Principios de Taxonomia

7.11 The Cladistic Bifurcation of a Stem Species Always Means the End of the Stem Speciesj211
monophyly (Hennig, 1966). The logic of cladistics does not allow the survival of the
stem species because sister taxa always must be of the same rank. From this, it follows
that every bifurcation must represent the end of the stem species.
A stem species can never survive a branching event. Advocates of the view that the
stem taxon continues to exist argue that the stem species has not experienced any
change in its traits during the branching event from group A to groups C and E
(Figure 2.1). Thus, only the side branches (B and D) would exhibit new traits and
would therefore have to be considered as new species. However, making a distinction
between a side branch and a main branch is an anagenetic view, and anagenesis is
excluded by cladists. In cladistics, side branches as opposed to the main branch do not
exist because there is no logical reason for this distinction. The logic of a side branch
in contrast to a main branch inherently implies that two branches would exist with
different ranks. What justification would there be for this? Only the branching is
crucial, and the result of a branching event is always two new branches of equal rank,
rather than only one.
Bifurcation is the disruption of gene flow. This is not associated directly with trait
alteration, even though as a consequence of bifurcation, trait alterations often arise
very rapidly. However, speciation is defined solely by bifurcation. The fact that trait
alterations that distinguish the two daughter branches from each other and from the
stem species then also occur is a secondary consequence that is not related to the
definition of cladistic branching.
The awareness that speciation only means bifurcation, not necessarily the alteration
of traits, is clearly counterintuitive. It is particularly difficult to accept the
extreme situation: a small population at the periphery of a large geographical range
becomes separated; then not only this peripheral population is considered a new
species, but also the complete rest of the species. The demand that both branches of a
bifurcation event must become new species, rather than just one of the two branches,
requires classifying the stem species as terminated and considering its entire
remainder as a newly evolved species, even if it consists of millions of individuals
spread over the entire continent that did not undergo changes in any of their traits.
This almost sounds paradoxical, but it is logical.
Another consequence of the concept that a bifurcation always constitutes the end
of the stem species and the origin of two new species is the impossibility of
determining the actual age of a species. The law of cladistics demands that the
origin of a species is always based on the bifurcation of a stem species into two
daughter species. There is no other mode of species formation. Consequently, the
conclusion consistently arises that a species is always as old as the last bifurcation.
However, as many sister branches are short lived because the organisms they
represent become extinct, not all branching events are perceived by the human
observer. Therefore, it is always possible that between a known split and a currently
living species, another bifurcation has occurred that has not yet been detected (B in
Figure 2.2). However, such an overlooked split would necessarily indicate that the
currently living species a somewhat younger. In Figure 2.2, species A is younger than
species C. As the possibility of an undiscovered lateral bifurcation can never be ruled
out, the age of a species can never be determined. Thus, the question of how old a