[Taxacom] another biogeographic note for those interested

[John Grehan]

One could indeed invoke any kind of dispersal, but again one would have to
invoke great distances and yet only in ways that precluded overlap other
than the single center of sympatry. Vicariance does not impose this kind of
contradiction. And with respect to 'satellites' it's not just one island or
archipelago that is involved in each taxon, but sometimes several. So
again, one requires that dispersal went off in different directions for
each taxon, went a very long way (sometimes halfway across the Pacific) and
to multiple islands, but somehow not the same islands as the other taxa.

John Grehan

On Wed, Nov 30, 2016 at 10:41 PM, Stephen Thorpe <stephen_thorpe at yahoo.co.nz
> wrote:

> 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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