Message 2002-02-0010: RE: interesting style of definition

Fri, 15 Feb 2002 11:34:31 -0600

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Date: Fri, 15 Feb 2002 11:34:31 -0600
From: "Bryant, Harold MAH" <>
To: Mieczyslaw Wolsan <>,
Subject: RE: interesting style of definition

Issues concerning the potential ambiguity of the meaning of the terms
"stemming" and "ancestor" can be avoided by the following wording:

"The least inclusive clade that includes A and all extant organisms that are
more closely related to A than to B."

I have not been a fan of the use of "extant" in phylogenetic definitions
because of the potential effect (no matter how slight) of future extinction
on the clade referred to by the definition.  Jonathan Wagner's recent
suggestion is one solution to that problem that could be useful in
stem-modified node-based definitions.   However, if one wants the definition
to always refer to a crown clade, then one would want to give priority to
stability in that aspect of the definition, and if later extinction changes
the inclusiveness of the clade so be it.

Harold Bryant
Royal Saskatchewan Museum
2340 Albert Street
Regina, Saskatchewan  S4P 3V7
FAX 306-787-2645 

		-----Original Message-----
		From:	Mieczyslaw Wolsan []
		Sent:	Friday, February 15, 2002 5:11 AM
		Subject:	Re: interesting style of definition

		Sorry, there was a problem with my previous posting. Now
it's ok (I hope).

		1) "extant"

		This problem has already been addressed comprehensively in
the literature 
		(see, e.g., Bryant 1996, Syst. Biol. 45: 174-189).

		2) "stemming"

		If clade A stems from B, is it necessary for clade A to
contain B? I feel 
		it is not. And this is my point. Of course, a clade consists
of an ancestor 
		and all its descendants. The problem I find with the
discussed definition 
		is that it is not clear (for me) whether the specified "last
		ancestor" is contained in the clade. If I correctly
recognize the meaning 
		of the word "stemming" , the phrasing "clade A stemming from
B" referes to 
		two different clades: one containing B, and the other
without B.

		3) "ancestor"

		There has been much discussion related to the problem, also
on the list 
		(recently by Kevin de Queiroz, as far as I remember). The
draft PhyloCode 
		defines the term "clade" as "a group of species comprising a
		ancestor and all of its descendants". (I would say "a group
of lineages" 
		and would replace the word "comprising" by "composed of"
because I feel the 
		word "comprise" meant to contain or include, but not to
constitute or to 
		compose.) I think that the majority of systematists agree
with the draft 
		PhyloCode's definition. However, I do not think that
according to this 
		definition it is necessary for the ancestor to be a species.
I think many 
		systematists agree that it is not necessary for a clade to
begin with a 
		complete species. If one wishes to have a species as an
ancestor, this 
		should be explicitly stated in the definition.


		Mieczyslaw Wolsan
		Professor and Chair
		Department of Vertebrate Paleontology
		Institute of Paleobiology
		Polish Academy of Sciences
		Twarda 51/55
		00-818 Warszawa, Poland
		Phone: +48-22-697-8793
		Fax: +48-22-620-6225


Feedback to <> is welcome!