"crown taxa" (and basal)

pierre deleporte pierre.deleporte at UNIV-RENNES1.FR
Wed Apr 24 12:49:25 CDT 2002


A 08:28 23/04/2002 -0600, Una Smith wrote:

>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.

It happens that I just discussed the use of "basal taxa" in reviewing a
paper with colleagues.
When you have two sister-groups, it seems effectively that people tend to
call "basal taxon":
-  (1) the less numerous of the two groups (in terms of number of terminals),
-  (2) and this the more if it carries a character of interest for the
author which is considered plesiomorphic on other grounds (I mean on
grounds other than this simple sister-group relationship, which is not
historically informative in itself).

What puzzles me is that the two sister-groups are in fact equally "basal":
all their members in the analysis (data matrix) are in fact terminals, and
thus apical (not basal in any way), and the only "basal" thing in the
picture is the basal bifurcation of the tree, or the common ancestor
supposed to have been there in the past (but not its contemporaneous
descendants). Thus, .........

> >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.

...... agreed for "artifact", or bias. I would add also, in many cases:
confusion between the notions of "taxon" and "character state" (i.e.
plesiomorphy, inferred "basal" in the common ancestor relatively to a
corresponding apomorphy). A contemporaneous taxon carrying a plesiomorphy
is not "basal" by mere virtue of this feature, otherwise a characteristic
of a character is abusively extended to the taxon as a whole (or a
descendant is implicitly, and abusively, considered a "living fossil"
because it carries a plesiomorphy).

Things get even worse when in the discussion of their phylogenetic results
people tend to use the sister-group relationship, renamed "basal status for
one taxon", to argue about plesio-apomorphy for some characters or adaptive
syndrome... which IMHO boils down to circularity in case (2), and hardly
makes sense in case (1) - both sister groups can carry some plesiomorphies.
Hence this semantic approximation ("basal taxon") may reveal, or favor,
methodological confusion. The term "sister group" is as informative, and
not misleading in my view.

The special case of a "true living fossil", possibly of the common
ancestor, was debated on Taxacom previously.

Hope I did not "blow off" too much this time   ;-)

best,
Pierre




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