Tim Tyler  wrote:

> John Wilkins  wrote or quoted:
> >  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.
> > 
> > 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.
> 
> Theis is what is conventionally known at the
> "One-Migrant-per-Generation Rule".
> 
> In theory, one indivdial per generation migrating between two sexual 
> groups is enough to prevent their gene pools diverging. 
> 
> The figure is independent of the population size.
> 
> In practice, lack of random assortment within the groups results in
> the number of migrants that are possible while retaining group-level
> characteristics to be larger - but the point is clear nontheless - a 
> "suprisingly low" between-group migration rate is needed before two 
> almost-isolated groups will diverge very far from one another.

Thanks you. Of course, this depends on the two populations being
compatible to begin with - hybrid genes can fail to introgress if they
are less fit or viable. I really must work through this at a textbook
level sometime...
-- 
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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