Message 2001-09-0023: defining clades/ancestors

Fri, 31 Aug 2001 08:08:29 -0500

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Date: Fri, 31 Aug 2001 08:08:29 -0500
From: David Baum <>
Subject: defining clades/ancestors

Mike Keesey offers a well-thought out logical structure based on
individuals.  I want to point out a couple of features of his system that
some might view as problems (I am not sure I do).  They arise because of
the reticulate nature of individual genealogies.

1) A given set, S, of 2 or more organisms can have more than one MOST

2) A COMMON ANCESTOR OF S can also be a common ancestor of another group,
T, and can even be the MRCA of T (where S and T are non-overlapping)

3) An (ancestrla) organisms X can simultaneously be a member of more than
one node-based clade.

If you want to see a very thorough logical analysis of how reticulating
lineages can be chopped-up I recommend:

Kornet, D. J. 1993.  Permanent splits as speciation events: A formal
reconstruction of the internodal species concept.  J. Theor. Biol.

Another article that relates is one of mine:

Baum, D. A. and K. L. Shaw. 1995.  Genealogical perspectives on the species
problem.  Pages 289-303 in Experimental and molecular approaches to plant
biosystematics (P. C. Hoch and A. G. Stephenson, eds.), Missouri Botanical
Garden [I can send reprints].

This article develops the concept of genealogical exclusivity which
provides an alternative to monophyly for defining clades.  Thus, a
node-based clade would comprise:  "The most-inclusive, exclusive group
containing internal specifiers x, y....n."

I do not know if this approach would allay some of the concerns expressed
lately on this list, but it is a way to define clades without getting
bogged-down in the question of what kind of ancestor they descend from.
The big "cost" of this approach is that it is time-limited - only
individuals at the tips of the tree of life are members of clades.

I hope this helps the debate.


David Baum
Dept. Botany, University of Wisconsin
430 Lincoln Drive, Madison WI 53706
Fax: (608)262-7509


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