Message 2001-09-0015: Re: Apomorphy-based definitions

Tue, 28 Aug 2001 23:08:14 -0700 (PDT)

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Date: Tue, 28 Aug 2001 23:08:14 -0700 (PDT)
From: Nathan Wilson <>
To: PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

On Sat, 25 Aug 2001, T. Mike Keesey wrote:

> On Fri, 24 Aug 2001, Nathan Wilson wrote:
> > 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.
> How?

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. 

A&B give birth to C&D
E&F give birth to G&H
C&G give birth to I
D&H give birth to J

What is the most recent common ancestor of I & J?  You have no way to
decide between A, B, E and F.  If you prefer mating pairs, then you have
no way to decide between A+B and E+F. 

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).  This is the standard meaning of the term in graph
theory.  Again in the case of strict hierarchies of species this is not
an issue since each species has exactly one parent.

> > I came up with a precise graph
> > theoretic definition which I can dig up for you if you're interested.
> I, for one, would be quite interested.

Unfortunately, I just discovered that I can't get at the copy of my old
mail that I kept and I don't have a backup of it.  Sigh.

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.

In the above example, A, B, E & F are all Most Recent Common Ancestors of
the set of individuals, I & J.  C, D, G & H are all eliminated since none
of them are ancestors of both I & J.  Any ancestor of A, B, E or F is
eliminated by the second part of the defintion.



Feedback to <> is welcome!