[Taxacom] Rational holophyly and deological extinction

[Curtis Clark]

On 2009-03-16 11:50, Richard Zander wrote:
> I think there is a rational (sort of) justification for using holophyly
> in classification. It goes this way:  ...

First, ancestors. There are organisms alive today that are ancestors of 
other living organisms (I am one of those, although I take umbrage at 
being called paraphyletic), but the cases where the ancestor is 
generally considered to be of a separate species from the descendant are 
extremely rare.

Second, it is well-established that some species are ancestors of other 
species. If one assumes that species are real, this has some 
evolutionary meaning (if species are not real, it's religion), but it is 
not possible to interpret all of evolutionary history as a nested set of 
peripatric speciations.

Third, it is generally impossible to say with any certainty that any 
given fossil is the ancestor of an organism alive today (although it is 
quite credible to say that it is a relative).

Fourth, higher taxa can be ancestral to other higher taxa only if they 
are paraphyletic. Ancestor-descendant relationships are thus an artifact 
of the way a taxonomist chooses to draw up the taxa. One could argue 
that in the case of two lineages, one derived from an ancestral species 
and another from one of its peripatric descendants, the former is 
irreducibly paraphyletic, but I see that as equivalent to saying that I 
am irreducibly paraphyletic: not of much use except as a straw man.

We all know that there are ancestors in evolution. But history has shown 
that they are devilishly hard to pinpoint, much less to develop testable 
hypotheses about.

For a cladist to say that ancestors are unimportant is perhaps arrogant, 
but for a cladist to say that our time is better spent looking at 
sister-group relationships is a nod to reality.

-- 
Curtis Clark                  http://www.csupomona.edu/~jcclark/
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