IUBio Biosequences .. Software .. Molbio soft .. Network News .. FTP

Am I a Cladist?

Doug Eernisse DEernisse at fullerton.edu
Tue Feb 9 11:20:41 EST 1999


> Joe Felsenstein wrote:
> > 
> > 
> > A cladist could be:
> > 
> >    1. A person who wants to make classifications, ones that contain only
> >       monophyletic (if you're Mayr and Ashlock "holophyletic") groups.
> > 


Because I am sure Joe would disclaim interest in these aspects of
systematics, I would like to go on record as this type of cladist.

Or perhaps I am at least:

1. A person who prefers classifications that conform, as much as is feasible,
   to the "rule of monophyly" (i.e., all formally recognized taxa should
   be monophyletic).

Those of you "molecular systematists" who might be quick to dismiss such
issues as esoteric concerns of museum curators, consider the "Taxonomy
Browser" of NCBI's sequence databases. I, for one, am extremely
supportive of their efforts to provide a cladistic framework for
their taxonomic classification system. Every formal name is, as it should
be, a hypothesized clade that is the parent taxon of any of its 
descendent species or clades. Imagine the chaos if they had tried
to follow more conventional (i.e., pre-cladist) grade-based 
classification systems. Fortunately, when a lineage is listed, e.g.,

Eukaryota; Metazoa; Chordata; Vertebrata; Mammalia; Eutheria; Primates;
Catarrhini; Hominidae; Homo 

one can assume that each taxon is monophyletic. Even better, NCBI's
system dispenses with traditional Linnean ranks, for the most part.
This has long been advocated by many hard-core cladists.

Think these issues don't matter? I face them daily when writing lectures 
and study guides for courses introducing natural diversity. Texts that
consistently present students with an overview of organisms and that
integrate summary cladograms with a corresponding "cladistic" perspective
are rare, or don't exist at all. Instead, students learn classifications 
featuring paraphyletic divisions (e.g., "5 Kingdoms of Life") and text
discussion all-too-often resorts to grade-based notions of higher taxa
giving rise to other higher taxa, and linear progressive scenarios, etc.
Learning about natural diversity would be so much easier if students
didn't have to study a cladogram figure and then try to link it to a
non-corresponding classification scheme (i.e., with known paraphyletic
taxa at the same rank level as the taxon or taxa they exclude).
I suspect that students would gain a better overview of diversity if the 
text was consistently cladistic in its presentation. Yet, judging
from the slowness of textbook authors to provide such texts, I
must conclude that I am in the minority. A raving cladist, I suppose.

I am reminded of a quote used in an entirely different context
by the founder of quantum mechanics, Max Planck, in 1936:

    An important scientific innovation rarely makes its way by
    gradually winning over and converting its opponents... What
    does happen is that its opponents gradually die out and that
    the growing generation is familiarized with the idea from the
    beginning.

I wish this were true in the case of biologists thinking about
groups of organisms as clades, not grades, but am not optimistic it 
will happen anytime soon.

Of course, cladistics involves much more than just the "names" used in a 
classification. For example, discussion of "primitive" and "advanced" members
within a group is not cladistic. Cladists would instead put emphasis
on the alternative states of attributes, and much of cladistics has
to do with this study of character evolution. I would suspect that
such issues are of concern to many readers of this newsgroup whose
interests might be less theoretical. There are plenty of reasons
to be interested in how "characters" have evolved. And there is plenty
to do even once a particular cladogram is in hand (i.e., apart from the
phylogenetic analysis that resulted in the cladogram). Questions
such as "What was the ancestral state of this character in the
common ancestor of this taxon (i.e., clade)?" are character optimization
issues that depend on whether the question is approached from a
cladistic perspective or not. For example, cladists typically employ
a procedure of outgroup rooting optimization to address this question.
Alternatively, try addressing such a question given an unrooted network, 
for example. 








Now that

-- 
Doug Eernisse
Department of Biological Science
California State University
Fullerton, CA 92834-6850 USA
<http://biology.fullerton.edu/doug-eernisse/>





More information about the Mol-evol mailing list

Send comments to us at biosci-help [At] net.bio.net