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Re: Turtles and Crocodylians are not Reptiles - no? What are they?



On Mon, Jul 28, 2008 at 4:15 PM, Tim Williams
<twilliams_alpha@hotmail.com> wrote:
>
> I'm not trying to be tricky here with semantics - quite the opposite.   I'm 
> just trying to get the point across that "closer" in a phylogenetic context 
> does *not* necessarily mean "shared ancestry".  Further, "closer" should not 
> even imply "shared ancestry".

This is starting to become a repeat of discussions we had about a
decade ago (or more?) on this list. Basic upshot: "closer" can mean a
lot of things. Like it or not, closeness of common ancestry is one of
them.

> To use the example of birds and deinonychosaurs again... _Archaeopteryx_ and 
> _Passer_ are both birds, and Deinonychosauria is sister taxon to the bird 
> clade.  Thus, _Archaeopteryx_ and _Passer_ share a more recent common 
> ancestor than either does to any deinonychosaur.  But if you look at the 
> overall topology, the number of nodes separating _Archaeopteryx_ (a basal 
> bird) from _Deinonychus_ is far fewer than separate _Archaeopteryx_ from 
> _Passer_.

That seems like a very unstable metric to me, since it relies on which
taxa are used in the topology in question. For example, look at this
topology: ((Archaeopteryx, Passer), (Troodon, (Microraptor,
(Unenlagia, (Velociraptor, Deinonychus))))). Here there are two steps
from Archaeopteryx to Passer, but seven from Archaeopteryx to
Deinonychus. Obviously, I've skewed the taxon list to get this result,
but, still, the numbers are going to vary from topology to topology,
especially considering that we only know a fraction of the actual
taxa.

Chronological distance is more stable (although not completely stable,
of course). Generational distance is somewhat less stable than
chronological distance, but probably closer to the gist than anything
else.
-- 
T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039
http://exopolis.com/
--
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