Species

246 Species

When some of the tokogenetic relationships among the individuals of one species

cease to exist, it disintegrates into two species and ceases to exist. It is the common

stem species of the two daughter species.^21

This has become known as the Hennig Convention. It has been strongly criti-
cized from all sides, by biospecies proponents, evospecies proponents, and other
phyogeneticists,^22 not least because it seems to be an arbitrary way to delimit species
taxa. Mayr, for example, takes Hennig to be making a substantive claim on the ways
species are formed at speciation, and criticizes it on the basis that the “parent” spe-
cies can remain unchanged even though it is no longer monophyletic. But it seems to
me that the critics have overlooked the most charitable interpretation of the Hennig
Convention—it is a convention about naming and denotation.^23 In short, the name
of a species is extinguished at speciation. This follows from Hennig’s views about
the task of systematics. Using (and citing) Woodger and Gregg,^24 and the views of
Woodger in particular, about sets in classification,^25 Hennig strives to ensure that
there is no ambiguity of reference in the sets named in systematics. Since as soon
as a set is divided there is ambiguity about which of the two resultant sets is being
referred to by a prior name, Hennig proposes to extinguish the now-ambiguous name
and create two new ones. However, he seems to equivocate over whether or not they
are new entities.
The Hennigian concept of species has been recently expanded and defended by
Meier and Willmann,^26 who have proposed a modified Hennigian Species Concept:

Species are reproductively isolated natural populations or groups of natural popula-

tions. They originate via the dissolution of the stem species in a speciation event and

cease to exist either through extinction or speciation.^27

(^21) Hennig 1950, 102, translated in Meier and Willmann 2000, 30.
(^22) See the citations in Meier and Willmann 2000, 31.
(^23) Rieppel 2011 argues against my interpretation, and backs it up with a passage from Hennig:
(^) If one focuses on the genealogical relationships [rather than on morphological or ecological
similarity], then one has to accept that a one-to-one relation exists not only between the fos-
sil [i.e., ancestral] species A and one of the extant [i.e., descendant] species (B or C), but also
between A on the one hand and B + C on the other. In spite of biological identity between A and
B there exist genealogical relations that also include species C, from where it follows that the
fossil population (species A) cannot be considered identical with only one of the extant popula-
tions (with only B or C respectively) ...
(^) Rieppel goes on to say
(^) What Hennig wanted to say was that the ancestral species (A) stands in a one-to-one relation
not only with one (B) or the other (C) of its descendant species but also with the union of A and
B, where (A ∪ B) is the monophyletic taxon that emerges from the speciation process.
(^) Rieppel may be correct about Hennig’s intentions, but Hennig’s phrase “cannot be considered” still
sounds to me as if it is about denotation and reference. In any case, the charity offered here is not
based on the assumption that Hennig himself was entirely clear on this matter.
(^24) See the excellent discussion in Wheeler and Platnick 2000.
(^25) Wo o dge r 1937.
(^26) Meier and Willmann 2000, 31; cf. Willmann 1985a, 1985b, 1997, 1997.
(^27) Quoted from Willmann 1985a, 80, 176.