"crown taxa"

Una Smith una at LANL.GOV
Tue Apr 23 08:28:30 CDT 2002


Una Smith wrote:
>>Crown taxa are those members of a clade that share (recognizable)
>>synapomorphies.  Stem taxa are members of the (larger) clade that
>>instead have plesiomorphies.  Usually, crown taxa are numerous,
>>stem taxa are few (this is a product of the concept of crown and
>>stem, not a definition thereof).

Pierre Deleporte wrote:
>Crown taxon would thus be strictly synonymous to monophyletic group
>(= clade), and stem taxon to paraphyletic group ?

Crown and stem taxa are OTU's, hence presumed to be monophyletic.
A crown clade is monophyletic.  There is no stem clade, but there
may be stem taxa "basal" to a crown clade.  Stem taxa are members
of the larger monophyletic clade that includes the crown clade.

>But then why would they regularly be more numerous in crown than
>stem?

It is an artefact, because people tend to recognize crown taxa and
clades only when they are numerous.

        Una Smith

Los Alamos National Laboratory, MS K-710, Los Alamos, NM  87545




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