make the differences
between taxa. Hennig has no system of absolute taxonomic levels, although he did
also attempt to develop such an absolute set of ranks based upon time of evolutionary
divergence.^1 There are just taxa, and they are arrayed in a flexible local hierarchy.^2 In
the classical logic, the base-level taxonomic rank—the infimae species—was a taxic
entity that was not itself the genus of any other species and which contained only
individuals. Likewise, Hennig has terminal taxa, and these he calls species, following
Linnaean tradition. Where the infimae species and the Hennigian species differ from
the Linnaean species, however, is that the former are derived from the general group
being sequentially divided into subgroups on the basis of characters shared (a single
dichotomous key in the early naturalists’ practice, on an implied parsimony criterion
of many characters for Hennig), while Linnaeus assumed fixed taxon ranks. Linnaean
species are an absolute rank, and so also are the higher taxa they comprise.
Species concepts based upon the phylogeny of the groups of organisms are called
“phylogenetic,” but there are several phylospecies concepts, as I will call them, and
there are several sub-versions of them in turn. All proponents of phylospecies con-
cepts claim both Darwin and Hennig as their inspirations, but it is arguable how
closely each of the modern views relate to those initial expressions of classification
of taxa by descent.
In some ways, phylogenetic classification is an outgrowth of the ideas expressed
by Haeckel and others during the late nineteenth century. Haeckel coined the term
monophyly, which Hennig later appropriated and more strictly defined. This is a

(^1) Hennig 1966, 184 –192.
(^2) Nelson 1989.