The topic of natural selection is of extreme interest. Unfortunately, a lot of unsubstantiated claims are made about the potential of natural selection and the actual effect of natural selection. Few people seem willing or able to take the subject by the horns and ferret out the real from wishful thinking. We could discuss this at length if anyone so desires, but such a discussion cannot be based upon assumptions, projections, etc based on what we learned in grammar school.  

Here is some background info

Natural selection will tend to adapt a population to local environmental conditions but immigrants from other populations will introduce genes adapted to other conditions. Gene flow between populations may prevent them from evolving into different species. But as emphasized by Sewall Wright in particular, gene flow can also be a creative force in evolution. The movement of individuals and even entire populations may spread superior genes and combinations of genes throughout a species once they become common in one location.  

Recently theories have shown that frequent extinctions and recolonizations of local populations can also be an important source of gene flow. Even if there is no exchange of individuals between established populations, there will be little differentiation of local populations due to genetic drift if the average time that a population persists in one area is less than the time it takes for genetic drift to fix neutral alleles, which is of the same order of magnitude as the effective population size.  

Viral and bacterial diseases sometimes become less virulent, and this is often attributed to group selection: genotypes of the parasite that prolong the survival of the host will be favored by group selection because greater host longevity will provide greater opportunities for dispersal of those genotypes. This view is widespread among parasitologists, but there is currently no evidence that group selection due to differential survival of hosts is in fact responsible for the loss of virulence of parasitic diseases.  

The evolutionary importance of gene flow depends on its effects averaged over a large number of generations. The time scale of change associated with a particular evolutionary force is the time needed for that force to cause a substantial change in gene frequency. Roughly speaking, the time scale associated with natural selection is the inverse of the difference in relative fitnesses. For example, a 1% difference in the fitesses of different genotypes would cause a significant change in gene frequency in approximately 100 generations.  

Studies of Drosophila pseudoobscura show the same difference between direct and indirect estimates of levels of gene flow. This species is found throughout the western United States and most of Mexico and Central America and was studied extensively by Dobzhansky, in collaboration with Wright, with the intention of estimating dispersal rates and local population sizes and testing the shifting balance theory. Dispersal distances in D. pseudoobscura depend on habitat. Adults disperse an average of approximately 500 m during their lifetimes in what appears to be optimal conditions in forests and several kilometers in deserts, which are much poorer habitats. Yet there is little differentiation of any of the North American populations, which is difficult to account for with observed dispersal distances.  

Slatkin, M. (1987). Gene flow and the geographic structure of natural. Science, 3576198(787), 236. 

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