Message 2000-07-0007: Re: Stem-based taxon definitions

Mon, 31 Jul 2000 11:40:53 -0400

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Date: Mon, 31 Jul 2000 11:40:53 -0400
From: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU>
Subject: Re: Stem-based taxon definitions

Actually, I don't see how this solves the issue at all. If you
change "species" to "individual", you still get a polyphyletic group,
simply a polyphyletic group of individuals, not species. Changing the
"taxonomic rank" (as it were) of the problem does not seem to make the
problem go away at all. This just makes the discrete level of organization
at which the problem occurs smaller, and thus less likely to be perceived.

This is irrelevant.  Every group is going to be paraphyletic or
polyphyletic at some level, and no one (except perhaps Baum and Shaw) seems
to have a problem with the idea that species (and thus the clades derived
from them) are paraphyletic or polyphyletic in terms of their component
organisms, cells, or genes.  See the discussion of phyly in my 1999 paper
(in Species: New Interdisciplinary Essays).  Moreover, I didn't advocate
changing the phrasing to "ancestral individual."  The solution is to leave
the type of ancestor unspecified, thus allowing the ancestor to be part of a
species (the part that's more closely related to A than to B; this is the
relevance of the paper by de Queiroz and Donoghue, 1988).

    Also, by my interpretation, a common ancestral individual cannot give
rise to a clade (by your definitions in the citation below), but rather to a
clan or a clone. I would argue (and I have, although not, as yet, very
eloquently on this list) that only a species can give rise to a clade. Thus,
if you wish to talk about the ancestor of a clade, you are talking about a
species. You yourself below mention just replacing "ancestor" with
"ancestral species," at which point we arrive back at my original point.

Yes, I suppose you're right about the last part, replacing "ancestor" with
"ancestral species" in the definition of "clade" won't solve the problem. 
On the other hand, the problem you raised with the wording of stem-based
definitions is based on a wording not used in the PhyloCode.  You stated the
phrasing as "species A and all species sharing a more recent common
ancestral species with A than with B."  This is not the wording used in the
PhyloCode, which states the definition as follows:  "Y and all oganisms [not
species] that share a more recent common ancestor [not ancestral species]
with Y than with W."  The use of "species" and "ancestral species" in the
wording you criticized is the source of the problem (i.e., the reason that
species F wouldn't be included in the clade in your example).  The wording
actually used in the PhyloCode wouldn't result in a reference to a
polyphyletic taxon.  

Kevin de Queiroz
31 July 2000


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