[Taxacom] Dispersal... again

[Jason Mate]

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. 
 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. Mayr?s only concern was how barriers interrupted gene flow and lead to speciation. 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. The point is that he never spoke out against vicariance, and since an ocean, mountain or desert are physical barriers, it follows that they can lead to allopatric speciation.

Best
Jason