Biogeography's data

Robert Mesibov mesibov at SOUTHCOM.COM.AU
Wed Apr 17 20:25:15 CDT 2002

Curtis Clark:

 > The fact that species X also has feature 3 tells us nothing about its
 > evolutionary history. In both cases, we need context--comparison with the
 > distribution of character states in other taxa, the distribution of taxa in
 > other areas.

Richard Jensen:

 >I disagree. Knowing that a plant has vascular tissue (feature 3 in the
specimen I
 >am examining) provides evidence of relationship to other plants also having
 >vascular tissue - that's a trace of its evolutionary history.

I think if Richard reflects for a moment, he'll see that he doesn't
disagree. Character states (like, "has vascular tissue") are basic data for
hypothesising evolutionary relationships between organisms. Locations
(like, "cedar glades in middle Tennessee") are basic data for hypothesising
spatial relationships between organisms. An isolated fact (character state,
location) isn't very helpful in working out relationships of any kind. You
need context, both in systematics and in biogeography.

I do disagree, however, with Richard's implied suggestion that the
biogeographic question to be answered about his organism in the cedar glade
is: "newly evolved there, left as a relict, or arisen through vicariance?"
What that's doing is attempting to assign a kind of biogeographical
character state, adding another bit of info to the description of the
organism. IMHO, that isn't what most biogeographers aim for in their
research. The goal, instead, is to work out likely spatial histories of
areas and their taxa.

It may some day be possible to say something detailed and well-supported
about the biogeographical history of middle Tennessee cedar glades, i.e.
their flora and fauna. That 'something' may not include information about
the spatial history of the particular endemic Richard mentions. This gap
doesn't undermine or invalidate the broader spatial-history hypothesis,
just as incomplete character-state information doesn't undermine or
invalidate an otherwise well-supported phylogeny.

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

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