[Taxacom] No more whingeing?
michael.heads at yahoo.com
Thu Feb 2 14:04:12 CST 2012
You wrote that our view 'somehow obviates the need to consider dispersal as having created such
widespread populations in the first place'. Widespread groups may be the result of vicariance or range expansion.The idea that we neglect dispersal (range expansion) is not correct. If there was no range expansion there would be no overlap of any groups; every area on Earth would only have one clade, and it would be endemic to that area. Obviously this is not the case.
You suggest that my new book 'will have some fine pro-vicariance examples and arguments' (thanks!), but I also suggest major episodes of range expansion, to account for overlap. For example, there are about 8 widespread clades of primates in the tropical New World that show extensive overlap through the region, 10 in Africa and 9 in Asia. The overlap seems to have developed after the divison between the New World group and its Old World sister (Early Cretaceous opening of the Atlantic), but before the differentiation within each New World widespread group (e.g. at the Romeral fault zone in the western Andes). The latter differentiation events correlate with events in the Late Cretaceous and so the overlap (range expansion) could be mid-Cretaceous. This would correspond with the last, great sea level maximums (100-200 m above present levels) and would make sense if range expansion took place along the greatly increased coast lines.
The difference between panbiogeography and dispersalist theories is that we use geology, climatic change etc. to explain vicariance and dispersal, whereas dispersal theory uses chance to explain both.
Wellington, New Zealand.
My papers on biogeography: http://tiny.cc/HeadsPubs
My new book 'Molecular Panbiogeography of the Tropics': http://tiny.cc/MolPanbio
From: Kenneth Kinman <kennethkinman at webtv.net>
To: taxacom at mailman.nhm.ku.edu
Sent: Thursday, 2 February 2012 5:35 PM
Subject: [Taxacom] No more whingeing?
I would tend to agree in general with your point below regarding
panbiogeographer's mindset. However, an even more fundamental problem
(in my view) is that their broader arguments often tend to rest on a
false dichotomy of vicariance versus dispersal----as if "never the twain
shall meet." In a majority of cases, a combination of vicariance and
dispersal probably provides the best answer, but the dispersal component
is often disparaged by extreme panbiogeographers (exemplified by John
Grehan in particular).
Even Michael Heads (who seems less extreme in this regard) seems
overly influenced by vicariance and overly critical of dispersalist
hypotheses in a lot of cases. In particular, note his arguments in the
paragraph "Origin of the ancestor". It seems to repeat the excessively
anti-disperalist notion that you can repeatedly shatter a widespread
species (like a mirror) into vicariant subtaxa, and that this somehow
obviates the need to consider dispersal as having created such
widespread populations in the first place. Michael Heads' view on this
point is hard enough to swallow (much less John Grehan's seemingly more
extreme extrapolation of the "shards of a mirror" argument to an absurd
Such arguments are based on a false dichotomy and thus remind me
more of a political campaign than a scientific debate. My view is that
panbiogeographers (some more so than others) tend to be so
anti-disperalist that they become excessively pro-vicariance. In most
cases, I am convinced that the answer is most often somewhere in between
what either extreme dispersalists or extreme panbiogeographers might
I have no doubt that this new book will have some fine
pro-vicariance examples and arguments, but having read what was written
under "Origin of the ancestor", I also expect there will be examples and
arguments that are overly anti-disperalist as well. It is just a matter
of separating the wheat from the chaff, and my experience with
panbiogeographers is that they often tend to generate just as much chaff
as many of the dispersalists they criticize (some more so than others).
Not really. The Chapter 1 excerpt makes it clear (in the
section "Island biogeography with metapopulations and without founders")
that Heads' case rests on including plant dispersal among oceanic
islands separated by hundreds of miles under his category of "ordinary
movement, seen every day". Somehow, in panbiogeographers' minds, events
that happen every few thousand years can't happen over longer distances
every few tens or hundreds of thousand years.
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