wrote:

> Tim Tyler  wrote:
> 
> >Yes it does.  The species is divided geographically - and then diverges
> >due to drift and/or differing selection pressures - and over time that
> >causes mating incompatibilities with members of the ancestral population -
> >and results in a new species.
> 
> Most scientist belief that. But why should a population split by a
> geographical barrier while slowly adapting become unable to mate? Do
> the Europeans show mating incompatibilities with the American Indian
> or the Australian Aborigine? Man has done a thorough job breading dogs
> apart but most still happily mate and produce offspring.
> 
> Mats Liljedahl

Incompatibilities accrue over time - random changes tend to make each
population diverge if the population size is small enough. Moreover,
many incompatibilities arise as byproducts of selection to local
conditions - for example, in sparse environments there may be selection
pressure for shorter gestation; if this is bred with the older
developmental cycle genes, then the result may be inviable.

But a very small amount of gene flow can maintain compatibilities - I
read once as little as one percent er annum can keep two populations
homogenous in that respect. I can't attest to that, but it is pretty
small. Others will be able to fill that lacuna in.
-- 
John Wilkins
john_SPAM{at}wilkins.id.au   http://www.wilkins.id.au
"Men mark it when they hit, but do not mark it when they miss" 
                                               - Francis Bacon
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