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Date: Tue, 26 Feb 2002 10:23:20 +0100
From: Michel Laurin <laurin@ccr.jussieu.fr>
To: PhyloCode@ouvaxa.cats.ohiou.edu, phyloadvisors@ouvaxa.cats.ohiou.edu
Subject: Re: Crown clade definitions (was: Re: interesting style of definition)
Hi, >Supposed clade _X_ is defined as the clade stemming from the most recent >common ancestor of E and all extant organisms sharing more recent ancestry >with E than with A. _X_ would be understood to be synonymous with Clade(D >+ E). B and C are extinct relatives of _X_, but not members. > >But, then, on a remote island somewhere, organism F is found. It fits in >thusly: > >--+--A > `--+--+--F > | `--B* > `--+--C* > `--+--D > `--E > >All of sudden, the composition is drastically changed, and B and C are >members of _X_. Furthermore, this is not due to any new understanding of >the phylogeny (the old topology is still held to be true). > >This potential problem could be avoided, though, if this wording is >adopted: > >"_X_ is the clade stemming from the last common ancestor of E and all >currently published and extant species sharing more recent ancestry with E >than with A." Here, I suggest a minor change of phrasing if we wanted to adopt such definitions because the expression "all currently published and extant species sharing more recent ancestry..." might be ambiguous; some might think that it includes the species that are published (whether or not they are extant), as well as the species that are extant (without necessarily being published). Thus, it would be better to use the phrasing "all currently published extant species sharing more recent ancestry..." As to the question of how people use the term "related", I agree with Harold that to my knowledge, all authors who will care to formulate phylogenetic definitions use this term in the cladistic sense. I even think that most cladists (even those who don't accept phylogenetic nomenclature) use it in this sense. I also agree with the comment made by Jonathan that there is no unique solution to the question of phenetic distance, and the discovery of new intermediate forms would likely require changing classifications made using this criterion. Sincerely, Michel -- ********************************** Michel Laurin Equipe 'Formations squelettiques' CNRS - UMR 8570 Case 7077 Universit=E9 Paris 7 - Denis Diderot 2, place Jussieu 75251 Paris cedex 05 =46rance Tel. (33) 1 44 27 36 92 =46ax. (33) 1 44 27 56 53 http://phylogeny.arizona.edu/tree/laurin/Laurin_Home_page.html **********************************