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Modes of definition

--Original Message-- From: T. Mike Keesey <tkeese1@gl.umbc.edu>08 December
1998 22:30

>Phylogeny-based definitions are far more stable than feature-based
>definitions. What if the diapsid skull condition were found to have
>evolved twice? Diapsida would become a polyphyletic taxon. By the
>phylogenetic definition, however, it remains a valid taxon (although its
>membership may change) no matter what.
>> [ archosaur@usa.net ]: So what constitutes being the common ancestor?
>Being the most recent critter which gave rise to all of the anchor taxa,
>natch. How can you tell if a particular organism is the MRCA? You can't.
>But, if an animal were found to have all of the synapomorphies of
>Reptilia, but none of the synapomorphies of any reptilian subclades, and
>occured before any other reptiles, there'd be a possibility.

I'm beginning to see point of using MRCA's (and the other things - is it
spur groups?) since they may at least be valid, even if one can't be sure of
the characters or membership.  However they are not absolutely immune from
problems.  They may grow to encompass a far bigger group than at first
envisaged; I've also seen at least one MRCA where the two groups may well
turn out to be nested.  Funnily enough, in this case, a particularly useful
character would do the trick quite well!