lonely Dan Janzen

Richard Jensen rjensen at SAINTMARYS.EDU
Fri Mar 26 08:38:55 CST 2004

Don Colless makes an excellent point about classifications.  The debates about stability and predictivity have never, despite claims to the contrary, been laid to rest.  The debates often centered on phenetic versus cladistic classifications and there are numerous examples in the literature where these approaches both agree and disagree.

When it comes to predictivity, one must ask "What is it I wish to predict?"  As most of us have recognized all along, if phylogeny is the ultimate goal, then clearly cladistic classifications are likely to be better predictors; phenetic classifications were never (except perhaps in some very early discussions; Colless early on noted
circumstances under which phenetic classifications might be expected to provide good approximations of phylogeny) intended to reflect phylogeny.  But the argument that cladistic classifications are better predictors of organisms sharing particular attributes has to be tempered by asking what attributes are being sought?  Cladistic
classifications often have very high levels of homoplasy and if the attribute you are interested in is one that shows a lot of homoplasy, then a cladistic classification will be no better than (and perhaps worse than) a phenetic classification when used to predict which taxa have which attributes.

As has been emphasized by a number of participants in these discussions, we have to ask what it is we want the classification to do and recognize that, at this point in the development of our science, there is no single system of classification that is best for doing the various things that both the classifiers and the users of the
classifications what done.



Don.Colless at CSIRO.AU wrote:

> Richard Pyle wrote:
> ---------------------------------------------------------------------------
> > many - and I dare say some day nearly all - users also make very
> > extensive use of the marvelous inferencial (=predictive) power of the
> > phylogenetic components of your classification systems.    You are
> > all aware that the more phylogenetically-based your system(s), the
> > more inferentially powerful it is for us users.
> ----------------------------------------------------------------------------
> that the Linnaean system was suboptimal for communicating inferred
> phylogenies)
> ----------------------------------------------------------------------------
>  the use of nomenclature as a tool for the strict
> representation of phylogenies inferred from the latest lump of data.
> -----------------------------------------------------------------------------
> He also offered several wise and temperate comments. But the above quotes illustrate a common non-sequitur that is achieving the status of received truth. To wit:
> A. Classifications should be predictive.
> B. Cladistic classifications offer the best available predictions of phylogeny.
> C. Ergo, classifications should be cladistic.
> The non-sequitur lies in just whether it is just phylogeny that we wish to extract from a classification. There are, after all, many other ways of providing that information, which on its own is about as interesting as the ?begat?s in Genesis, ch. 10-11.
> I would dare to suggest that a worker interested in an insect?s ability, say, to transmit disease could not care less if the relevant taxon is mono- or paraphyletic. And that that goes for our classifications at large. Also note that classical taxa are generally mono- or para-; we do frown on convergence, and have our ways of minimising it.
> Moreover, the much maligned Linnean system is an excellent tool for marking where useful or important information is located. Generic, family, etc., ranks are NOT allocated by an algorithm, as so much beloved by cladists. They result (optimally) from a consensus of conscientious, trained, taxonomic opinion.
> Don Colless,
> Div of Entomology, CSIRO,
> GPO Box 1700,
> Canberra. 2601.
> Email: don.colless at csiro.au
> Tuz li munz est miens envirun

Richard J. Jensen              | tel: 574-284-4674
Department of Biology      | fax: 574-284-4716
Saint Mary's College         | e-mail: rjensen at saintmarys.edu
Notre Dame, IN 46556    | http://www.saintmarys.edu/~rjensen

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