One origin? (viral evolution)
tdibenedetto at OCEANCONSERVANCY.ORG
Mon Jul 23 16:29:11 CDT 2001
From: Curtis Clark [mailto:jcclark at CSUPOMONA.EDU]
The peripatric model of speciation virtually assures that the
ancestral species, which continues through the speciation event, becomes
paraphyletic, despite the fact that it might be in no way changed by the
speciation....Paraphyletic higher taxa are the result of human decisions.
I think that paraphyletic species are artificial human constructs as well.
If a tiny fraction of a species becomes isolated from the rest of the
species, and develops apomorphies, it can be recognized as a new species. We
would name it and represent that as a new branch on a tree. If we continue
to refer to the rest of the species by the same name as we referred to the
ancestral species, then we have made a human decision regarding naming, and
have created a paraphyletic species. If we give a new name to the persisting
part of the ancestral species, then we have made a human decision regarding
naming and have not created a paraphyletic species.
The standard objection is that the persisting species is "essentially"
unchanged; it is in fact identical to what it would have been if that
fraction had simply died, rather than go off and start a new branch. This is
absolutely true of course. But I would contend that something has changed
for the persisting/ancestral species; something of great and overriding
importance to systematists. It's relationships have changed. Curtis, aren't
you one who firmly believes that we should classify on the basis of
relationships rather than essentialistic attributes (believing that more
than I do)? The ancestral species now has descendant branches, whereas it
did not before. The taxon, the clade that began with the original ancestor
of the ancestral species, now includes more than one species, it subtends a
branch with branches. The ancestral taxon (species) is now a higher taxon.
Is this not what we mean by a higher taxon?
If (when?) the persisting main species develops new apomorphies, we will
have three different identifiable groupings; the new species, the persisting
species (now identifiable as such), and the ancestral species (no longer
extant). But what are we to do in the interim, before the persisting species
develops apomorphies? Two possibilities come to mind.
1. In the unusual situation in which we can actually watch this process
happen,and thus know the reality independant of normal methods of analysis
(as this example implies), we could use a sub-species rank, or some other
categorization to refer to the new fraction until such time as the
persisting branch is differentiated from the ancestral branch. Then the
smaller part would be bumped up in rank to the species level, the newly
apomorphic persisting species would also be recognized as a new species, and
the the ancestor would be seen as the ancestral branch of a higher taxon.
2. In the normal course of events, when we dont really know what the
ancestor looked like, we would simply go ahead and name the new species and
the persisting species as two different species. The stuff that was alive
yesterday would be part of the ancestral branch of a higher taxon, even
though it was "essentially" the same as the new species.
Ultimatly, we need to decide what our basis for classification is. If it is
to be relationships, then we should recognize that when the fraction went
off and started a new branch, then the relationships changed. The ancestor
is no longer a terminal taxon, it is an inclusive higher taxon, and its
parts should be both recognized as terminals.
If we are to classify based on attributes, then we might need to delay the
reclassification until the persisting species differentiates itself from the
ancestor. In either case, we need to make a human judgement about how to
classify a lineage that his diverged. Reality does not compel us to accept
More information about the Taxacom