Species

242 Species

critical notion in the context of phylogenetic systematics, or, as it is popularly known,

cladistics. In Hennig’s definition, a monophyletic group is an ancestral species and

all of its descendent species.^3 Under more formally defined notions of phylogeny,

sometimes called pattern cladism, a monophyletic group is a proper subset of some

set of taxa, without necessarily implying a particular historical relationship.^4 On this

account, a monophyletic group is definable in terms of it having unique characters

that are not shared by other taxa. These are called apomorphies in Hennig’s termi-

nology, or “derived characters.” This is a relative term: an apomorphy for one set of

taxa is a plesiomorphy (an underived, or primitive, character) for a more inclusive set

of taxa. If one or more taxa share an apomorphy, it is called a synapomorphy, and if

only a single taxon carries an apomorphy, it is called an autapomorphy.

There are fundamentally three phylospecies concepts (Figure 9.1). The first,

defined initially by Hennig, is sometimes called the Hennigian species concept.^5

It rests on a conventional decision Hennig made to be consistent with his broader

philosophy of classification, and which we shall call the Hennigian convention. On

this account, if a new species arises by splitting from a parental species, then we

shall say that the parental species, no longer being monophyletic (that is, now being

paraphyletic), has become extinct, and there are now two novel species.

(^3) There is a rival conception, holophyly, which was coined by Ashlock 1971 (see the response by Nelson
1971) to free up the term monophyly to mean a more loose sense of “clade” in which paraphyly could be
tolerated; Hennigian monophyly would thus be holophyly on Ashlock’s view. In this writer’s opinion, this is
a concession to the “evolutionary systematics” of Mayr and collaborators, which seeks to group by similar-
ity as well as genealogy [Mayr and Ashlock 1991]. This point was made early on Pratt 1976, 380:
(^) ...for Mayr the principle of unity in classification is simply evolutionary relationships. He
wishes rather to erect groupings on the basis of evolutionary relationships in so far as these are
correlated with resemblances and differences between genotypes. [Italics original]
(^4) Vanderlaan et al. 2013 discuss the development, and many meanings, of the term monophyly.
(^5) Meier and Willmann 2000.
Hennigian
species
Synapomorphic
species
Diagnostic
species
A BCA B A B
C
(A) (B) (C)
FIGURE 9.1 Phylogenetic species concepts. A. Hennigian species are formed at speciation
and extinguished at the next speciation event or extinction. The cladogram is a history as well
as a classification. B. Synapomorphic species are terminal nodes in a cladogram or evolu-
tionary tree specified by synapomorphies. They are recognized by a lack of further discrete
patterns of ancestry and descent (i.e., by having no further synapomorphies). C. Diagnostic
species are the smallest terminal nodes in a cladogram, marked by their autapomorphies
(unique sets of characters depending on the authors). All three conceptions may also specify
that species are either monophyletic or not.