Message 2001-09-0020: Re: Apomorphy-based definitions

Wed, 29 Aug 2001 18:11:49 -0400 (EDT)

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Date: Wed, 29 Aug 2001 18:11:49 -0400 (EDT)
From: "T. Mike Keesey" <>
To: Nathan Wilson <>
Cc: PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

On Wed, 29 Aug 2001, Nathan Wilson wrote:

> > Right -- although, you *could* use temporality as a "tiebreaker". That
> > doesn't really mesh with the idea of classification by phylogeny, though.
> I considered that, but I don't think it's a good idea.  You can get into
> really weird situations that way if you start considering species like
> redwood trees or some species of fungi which live 1000's of years, but
> potentially mate every year.

Theoretically you would measure it from the moment of fertilization, I
suppose. But, as I said, this does not really work well for a system that
is supposed to be based on phylogeny alone.

> > > A 'Most Recent Common Ancestor' of a set of individuals is any
> > > ancestor of those individuals is an ancestor of all of them and
> > > which has no decendent with that property.
> > Very nice. I think I like that a lot better than using "species" (with all
> > its hundred definitions) as the smallest unit of taxonomy. The only
> > improvement I could think of would be something to account for situations
> > where one or more internal specifiers are ancestral to one or more other
> > internal specifiers.
> I don't think there's any problem with internal specifiers being ancestral
> to each other.  Is there a case your thinking of?  I guess there's an
> issue if you don't consider an individual an ancestor of itself.

That's the issue I am referring to. I would not, for example, consider
myself my own ancestor, and would seek a more explicit way to phrase it.

> I don't know of a better word to express that concept.

I will take a crack at it:

Most Recent Common Ancestor (MRCA):
Given set S of two or more individual organisms, and given individual
organism A, A is a MRCA of S if and only if:

1. A is a member of S, and A is ancestral to all other members of S
2. A is not a member of S, and A is ancestral to all members of S
3. Statements 1 and 2 are false for all descendants of A

(I tried to do it in prose, but it was very awkward.)

So, that takes care of node-based definitions (all MRCAs of the set of
internal specifiers and all of the MRCAs' descendants). Apomorphy-based
definitions, if defined specifically enough, should be even more
straightforward to apply to the individual level (except maybe in rare
instances of parallelophyly?). What about stem-based definitions?

> > Of course, in practice, issues like this are not likely to cause much of a
> > difference, if any. But it does seem like a more solid theoretical ground
> > to rest upon.
> Actually I believe this issue could make a significant difference.  For
> one thing it eliminates the 'what about species?' issue.  It also allows
> for the defintion of sub-species groupings.

Hmm, true. I guess I am used to thinking in terms of paleontology, where
it probably would not make much difference. You're right, for extant
organisms, it could matter more.

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