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Re: Two new FAQs: Everything You Wanted To Know About Cladistics

Mike Taylor (mike@tecc.co.uk) wrote:

<Could someone please briefly explain why we avoid polychotomies? Is the 
truthful answer just that
life is easier if we don't bother thinking about them?  :-)>

  This is a complex answer actually...

  One simple answer is that it avoids a strict representation of divergence, 
i.e., branching. We
know life branches, but a polytomy does not represent this. On the other hand, 
the neontological
one, with which I agree, polytomies are in fact a fact of nature: within a 
genetic stock one will
find a close patterning of chaotic evolution. As I suggested in my large post 
earlier today, a
taxon like *Echinodon* represents the primordial ooze from which cerapodans 
diverge, quite likely,
and this led to the lumping of "fabrosaurs", "hypsilophodonts", classic 
"prosauropods", etc. into a monophyletic whole.

  The problem with the neontological "basal chaos" of polytomy, is that the 
polytomy occurs only
as a small genetic group, and by the time distinctive groups of synapomorphies 
or more clarifying
synapomorphies with other groups appear, these forms have most likely diverged 
from the genetic
basal stock to prove a distinctive trend. By the time an animal shows these 
distinctions, it is no
longer part of that morass.

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

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