Striking a balance, weighting and Cladistics

[Pierre Deleporte]

Date: Fri, 16 Feb 2001 10:52:49 +0100
To: Mark Garland <magarland at NETZERO.NET>
From: Pierre Deleporte <Pierre.Deleporte at univ-rennes1.fr>
Subject: Re: Striking a balance, weighting and Cladistics
In-Reply-To: <003c01c097ce$5ec50040$39360404 at garland>
References: <9188D24F318ED31198E300A0C9D81E04796474 at smtp.dcccmc.org>

on 15/02/01, Mark Garland wrote :

>Okay, here's a silly question:  How "big" can a homology get?  You mentioned
>feathers; how about limbs? Organisms?  Is a human homologous to a chimp?
>
>As the size of the things being homologized does not matter, there should be
>no problem in homologizing organisms. Right?


Right! A human is homologous to a chimp is homologous to a starfish...
If we didn't consider organisms as homologous by common ancestry
(monogenism), we wouldn't engage in reconstructing a unique tree of life in
the first place.
But with the single character "being an organism", the only phylogenetic
tree we can reconstruct is a mere bush.

Now, as soon as we can "split" organisms into little chunks, we can use
these different characters as cues for reconstructing the phylogeny.
Provided that we judge them homologous, and mutually independent.
Thus, as soon as we begin to use these "detailed" characters, we discard
the mega-character "organism" for reasons of non-independence relatively to
its parts. This is not question of "importance", but of plain logics.

And this question still doesn't bear on the problem of possible
differential weighting of independent characters.

Pierre

Pierre Deleporte
CNRS UMR 6552 - Station Biologique de Paimpont
F-35380 Paimpont   FRANCE
Téléphone : 02 99 61 81 66
Télécopie : 02 99 61 81 88