[Taxacom] biogeography and tectonic commonalities

[John Grehan]

In recent posts Ken and Jason have outlined their preference for chance
dispersal explanations for allopatry. Ken in particular has emphasized how
a center of origin and chance dispersal explanation may be made for
individual taxa. I think this raises an important point for biogeography.
Chance dispersal explanations can indeed be imagined for each individual
case, but when there is a correspondence of different taxa with different
ecologies having the same biogeographic patterns and breaks that also show
boundaries with tectonic features, such chance explanations thenhave to
account for the phylogenetic and tectonic correlations. In other words,
chance events over a multitude of taxa are supposed to result, but chance,
a shared tectonic correspondence as well as allopatry.

One may certainly decide to put their faith in historically unrelated
'chance' events giving the same result time and again, or one might
consider that the shared tectonic correspondence may indeed be historically
informative. This was the explicit point made by Croizat and it was a view
considered to be an anathema then, but as one looks through the
biogeographic literature to individual studies of taxa, one does find
various researchers coming to Croizat's point of view regarding the
importance of tectonics as noted below in an excerpt from Heads (2012).

"The break between India and Burma

A break here, in the Bangladesh/Bay of Bengal region, was cited above in
Scandentia, lorisids, Trachypithecus/Semnopithecus, and gibbons. For
Scandentia, Olsen et al. (2005) wrote that the circumstances leading to the
disjunction remain a “mystery.” The break occurs in groups that show a wide
range of ecologies, and so, as with other breaks in Burma, it may be
related in some way to the plate boundary in the region (the Indian plate
is subducting beneath Burma and sundaland; Stork et al., 20080 (Fig. 5-14).
In tree, for example, Aglaia cucullata (Meliaceae) ranges from the Ganges
delta east to the Mekong delta and New Guinea, in mangrove, tidal
estuaries, Nypa palm swamp, and riverine forest , from 0 to 20 m (Mabberley
et al., 1995), and there is no ecological reason for its absence from the
large areas of mangrove forest in India west of the Ganges delta. In other
primates, the same limit occurs in groups of Macaca such as M.
fascicularis. Macaca ranges between Morocco and Sulawesi/Japan, and other
groups with similar Tethys oriented distribution also show breaks here. The
freshwater crab family Potamidae ranges between Morocco and Borneo and
comprises two clades, one western and one eastern, with the major division
in Bangladesh/Burma where the two groups overlap. Shih et al. (2009)
attributed the break to orogeny in Burma cause by India-Asia collision, and
whatever the details, it probably did involve some kind of activity on the
plate boundary."

John Grehan