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Re: TETRAPODS, PHYLOGENETIC TAXONOMY, AND CLEAR DEFINITIONS



I understand Mickey wants us to wind this down, so here's my last missive
on the subject:


>
>So we have a second diagnostic procedure here: anything that can be
>diagnosed as belonging to a subgroup of Dinosauria also belongs to
>Dinosauria.  Reasonable.
>

But, again, I think you're approaching the subject with an eye toward
*defining* taxa on the basis of these characters.  Ankylosaurs must possess
some intrinsic physical feature that makes them a dinosaur, and if the
features found exclusively in ancestral dinosaurs don't apply to
ankylosaurs, we have a problem.  That's what we no longer do.  A taxon
belongs to a group if it's descended from the last common ancestor of the
group - period.  Diagnoses are used as evidence for relationships - not to
define taxa, and *certainly* not as identification keys, since diagnoses
change every time a new taxon is considered, or a new specimen is
discovered, or new characters are added to the analysis, or whatever.

As an aside, a war was actually fought over this distinction.  The Royal
Navy, shortly after the American Revolution, actually maintained an
ancestry-based view of people, which is why they were impressing American
sailors.  Once an Englishman, always and Englishman - the accent and
adopted nationality was irrelevant.

Please, read that article by Tim Rowe on definition and diagnosis - it
will, I think, clear up some of the misunderstandings you're having.  I
also recommend the following:

Ghiselin, M.T.  1997.  Metaphysics and the origin of species.  State Univ.
of New York Press, not sure of the city.  You can get it through amazon.com.

de Quieroz, K.  1995.  Phylogenetic approaches to classification and
nomenclature, and the history of taxonomy.  Herpetological Review,
26(2):79-81.



chris


-=--=--=--=--=--=--=--=--=--=
Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566

cbrochu@fmppr.fmnh.org