Re: synapomorphies not created equal

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Sat, 1 Sep 2001 Dinogeorge@aol.com wrote:

> Incidentally, any clade is united not by a list of synapomorphies but only by
> a single synapomorphy; in any list of synapomorphies, some must inevitably be
> synapomorphies of nested groups contained in the clade, and others must be
> plesiomorphies of clades that contain it. This is because it is extremely
> unlikely that a set of apomorphies would all evolve at exactly the same time.
> Apomorphies appear in a lineage in serial order, sometimes in rapid
> succession, sometimes more slowly. When they appear in rapid succession and
> the fossil record is poor (as usual), it presents the illusion that they
> appear all at once.

Very true; in fact, to take this a bit further, the fossil record is so
spotty that it is very, very likely that *all* synapomorphies used to
diagnose a given stem-based clade are in fact synapomorphies of clades
within it, and that *all* synapomorphies used to diagnose a given
node-based clade are in fact plesiomorphic for that node. Not that it
really matters, as far as determining the phylogeny of known forms is
concerned, but something to keep in mind, nonetheless.

_____________________________________________________________________________
T. MICHAEL KEESEY
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