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Date: Fri, 31 Aug 2001 08:08:29 -0500
From: David Baum <dbaum@facstaff.wisc.edu>
To: PhyloCode@ouvaxa.cats.ohiou.edu
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 RECENT COMMON ANCESTOR. 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. 164:407-435. 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 David Baum Dept. Botany, University of Wisconsin 430 Lincoln Drive, Madison WI 53706 dbaum@facstaff.wisc.edu (608)265-5385/-7929 Fax: (608)262-7509