Re: Apomorphy-based definitions - who needs them?

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Fri, 25 Jan 2002, Ken Kinman wrote:

>      The other word that gets carelessly tossed around a lot is "arbitrary".
>   If apomorphy-based definitions are so arbitrary, better tell the
> PhyloCoders to forbid them.  But I simply have never understood why cladists
> can't see that node-based and stem-based definition are just as arbitrary.

Okay, "arbitrary" is probably the wrong word there -- try "imprecise".

Organisms A and B will definitely have one most recent common ancestral
species. Organism A will definitely have one least recent ancestral
species which is not an ancestor of B. (Unless A belongs to a species
which is ancestral to B, or A and B belong to the same species, of
course.)

But apomorphy X may not be binary -- it may not be clearly present or
absent, but may appear in transitional phases. For that reason,
apomorphy-based clades run the risk of being imprecise.

As someone else pointed out, too, determining whether an organism belongs
to a node- or stem-based clade does not require that a specific part of
the anatomy be known -- it can be determined (albeit not always with
absolute certainty) from whatever is known.

Apomorphy-based definitions may indeed be useful sometimes, but they
present difficulties not inherent in the other two type of definitions,
and for that reason seem to be pretty unpopular. Certainly taking a clade
which has already been given *two* node-based definitions and redefining
it based on an apomorphy seems superfluous.

_____________________________________________________________________________
T. MICHAEL KEESEY
 The Dinosauricon        <http://dinosauricon.com>
  BloodySteak             <http://www.bloodysteak.com>
   personal                <keesey@bigfoot.com> --> <tmk@dinosauricon.com>
    Dinosauricon-related    <dinosaur@dinosauricon.com>
     AOL Instant Messenger   <Ric Blayze>
      ICQ                     <77314901>
       Yahoo! Messenger        <Mighty Odinn>