Biogeography's data, again
mesibov at SOUTHCOM.COM.AU
Thu Apr 18 09:59:07 CDT 2002
My post yesterday in response to Richard Jensen's was written when I was
half asleep. With your indulgence I'll try again to respond properly:
It all comes back to the difference between a classification and a system.
It's possible to classify organisms by putting like with like, and if you
want you can arrange your classification hierarchically by what might be
called arbitrary features. (Legs/no legs. Under 'legs', 4 legs/other number
of legs. Etc.). You classify according to the properties of the objects
being classified. You systematise according to the relationships you see
between the objects. The relationships can be arbitrary ones.
Darwin said they shouldn't be arbitrary. The relationships in a (robust,
scientific, modern, etc) system for organisms should be natural, and to be
natural they should be strictly genealogical. All organisms on Earth,
fossil and extant, are related to one another through descent. The
relationships are there, said the Darwinians, let's use them in our system.
(No, I'm not promoting Phylocode.) The move from classification to
systematisation took a long time in biology. It's still happening, slowly,
in biogeography, and there are signs that it's begun in geology, too. In
both cases the relationships are, again, historical ones.
If you've studied geology you know how obsessed geologists are with
classification and naming things. Some geolomorphologists have begun to
think outside the classification box. Picture a horst-graben complex. The
hills and the basins are historically related because both arose in the
same faulting event. The sediments that accumulate in the basin are
'descended' from that event. Can rock or landscape units be arranged in a
natural, historical system? Yes, as soon as earth scientists get their
heads around that idea. The main program in geology, though, is to put
together a gigantic historical narrative about the Earth's rock units. Some
people think the main program in biology is to come up with a gigantic
historical narrative about Life. If we get there before the geologists,
it's because we've been systematising while they've been classifying.
Biogeography, as I understand the word, began with an idea best expressed
by Wallace in 'Island Life' 120 or so years ago. If you believe in a
special creation, then God made all the animals and plants and put them
just where we find them. There's nothing to explain. If you believe in
evolution, then you need to explain BOTH the diversity of life AND its
locations. Many biologists, sadly, work on the first explaining and never
give a thought to the second. As I said in an earlier post, location tends
to be an afterthought in making phylogenetic hypotheses, even though it's
glaringly obvious that Earth and Life evolve together, and even
microevolutionary mechanisms involve spatial ideas.
The 'units' in biogeography are the inventories of life in particular
places (e.g., the flora and fauna of cedar glades in Tennessee), not
individual species and their individual ranges. Phytogeographers
(especially) have been classifying such 'units' for many years and have
even arranged them in hierarchical systems. Analytical biogeographic
methods, such as cladistic biogeography and panbiogeography, have as their
aim the putting of 'units' into a natural, correctly historical system.
It's a very difficult task, and many non-biogeographers raise a quizzical
eyebrow when they hear the acrimonious debates between biogeographical
'schools'. Please understand, however, what biogeographers are trying to
do. They are NOT primarily trying to write spatial histories to add to the
genetic histories in the dossiers of individual species.
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 3 64371195
More information about the Taxacom