Species

244 Species

Phylogenetic approaches to species are founded on one of three criteria—

synapomorphy, autapomorphy, or prior phenetic identity as operational taxonomic

units (OTUs). Roughly, synapomorphy acts as the classical logical notion of genera,

autapomorphy acts as the classical notion of differentia, and in the case of phenetic

OTUs, as inputs into a cladistic analysis, while species here are types that are known

prior to the formal division. Therefore, it is something of a misconstrual to think that

there are only two “phylospecies” concepts besides the Hennigian account.^14

Hennigian, or Internodal, Species

In the work that began the cladistic revolution and which remains the source for
much cladistic thinking, so clearly and consistently was it expressed, Hennig treated
species in an unusual manner.^15 He begins by defining “semaphoronts,” or “charac-
ter bearers” as the elements of systematics. These are effectively stages or moments
in the lifecycle of organisms of the taxon. Individuals themselves, or more exactly
the ontogenetic lifecycles of individuals, are related through reproductive relation-
ships he called “tokogenetic” relationships. When tokogenetic relationships begin to
diverge, they form species, which arise

... when gaps develop in the fabric of the tokogenetic relationships. The genetic rela-

tionships that interconnect species we call phylogenetic relationships. The structural

picture of the phylogenetic relationships differs as much from that of the individual

tokogenetic relationships as the latter does from the structural picture of the ontoge-

netic relationships. In spite of these differences in their structural pictures, the phylo-

genetic, tokogenetic, and ontogenetic relationships are only portions of a continuous

fabric of relationships that interconnect all semaphoronts and groups of semaphoronts.

With Zimmermann we will call the totality of these the “hologenetic relationships.”^16

Hennig illustrated this with a now-famous diagram (Figure 9.2).

Hennig is sometimes read as asserting that species cease to exist when they are

divided; that is, when the hologenetic relationships cease to be a single set (the spe-

cies sets are represented in the diagram by the large ellipses). Although Hennig

assumes that species are reproductive communities of harmoniously cooperating

genes, as Dobzhansky had said, and that species are reproductive groups, as Mayr

had said, Hennig nevertheless assumes that species are phylogenetic lineages. In

fact, he says that species are defined over spatial dimensions as well, as

... a complex of spatially distributed reproductive communities, or if we call this rela-

tionship in space “vicariance,” as a complex of vicarying communities of reproduction.^17

(^14) Brent Mishler pointed out to me that what I had been calling a single class of species concepts, under
the term “Monophyletic Species Concepts” (now Phylogenetic Taxon species) was actually pretty
diverse, and that some (e.g., Cracraft) did not think species had to be monophyletic. I have attempted
to disentangle these views from each other in the text.
(^15) Hennig 1966, 28–32.
(^16) Op. cit., 30.
(^17) Op. cit., 47.