Goalposts

[Robert Mesibov]

I find it helpful in reading TAXACOM threads to keep the "goalposts" in
view. At one end of the playing field is the classification goalpost. When
shooting for this one, you aim to sort your organisms into a classification
which is useful for identifying and for unambiguous labeling. A useful
classification may or may not have predictive value about organisms not yet
fitted into it, but in any case you are taking a risk when making such
predictions, because as we all know, Nature is full of surprises. I'm sure
no one on this list would rely on a classification as a substitute for
fieldwork or careful biological study.

At the other end of the field is the phylogeny goalpost. Here the aim is to
imagine the evolutionary history of life. The hypothetical histories we come
up with can never be tested against the truth, because that truth is
unknowable. The best we can do is make sure a phylogenetic hypothesis isn't
in serious conflict with morphological, molecular, paleontological or
biogeographical data. If it is, try again.

There often seem to be brawls in the middle of the field between players who
aren't 100% sure which goalpost they should be kicking towards. At the
methodological level, especially in molecular studies, this confusion is
entirely understandable, because we are still largely in the dark about how
genomes have evolved in general and in specific cases, and because models of
molecular evolution are often untestable.

At the philosophical level, there are many people in that mid-field struggle
who are pursuing the Darwinian dream of a classification which also
accurately reflects evolutionary history. IMHO, shooting for both goals at
once is worthwhile at higher taxonomic levels but a Quixotic fantasy at
higher "magnifications" of biodiversity. Down at the level of species,
populations and genetic diversity there's vastly more information to deal
with. Some of that information may be phylogenetic noise when you're looking
for a classificatory signal, and some of that classificatory signal may be
noise when you're looking for a phylogeny.

(As an aside, there's an analogy here with biogeography. Continent-scale or
ocean-basin-scale patterns are easy to understand. Fine-scale biogeography,
as attempted these days in a one-eyed molecular fashion in phylogeography,
is intellectually terrifying, and may be impossible to understand, ever.
There are just too many possibilities.)

This philosophical ramble was inspired by the current Grehan threads. I
think John is defending methodology appropriate for studying  higher-level
taxa, and others are defending methodology appropriate to sequence analysis.
It isn't clear to me that either side knows which goalpost they're facing.
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Dr Robert Mesibov
Honorary Research Associate, Queen Victoria Museum and Art Gallery
and School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 6437 1195

Tasmanian Multipedes
http://www.qvmag.tas.gov.au/zoology/multipedes/mulintro.html
Spatial data basics for Tasmania
http://www.geog.utas.edu.au/censis/locations/index.html
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