That is if one person from a population of over a million people leaves and goes to another population from the point of view of allele frequencies it will make no difference. Migration that has an impact is either a single large wave or a moderate size that is continuous. The Irish immigration into the US in the early part of this century. The northern African migration into southern Italy. Because migration is usually over a period of years and thus spans more than one generation- generations are overlapping.
Selection can act on alleles, or genotypes.
The effect is to move the population away from equilibrium. The direction depends on how exactly the selection acts. We are still accepting applications for full or part time summer help at the Daniel Turf Center. Duties include mowing, trimming, irrigation, and occassional help with research projects. Work schedule is flexible around summer classes, vacations, etc.