[Taxacom] Dispersal... again

[Michael Heads]

Hi Jason,
?
I've replied to?your points?below.
?
Michael

Wellington, New Zealand.


My papers on biogeography are at: http://tiny.cc/RiUE0


________________________________

From: Jason Mate <jfmate at hotmail.com>
To: Taxacom <taxacom at mailman.nhm.ku.edu>
Sent: Wednesday, 26 October 2011 11:29 AM
Subject: Re: [Taxacom] Dispersal... again







Michael,

If you reduce speciation to the point of the first mutation from which, in retrospect, the speciation process "started" (i.e. like Hennig?s beautifully simplistic graphic) it happened in one individual at a certain point in space and time. So unless you are suggesting that vicariance has divided this individual in half, it must have successfully multiplied and dispersed to form a population sufficiently numerous to divide succesfully. Only afterwards can a barrier divide your original pop?lation and lead to allopatric/vicariant speciation. 

MH: Yes, that's the center of origin/dispersal model of speciation, as proposed by?Mayr and Hennig. But in the vicariance model, speciation doesn't work like that - there is no point center of origin. It is not caused by a single, freak, random mutation followed by gene flow of this through the population. Instead, a widespread ancestor 'breaks down' into subordinate groups by repeated, non-random mutation.??(For?new, post-modern synthesis views on genomic evolution, see:?Lynch,
M. 2007. The frailty of adaptive hypotheses for the origins of organismal
complexity. Proceedings of the National
Academy of Science USA, 104, S8597-S8604. Lynch, M. 2007. The evolution of
genetic networks by non-adaptive processes. Nature
Reviews Genetics, 8, 803?813. Stoltzfus, A. 2006. Mutationism and the dual causation of
evolutionary change. Evolution and
Development, 8, 304-317). 

?You wrote: '...vicariance (allopatry in Mayr?s parlance...'.? For Mayr, allopatry was not caused by vicariance, but by chance dispersal.Michael you are reading Mayr in the narrowest possible sense and being quite unfair to the man. 

MH: See the 1965 quote from Mayr I gave earlier.

Mayr?s only concern was how barriers interrupted gene flow and lead to speciation. 

MH: In dispersal theory it's not the formation of the barrier or any change in it that interrupts gene flow - the barrier was there before the differentiation. One founding individual crosses the barrier, by chance,?and then, by chance, no others do. The factor interrupting the gene flow is chance, not a change in an extrinsic environmental factor.???

He didn?t care about the nature of the barrier:
"Not all population discontinuities in nature originate by the colonization of islands. Sometimes they arise by the contraction of a species range and the separation of a previously continuous area into separate ranges." (Mayr 1976)

Barriers could be anything that physically reduced or eliminated gene flow. Of course, he may have been just thinking of mountains, rivers, deserts.... but since continental drift was still in its infancy in the 60?s then he is hardly to blame. 

MH: He wrote that in 1976. 

The point is that he never spoke out against vicariance

MH: See the 1965 quote, where he clearly indicates that the majority of groups attained their distribution by spread (i.e. not vicariance). He would also interpret the others (the very local endemics) as due to peripatry (dispersal). Also see any of his studies of actual groups (e.g. the bird faunas of North America, the pantepui, the Pacific islands etc., all interpreted in terms of the different centers of origin). For?his critique of vicariance biogeography in The Auk, 1982. 


, and since an ocean, mountain or desert are physical barriers, it follows that they can lead to allopatric speciation.

MH: Yes, it's obvious now, but he chose not to follow that line.



Best
Jason
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