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Date: Fri, 28 Jul 2000 20:09:29 -0500 (CDT)
From: "Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>
To: PhyloCode@ouvaxa.cats.ohiou.edu
Subject: Stem-based taxon definitions
For all, a triviality to consider:
In considering my reservations regarding some of the more common
phrasings of stem-based definitions, it occurred to me that perhaps it would
be best to recommend (in the Code) that definitions be phrased in a common
format which most closely matches the "most recent common ancestor of X and
Y and all of its descendants" format whcih seems most popular for node-based
taxa. I suggest the following, which is similar to some other phrasings:
"The first ancestral species (or ancestor, if you please) of X which
is not ancestral to Y, and all of its descendants."
This avoids the problem of potential polyphyly in the traditional "X
and all taxa sharing a more recent common ancestor with X than with Y"
phrasing, as well as avoids the slight ambiguity I see in formulations
involving the phrase "the most inclusive clade."* This definitional format
makes EXPLICIT reference to an ancestor and its descendants, without relying
on other terms which must be defined separately. It also points out that the
"stem-based clade" is still a clade, not a half of one.
(* I should note that at least one learned member of this list
embrace this ambiguity... I should also note that I owe him some explanation
which I have not had the time to provide... sorry Dr. Wolsan)
It might be more agreeable to use "the oldest" instead of the first,
although I believe that, in either case, the concept of relative time is
served well enough.
Wagner
--------------------------------------------------------------------------------
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