Message 2001-09-0024: Re: defining clades/ancestors

Fri, 31 Aug 2001 14:38:15 -0400 (EDT)

[Previous by date - defining clades/ancestors]
[Next by date - Re: defining clades/ancestors]
[Previous by subject - Re: current usage (blunt talk)]
[Next by subject - Re: defining clades/ancestors]

Date: Fri, 31 Aug 2001 14:38:15 -0400 (EDT)
From: "T. Mike Keesey" <>
To: -PhyloCode Mailing List- <>
Subject: Re: defining clades/ancestors

On Fri, 31 Aug 2001, David Baum wrote:

> Mike Keesey offers a well-thought out logical structure based on
> individuals.

Thanks -- I think Nathan Wilson deserves most of the credit here, though.
(I mostly just summed his formulations up -- added a couple of things,
such as the possibility for multiple internal specifiers for stem-based

> I want to point out a couple of features of his system that
> some might view as problems (I am not sure I do).  They arise because of
> the reticulate nature of individual genealogies.
> 1) A given set, S, of 2 or more organisms can have more than one MOST

That was the concept Nathan Wilson raised in the thread on Apomorphy-Based
Definitions, since it is impossible, in some cases, to select one
individual, or even a single breeding pair, as the MRCA based only on
phylogeny. (You *could* factor in time, but, as we discussed, this is not
relevant to phylogeny.)

The set of MRCAs for any set of 2 or more organisms would all be members
of the same species, as far as I can see, so this doesn't bother me much
more than having a species as the MRCA. I know a clade is supposed to be a
*single* ancestor plus all of its descendants, though ... but, then again,
I have difficulty seeing a species as a discrete, single entity.

> 2) A COMMON ANCESTOR OF S can also be a common ancestor of another group,
> T, and can even be the MRCA of T (where S and T are non-overlapping)

But, if it were a MRCA of T, the node-based clade based on T would
include S, since a node-based clade is all MRCAs plus all of their

If it isn't a MRCA of T -- well, what's so weird about that? Any two
organisms, or two sets of organisms, will likely share a great number of
common ancestors.

> 3) An (ancestrla) organisms X can simultaneously be a member of more than
> one node-based clade.

You mean a single organism can be a MRCA of more than one node-based
clade? True, although one of those clades (the one with the most
MRCAs) would include the other(s) -- I think ... (My head hurts now.)

Thanks for the references--
 Home Page               <>
  The Dinosauricon        <>
   personal                <> --> <>
    Dinosauricon-related    <>
     AOL Instant Messenger   <Ric Blayze>
      ICQ                     <77314901>
       Yahoo! Messenger        <Mighty Odinn>


Feedback to <> is welcome!