"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

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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