Message 2000-07-0005: Re: Stem-based taxon definitions

Mon, 31 Jul 2000 00:29:45 -0500 (CDT)

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Date: Mon, 31 Jul 2000 00:29:45 -0500 (CDT)
From: "Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>
To: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU>
Subject: Re: Stem-based taxon definitions

At 01:44 PM 7/30/00 -0400, Dr. de Queiroz wrote:
>>Ah yes, now I remember this posting, though I was too busy to respond at
>the time.
        I understand. Technically, I am too busy to have this discussion
now, but I don't want to fink out.

>The problem here stems from reference to a common ancestral
>species (for more on this problem see de Queiroz and Donoghue, 1988,
>Cladistics 6:61-75, especially Fig. 2),
        I shall have to see if I have a copy of this article. Thanks for the

>so all that has to be done is to
>delete that term from the definition (i.e., "A and everything sharing a more
>recent common ancestor with A than with B").  Note that this is how the
>definition is stated (i.e., without reference to an ancestral species) in
>the example given in the PhyloCode (Note 9.4.1).  No change is necessary.  
        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.
        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.

>The problem is that other people prefer not to spell it out that way, and
>some of those people were contributors to the PhyloCode effort.  I don't
>really see this as a big problem in that most people use clade for an
>ancestral species (as opposed to a breeding pair) and its descendants.
        But surely you'd see a problem in a nomenclatural system in which
the taxa mean different things to different people? I thought this was
exactly the sort of thing the phylogenetic systematics revolution set out to
end? That was my point, anyway...

>Indeed, I have more or less formalized this distinction in my 1999 paper (in
>Species: New Interdisciplinary Essays: see Note 3 regarding the distinction
>between clades, clans, and clones).
        And I am very grateful for the distinction. It is one which I
believe belongs in Phylogenetic Taxonomy.

>Moreover, the term "clade" is defined
>in the glossary of the PhyloCode, though not in a way that entirely solves
>the problem.  The simplest solution would be to modify that definition,
>replacing "ancestor" with "ancestral species"--a wording that I believe I
>advocated but on which I compromised to accommodate the views of the
>"anti-species" contributors.  
        I wonder if a compromise truly can be reached, or for that matter
whether it is worth compromising the integrity of the "speciose" system for
those who do not accept species. This is not to marginalize those whose
worldview does not accomodate species, I respect their dedication to
principle and iconoclasm. Anyway, I suppose the debate has already been held...

        Part of my comfort with the format I advocate is also that it
emphasizes that the "stem-based clade" is still an ancestor and all of its
descendants. It is not "everything closer to X than to Y". It is not
necessarily never equivalent to a node-based clade (as stated by Padian,
Hutchinson and Holtz in a recent review of theropod dinosaur taxonomy), nor
is a node "equal" to its "component" stems (as suggested by Sereno in his
recent "rationale" for PT). A stem-based definition (or an
autapomorphy-based definition, or a true crown-clade definition, etc.) is
simply a different way of specifying which ancestor the named clade starts with.
        Not that the "most inclusive clade" format does not do this to some
level, it is just perhaps not as forceful. I could as easily use that format
and just deal with the ambiguity. However, the common node-based and
autapomorphy-based definitional formats include the phrasing "ancestor...
and all of its descendants." Call it aesthetics, but I like the continuity.
Oh well, a clade by any other name, I suppose...

     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi


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