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Re: Polytomy question

On 10/25/05, Manuel Parrado <meparrado@yahoo.com> wrote:
> My question is about polytomy in evolution.
> Phylogenetic analyses, especially of vertebrates,
> usually show progressive trees in the sense that one
> lineage splits an changes over time and splits again
> and so forth.  However, there are organisms that the
> fossil record shows have changed very little over time
> e.g., the coelacanth.  I assume there are many others
> that have achieved this level of morphological
> stability (perhaps turtles?).  I would assume that
> some of these organisms must have sprouted further
> lineages on several instances separated by millions of
> years due to allopatric speciation and other
> mechanisms.  Yet, phylogenetic trees rarely show
> polytomies of this kind and I don't recall seeing a
> single instance of this in any dinosaur tree.

I think that's because there aren't very many (if any) species that
are truly that conservative. Coelacanths may not have changed as much
as we have since our last common ancestor, but they have changed
(notably gaining some adaptations for living in deep seas). And look
at an alligator snapping turtle, a Galapagos tortoise, a leatherback
sea turtle, and a diamondback terrapin and tell me that they all look
Mike Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com