Farewell to Species - reticulation

B. J. Tindall bti at DSMZ.DE
Tue Feb 15 14:41:22 CST 2000

>Richard Jensen wrote:
>a phenetic classification is often to be preferred when it comes
>to stability and information content.
Thomas DiBenedetto <TDibenedetto at DCCMC.ORG>
>It is my understanding that phenetics never found widespread favor precisely
>because it was not a stable system, and that it failed to provide
>information in its most useful context. Although the true phylogeny is not
>something we can ever be certain to have discovered, it does stand as an
>objective goal which can act as a stabilizing factor to the various studies
>which attempt to approximate it. Using "overall similarity" as the criterion
>for a classification seems to be obviously less stable, since the criterion
>itself entails a subjective judgement.

I fail to see why a phenetic system is any more unstable than a cladistic
system. I would agree that different ways of handling the data sets may
give different arrangements, but this applĂ­es to both methods. Secondly all
of what we do has a subjective nature, not just our appreciation of
"overall similarity". If you take a particular set of characters for a
cladistic analysis, then it is your subjective judgement which ones you take!

>Furthermore, I think that there is a widespread sense in the biological
>community that the classification around which we build our naming system
>should be reflective of phylogeny. The hierarchy of genealogical
>relationships is, to me, obviously the context in which one must anchor ones
>understanding of all of the characteristics of taxa. Evolution should be the
>central organizing prinicple for the study of biological phenomena, and it
>is beyond my comprehension why anyone would advocate for a non-phylogenetic
>classification to be the basis of our naming system.
>It is interesting that, beyond the issues of stability and information
>content (both of which I think are very weak arguments for phenetics) the
>best that can be said of a phenetic classification is that it sometimes
>yields good historical groupings, despite the fact that it doesnt try to.
>I would prefer to try for good historical groups, and to get them as often
>as possible.

I thought about this some time ago and it would seem to me to be pretty
logical that organisms with a recent common ancestor are also fairly
similar (share a high degree of overall similarity), or as Sneath said
recently there is a causal relationship between evolution and overall
similarity. Of course the Archaea-Eukaryote-Eubacteria split was initially
based on phenetic evaluation. The problem with similarity values is that
one cannot easily recognise those characters which are not homologous, but
are due to things like gene transfer, convergent evolution, parallelism
etc. which distort the true evolutionary history. I agree that an
evolutionary basis is important in taxonomy, but I have a nasty feeling
that some of the questions raised 50 years ago may still be unanswered and
perhaps cannot be answered. Perhaps we should recognise that we are doing
the best we can in the light of the information we have.
Brian Tindall
>Tom DiBenedetto
>tdib at dccmc.org

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