Farewell to Species - reticulation

[Thomas DiBenedetto]

Richard Jensen wrote:
....a wide-spread species ... experiences the loss of a peripheral
population that undergoes rapid evolutionary change and is now readily
recognizable as a taxon in its own right.  Does the original wide-spread
species cease to exist?  Do we still have two taxa or three?
As has been explained to me by many cladists, after this "speciation"
event, we now have three taxa: the original wide-spread species (now
relegated to at least nomenclatural extinction) and its two "daughter"
species.
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I would disagree with those cladists, and with you as well.
I would say that we have two species, but three taxa. The original species
should not be "relegated to nomenclatural extinction", but recognized as a
higher taxon. My view of a taxon is that it represents a lineage (branch, or
system of branches)emanating from a common ancestor. The original taxon
still exists, and will continue to exist until all of its descendants are
extinct. The original taxon is, however, more complex than a simple lineage
branch; that original single branch has given off a sublineage. Thus the
original taxon (the branch emanating from the original common ancestor) is a
higher taxon. The name applied to the lineage emanating from that common
ancestor should always remain with the lineage (system) emanating from that
ancestor.
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R.J.:
My view is that, unless both daughter species have evolved
autapomorphies with respect to the original species, then after the split
there are still two species: the original is unchanged and there is the
new isolated species that takes off on its own evolutionary pathway.
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But the name of the original taxon was meant to apply to all descendants of
the common ancestor of the original taxon. So the original name applies to
all parts of this example; the singular branch as well as both sublineages
after the divergence. The original taxon is a higher taxon. You are right
that there are two species, but there is also a third taxon; the higher
taxon which encompasses the two species. And the original name must refer to
the higher taxon.
Apomorphies are the evidence by which we recognize lineage divergences. A
lack of evidence may make lineage identification difficult or impossible,
but that doesnt change the underlying reality that the divergence may have
happened. If we knew, independently of character evidence, that a divergence
has occurred, surely we would want to recognize that,,no?

Tom DiBenedetto
tdib at dccmc.org