cladism's greatest weakness

[Richard Zander]

----- Original Message -----
From: Barry M. OConnor <bmoc at UMICH.EDU>
To: <TAXACOM at USOBI.ORG>
Sent: Friday, September 17, 1999 12:45 PM
Subject: Re: cladism's greatest weakness


> At 8:22 AM -0700 9/17/1999, Ken Kinman wrote:
> ...  As for the problem of
> >"birdness", it is very similar to the problem we had and still have with
> >"mammalness"....
> >Unfortunately, strict cladists have used Hennig's assumption to mask the
> >fact that they are just substituting one kind of arbitrariness for
another...
>
> That's a problem with "naming and ranking" which will always be arbitrary.
> Monophyletic groups which are well supported by data are not arbitrary
> within the context of that data set;

Well, sure they are. The arbitrariness is in the use of an optimality
criterion alone. The assumption is usually that only one group is discussed
as supported by the data. Data, of course, may support several different
reasonable monophyletic groups - in cladograms almost as short of the
optimal one. Choosing the optimal one against all alternatives is arbitrary.

Two support criteria are commonly used. (1) Bootstrapping simply indicates
that data that refute the shortest tree are well dispersed in the data set.
(2) Bremer support is okay for morphological data sets where branches are
generally short, but in molecular sets it is nonsense since, for example,
Bremer support of say 10 on a branch that is 40 steps long leaves 30 steps
of data supporting an alternative, and I think reasonably supported branch.

My point is that optimality is fine for classifications, since taxa not
genuinely sharing close ancestors are surely well separated in the optimal
cladogram, but the details are dubious.


Richard H. Zander, Curator of Botany
Buffalo Museum of Science
1020 Humboldt Pkwy
Buffalo, NY 14211 USA
email: rzander at sciencebuff.org
voice: 716-895-5200 x 351