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Date: Fri, 24 Aug 2001 14:48:02 -0700 (PDT)
From: Nathan Wilson <velosa@cinenet.net>
To: "T. Mike Keesey" <tmk@dinosauricon.com>
Cc: PhyloCode mailing list <phylocode@ouvaxa.cats.ohiou.edu>
Subject: Re: Apomorphy-based definitions
On Fri, 24 Aug 2001, T. Mike Keesey wrote: > Perhaps I'm missing something (and this is a difficult thing to > conceptualize), but can't node and stem definitions really be taken down > to the level of the individual? > > Node: individual organisms A and B have a most recent common ancestral > individual C, the latest-occurring individual which is ancestral to both A > and B. The node-based clade consists of C and all of its descendants. (For > sexual organisms, C might be a breeding pair instead of an individual -- > not necessarily, though, in non-monogamous organisms.) The problem is that "the most recent common ancestral individual" is not well defined. Consider the case of two cousins. To start with they have at least the breeding pair you mention. However there is also the potential for siblings to marry siblings. In that case you end up with four most recent common ancestors. You sort of get around this when you talk about species since in theory species are only spawned by a single parent so a strict hierarchy is created. However, in cases where species are created by hybridizing two otherwise distinct species the node-based clade definition becomes undefined. I made this argument a while back on this list. In my opinion the node-based clade definition should be changed and based on the set of most recent common ancestral individuals. While technically this is not a 'clade', any clade that is well defined using the node-based clade definition includes the same individuals with the new definition. In addition, this definition handles hybrids with no problem and as you point out can be used at the sub-species level. I came up with a precise graph theoretic definition which I can dig up for you if you're interested. Enjoy! Nathan Wilson