Archaeopterygid bird from China

[Robert Mesibov]

This has been a truly intriguing discussion, with argument at several
levels. One of those levels is "centre of origin" and what it means. The
underlying evolutionary idea seems to be clear enough: it's where the
lineage under consideration first distinguished itself from its sister
lineage(s).

It's the spatial aspect that's slippery, just as it is for "endemism".
Everyone knows what "endemic" means: distributed in this place and nowhere
else. Clear, yes? Actually, no. A taxon endemic to central Florida is also
endemic to Florida, to the southeastern USA, to North America, to the
Western Hemisphere and to planet Earth. Which level are you interested in?

Try zooming in on the map and it's equally hairy, because every taxon is
disjunct in its distribution at finer spatial scales, and the disjunctions
get more numerous and more annoying the finer the map scale, until at the
limit you have point localities whose limits are the physical boundaries of
the individual.

For practical purposes, many biogeographers delimit the area to which a
taxon is endemic with a line drawn around all known localities. Clear, yes?
Well, no, again, because (a) for most taxa all peripheral localities aren't
known, (b) you get different lines when delmiting by hand, by minimum convex
polygon and by convex alpha-hull, and (c) for many taxa the time-window for
which the localities are known is inappropriate e.g., this plant used to be
far more widely distributed, but we only started recording it after the
farmers had cleared 75% of its range).

All of which raises the questions, why think about "endemism" at all? and
what difference does it make? Now try those questions out on "centre of
origin".
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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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