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Date: Wed, 29 Aug 2001 17:41:15 -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 Wed, 29 Aug 2001, T. Mike Keesey wrote:
> I will take a crack at it:
>
> Most Recent Common Ancestor (MRCA):
> Given set S of two or more individual organisms, and given individual
> organism A, A is a MRCA of S if and only if:
>
> 1. A is a member of S, and A is ancestral to all other members of S
> or
> 2. A is not a member of S, and A is ancestral to all members of S
> and
> 3. Statements 1 and 2 are false for all descendants of A
Looks good to me.
> What about stem-based definitions?
For stem-based clades, there isn't the dependency on a concept like Most
Recent Common Ancestor, so it's pretty straight forward.
Given the included individual, I, and the set of excluded individuals, E,
any individual, M, is a member of the stem-based 'clade' including I but
excluding E, s(I,E), if and only if all three of the following are true:
1) M is not a member of E
2) M is not an ancestor of any member of E
3) M is an ancestor of I or M has an ancestor, J, that is an ancestor of I
and 1&2 are true of J.
One of the interesting cases that gets included with this definition is:
-
/ + E
/ \ /
I M
You could exclude this case by adding the condition:
4) M is not a descendent of any member of E
However, the set would then be empty in the case where I is a descendent
of any member of E, so I prefer to leave that condition out. Once again
in the strict hierarchy case it's not an issue.
Enjoy!
-Nathan