Re: "Common ancestor" in cladistics

[Dora Smith <villandra@austin.rr.com>]

That confused me, too.   Studies of phylogenetic relations of birds use a
related confusing term of basal, ie, some groups of birds are "basal" to
others.   (Unfortunately which groups are basal to which varies completely
by the study.)

"Basal" and "ancestral"  and "common ancestor" as you use the terms here
both mean that this group split off from a common line earlier than
specified other groups that share the same common ancestor.

Methodologically we rarely have DNA from any common ancestors at all, so
what is done is to chart or measure genetic differences between existing
groups, on the theory that the groups with the most genetic differences are
teh most distantly related.

I think I myself am more puzzled than I initially realized as to how
researchers decide which groups are oldest - and that could partially
explain the muddle in the ordering.

For instance, we know that rheiformes (like ostriches) and a closely related
group are most distantly related to all other living birds, that chickens,
geese and ducks are next most related to all other groups of living birds,
and somehow passeriformes end up at the other end of the spectrum.    Some
researchers find rheiformes to be the oldest group, some find chickens,
geese and ducks to be the oldest group, and some actually insist that
passeriformes are the oldest group!

I know part of it is that researchers assume that the greatest genetic
differences took the most time to develop - but I'm not convinced that that
actually tells us which group happened first if we have no ancestral DNA to
compare it to.

Yours,
Dora Smith
Austin, Texas
villandra@austin.rr.com
----- Original Message -----
From: "Manuel" <mparrado@peoplepc.com>
To: <dinosaur@usc.edu>
Sent: Wednesday, July 28, 2004 1:55 PM
Subject: "Common ancestor" in cladistics


> I hope this is not an overdone question.
>
> Recently, I have started reading very avidly about dinosaurs and
paleontology in general.  I keep finding many authors (of books, websites,
articles, etc.) referring to groups of dinosaurs as having "a common
ancestor", "a more recent common ancestor than", "all descendants of the
common ancestor of this and that", etc...
> To help me get a feel for how paleontologists build phylogenic
(phylogenetic?) relationships, can somebody tell me whether there are some
genera or even species that have been identified as the so-called "common
ancestor" of others?  Is there such a thing as an actual common ancestor or
is that an abstract concept?  If there are, how are they identified to be
such?  Some examples of actual dinosaurs would be great.
>
> Thanks in advance.
>
> Manuel Parrado
>
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