Biogeographic units and classification

[John Grehan]

Regarding Hovenkamp's comments on the differences between
biogeography and biological systematics, the problem that
remains is the question of establishing natural classifications in
biogeography.
Regardless of the differences between biogeographic and biological
processes, biogeographers are involved in the construction of
biogeographic units and natural classifications.

The way I look at the application of systematics to biogeography
involves the question of natural units and natural classifications. In
biological systematics the status of "natural" lies in a shared history
(monpophyly) indicated by the presence of uniquely shared characters
(synapomorphy). If biogeographic units and classifications are not to use
these
principles, what are the alternatives for establishing that a
biogeographic unit is natural as opposed to arbitrary?

 In panbiogeography the biogeographic units are defined by
baselines -spatial characters uniquely shared by those
distributions whose tracks cross the baseline.

Whether or
not one might agree with this method, the criterion for natural
unit is at least explicit and  testable. In contrast,
Wallacean areas are not suported by any such criteria (Gary Nelson
suggests that even Wallace did not consider them natural). If
they are simply arbitrary constructs they would seem not to merit
any scientific standing.

In the case of panbiogeography at least, the principle of
synapomorphy appears to have been established as a matter
of methodological principle, rather than simply adapting problems
to methods developed in another field.

I would be very interested to know of criteria outside those of
monophyly and synapomorphy whereby biogeographic units and
areas might be considered natural. As far as I am able to discern
at present, all geograpic areas, including areas of endemism, biomes,
ecological areas, Wallacean areas, are all artificial constructions.
That this is a minority and eccentric point of view I would agree.

John Grehan