I think you're misunderstanding my point, but maybe I shouldn't have taken vertebrates as an example; we can of course imagine some island where massive selecti

I think you're misunderstanding my point, but maybe I shouldn't have taken vertebrates as an example; we can of course imagine some island where massive selection pressures lead to speciation. I would be interested if you have such an example. However, dog breeds are not species and, given their non-random mating, don't really tell us much about whether 200 years is long in vertebrate evolutionary time. As far as humans go, I will grant that allele frequencies may have changed if you pick an isolated subpopulation of humans (say, everyone in Greenland) but even that seems extremely unlikely. As an aside, there are researchers who are using the caste system in India as a way to explore genetic architectures within a civilization. As these castes have existed for considerably longer than the period we're discussing, meaningful allele frequency changes may arise.