Message 2001-09-0009: Re: Apomorphy-based definitions

Fri, 24 Aug 2001 14:48:02 -0700 (PDT)

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Date: Fri, 24 Aug 2001 14:48:02 -0700 (PDT)
From: Nathan Wilson <>
To: "T. Mike Keesey" <>
Cc: PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

On Fri, 24 Aug 2001, T. Mike Keesey wrote:

> 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.)

The problem is that "the most recent common ancestral individual" is not
well defined.  Consider the case of two cousins.  To start with they have
at least the breeding pair you mention.  However there is also the
potential for siblings to marry siblings.  In that case you end up with
four most recent common ancestors.  You sort of get around this when you
talk about species since in theory species are only spawned by a single
parent so a strict hierarchy is created.  However, in cases where species
are created by hybridizing two otherwise distinct species the node-based
clade definition becomes undefined.

I made this argument a while back on this list.  In my opinion the
node-based clade definition should be changed and based on the set of most
recent common ancestral individuals. While technically this is not a
'clade', any clade that is well defined using the node-based clade
definition includes the same individuals with the new definition. In
addition, this definition handles hybrids with no problem and as you point
out can be used at the sub-species level.  I came up with a precise graph
theoretic definition which I can dig up for you if you're interested. 

Nathan Wilson


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