[Taxacom] another biogeographic note for those interested

[John Grehan]

Star vicariance represents a significant biogeographic pattern and process
that is either generally overlooked, or explained away as dispersal from a
common center of origin. Star vicariance is exemplified by a pattern of
distributions that are largely or entirely allopatric except for a common
center of sympatry, giving the appearance of a multipoint star (depending
on the number of taxa involved).


Dispersal explanations attribute the region of sympatry as a center of
origin from which each of the taxa spread out. The problem with this view
is that it does not explain why each taxon managed to spread so far and
wide and yet keep out of each other’s ‘territory’ other than the region of
sympatry. Vicariance does not impose this quandary, but recognizes that the
allopatry is the result of vicariance of a multitude of taxa that
subsequently underwent local dispersal resulting in sympatry in a
relatively localized area. Sympatry is effectively evidence of dispersal.


 In “Biogeography and Evolution in New Zealand” Heads draws attention to
star vicariance with respect to several taxa, including a very nice example
in the plant genus Astelia which has two main clades around the Indian and
Pacific basins respectively. The Pacific group forms a star pattern with
New Zealand at the center. Even though the overlap of individual ranges
looks complex against present day geography, it is possible to offer
reconstructions of the possible ancestral range of each member group prior
to the dispersal that led to the present day overlap.


Even though the examples are presented for New Zealand, the star pattern
could apply to any region of the globe and as such should be a pattern that
any student of biogeography could recognize. At the very least it would not
be unreasonable for recognition of star vicariance to be a standard exam
question for graduate students (or any students for that matter). I’m have
not seen star vicariance presented in any university biogeography text
book, but admittedly I have not read every one that is out there. However,
it goes without saying that “Biogeography and Evolution in New Zealand”
should be considered as a standard university text book for any
biogeography course anywhere.


John Grehan