[Taxacom] another biogeographic note for those interested
Stephen Thorpe
stephen_thorpe at yahoo.co.nz
Wed Nov 30 21:41:32 CST 2016
John,
Why can't dispersal go in both directions? If the "center of sympatry" is a big area, whereas the satellites are small areas, then dispersal back from a satellite to the center is more likely than dispersal in the other direction of more than one species to any one small satellite. So we could start with one species in the center which disperses to each of the satellites independently, speciates on each satellite, and then each new species disperses back to the center creating sympatry at the center. If this is combined with tectonic events causing the distance between center and satellites to increase over time, then it will become less likely that any species which disperses back to the center will be able to subsequently disperse to another satellite, maintaining allopatry between satellites.
Stephen
--------------------------------------------
On Thu, 1/12/16, John Grehan <calabar.john at gmail.com> wrote:
Subject: [Taxacom] another biogeographic note for those interested
To: "taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Thursday, 1 December, 2016, 4:22 PM
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
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