Message 2001-09-0017: Re: Apomorphy-based definitions

Wed, 29 Aug 2001 15:28:40 -0400 (EDT)

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Date: Wed, 29 Aug 2001 15:28:40 -0400 (EDT)
From: "T. Mike Keesey" <>
To: Nathan Wilson <>
Cc: PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

On Tue, 28 Aug 2001, Nathan Wilson wrote:

> You didn't go quite far enough to capture cousins.  You only went to
> siblings.  Unfortunately my example can't be drawn in email as easily as
> yours, but here's the idea in text.

[example snipped]

THAT's what I was looking for! I'd had a feeling there'd be some ambiguous
situation, but couldn't think of one. (Surprised to see that this one
involves no inbreeding.)

> As a side note, I am using the term 'most recent' to mean not in a strict
> temporal sense, but in the sense of closest relative.  In otherwords, a
> parent is always 'more recent' than a grandparent even if the grandparent
> is younger than the parent (obviously the parent that is not the child
> of that grandparent).

Right -- although, you *could* use temporality as a "tiebreaker". That
doesn't really mesh with the idea of classification by phylogeny, though.

> Any way here's a definition that I believe works:
> 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.

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.

So, has this definition of MRCA been published anywhere?
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