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Date: Mon, 23 Oct 2000 14:21:12 -0400
From: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU>
To: PhyloCode@ouvaxa.cats.ohiou.edu
Subject: Re: Hybrid specifiers
Nathan stated: "As I understand the concept, a clade is the set of all descendents of a = particular hypothetical species. For this definition to be well defined = you need a solid definition of species. My personal belief is that there = is no such definition." I'm not sure how relevant this is to the issue we've been discussing, but = I would like to know why Nathan thinks there is no good definition of the = term "species" (since I have argued to the contrary in several papers). = =20 I'm also uncertain about whether Nathan ever answered my question about = the reference of the name Alpha. I get the impression from his last = message that he is viewing the clade as stemming from both Y and (Z). If = so, he is indeed "changing what is meant by the term clade." The = definition of the term "clade" is a common ancestor (or ancestral species) = and all of its descendants (not Nathan's "set of all descendants of a = particular hypothetical species," since this definition excludes the = ancestral species from the clade). If this more-or-less standard = definition is adopted, then there are two clades involved in Nathan's = example, one stemming from Y, the other stemming from (Z), and in this = particular case those clades are partially overlapping (have species 3 and = 4 in common). If Nathan wants to apply a name to the combination of these = two clades, that's OK, but we probably should call it something other than = a clade (such as a set of two partially overlapping clades). =20 It also occured to me that under one of the ways of stating a node-based = definition, the application of the name might not be ambiguous even in = this confusing case. That is, if Alpha is defined as the clade stemming = from the most recent common ancestor of 3 and 4 (as opposed to the least = inclusive clade containing 3 and 4), then unless the splits at Y and (Z) = occurred at the same instant (highly unlikely), one of the ancestors (Y or = (Z)) is more recent than the other, and thus the name refers to only one = of the two clades (though in practice it might not be easy to determine = which one). Kevin 20 Oct 2000