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Descent by Ancestry

Philidor11 (philidor11@snet.net) wrote:

<<The difference in approach being considered is:  are you
emphasizing the end result (bird/dinosaur) or the beginning of 
the separation between the two groups?  Doesn't it seem most
reasonable to emphasize the end result when deciding when
differences are large enough to distinguish a group?>>

Filipo Calzola (calzola@iol.it) wrote:

<no, i think it's more reasonable to consider the group as a
whole; in this case you have to arbitrarily choose to consider
one branch of the tree as a different group, which it is, based
on few characters seen only in the components of this group, but
_in ALL_ of them. Many derived featurs will be present in some
taxa, but this isn't important(well, at least as long as you
don't think they're enough to estabilish another group).>

  Filipo makes a point about considering a whole when working
with intergroup relationships, but because of the seemingly
transistory aspect of some groups, especially coelurosaurian
theropods and marginocephalian ornithischians, where the forms
don't fit into neat little boxes and categories, but into grades
where the distinctions are by one or two "characters," the whole
cannot be considered without subjecting it to human inference.
How we want to see it. Check out Sereno et al's series of grades
produced in so many papers of the last decade.

  The whole itself can be diagnosed by as few as a handful of
characters, however many are actually diagnostic at the _base_
of the group, depending on the amount of reversals and
transformations have occured in other derived members. The whole
is often diagnostic as a sort of paraphyletic definition, but
the base is secure no matter what its descendants have
transformed, reversed, lost, or whatever, even in their meager
little gene codes. A descendant does not always add on to the
predescesors' whole, but changes it ultimately, otherwise a
child would have a longer genome than it's own parents, by every difference.

Jaime A. Headden

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

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