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Re: Paraphyly and sister taxa (was Re: AMARGASAURUS SATTLERI)

Nick P. wrote...
>Not sure what you mean by (3) here.  Let's say "A" and "B" are stem-based
>sister taxa and that both are subgroups of node-based taxon "X", which
>includes both "A" and "B" and their common ancestor.  Then the common
>ancestor is a member of taxon "X" but is not a member of taxon "A" or taxon
>"B".  Where is the paraphyly here?

>I guess we need to clarify what we mean by "common ancestor" here.  Are we
>talking about an individual?  A population?  A species?  A family?

    I'm assuming that the common ancestor of A and B would be assigned to a
genus and species if found.  I think the difference in perception here has
to do with the difference between putting words and lines on a peice of
paper or computer screen and thinking about what it really translates into
materially, and I'm not trying to say that in a way that sounds
condescending.  I think there really is a complete conceptual difference
here between plotting out OTUs in a cladogram and thinking about what it
represents in real life.
    A, B, and common ancestor "X" were all equally real organisms.  They all
had cells and DNA and proteins.  No one was more real then another.   If you
took a time machine, collected all three organisms, and showed them to a
biologist, he or she would be unable to give you a good reason why one
should be called "A", and one "B", but the third could only be referred to
as "a member of taxon 'X' including A and B".  The only reason for not
assigning "X" an artificial label like "A"s and "B"s is because it makes
things difficult for a person mapping out organisms on a cladogram without
paraphyly.   That is an artificial decision.

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Jeffrey W. Martz
Graduate student, Department of Geosciences, Texas Tech University
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