Message 2001-09-0008: Re: Apomorphy-based definitions

Fri, 24 Aug 2001 17:10:20 -0400 (EDT)

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Date: Fri, 24 Aug 2001 17:10:20 -0400 (EDT)
From: "T. Mike Keesey" <>
To: "Alastair G. B. Simpson" <>
Cc: David Marjanovic <>, PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

On Fri, 24 Aug 2001, Alastair G. B. Simpson wrote:

> Okay, so there is a devil-in-the-detail of apomorphy-based
> definitions (when does a feather become a feather? and therefore, if I
> have a 'borderline feather', am I in Aves or not?*):  However, there is
> presumably a similar finest-scale fuzziness in determining membership in
> stem- and node- defined taxa too (especially, but not solely, in
> non-clonal lineages), so I don't think this 'problem' with apomorphy-based
> definitions should be invoked as a reason to prefer stem- or node-
> definitions over apomorphy-based ones.

Perhaps I'm missing something (and this is a difficult thing to
conceptualize), but can't node and stem definitions really be taken down
to the level of the individual?

Node: individual organisms A and B have a most recent common ancestral
individual C, the latest-occurring individual which is ancestral to both A
and B. The node-based clade consists of C and all of its descendants. (For
sexual organisms, C might be a breeding pair instead of an individual --
not necessarily, though, in non-monogamous organisms.)

Stem: A has ancestral individuals. Some are also ancestral to B, while
some are not. One of these ancestral individuals, C, is the
earliest-occurring one which is not ancestral to B. The clade is C and all
of its descendants.

Now, obviously, we are never going to have sufficient data to recognize
clades at this extreme granularity (unless I were to anchor a clade on me
and my cousin or something like that). But, unless I'm mistaken, this
works in theory, at least. If this is wrong, why is it wrong?

With apomorphy-based definitions, though, it depends on the level of
detail when specifying the apomorphy. Some might work even at the
individual level, others (like "feather") would certainly not.

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