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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
alike.
--
Mike Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com




What I find interesting is that every evolutionary
change is always modeled as a split.  How about
species that simply change over time without spliting
into two different lineages?  This is rarely captured
in any phylogenetic tree.  Is it because of lack of
evidence?  Would we know them if we see them (not me
of course but you get the point).

Thanks.


                
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