There are three possible outcomes of migration:

• If migration is powerful relative to selection, the rate of gain of A genes by immigration will exceed the rate of loss by selection. The local population will be swamped by immigrants. The frequency of the A gene will increase until it reaches pm .

• If migration is weak relative to selection, the frequency of A will decrease until it is locally eliminated.

• The third possibility is an exact balance between migration and selection. There will be an equilibrium (with local frequency of A = p*) if rate of gain of A by migration = rate of loss of A by selection.

(p_m -p*)m = p*s

p* = p_m (m / (s+m))

In the first case, migration unifies the gene frequencies in both populations, migration is so strong relative to selection that it is as if selection were not operating. In the second and third cases, migration is not strong enough to unify the gene frequencies and we should observe regional differences in the gene frequency; it would be higher in some places than in others. In the third case there is a polymorphism within the local population; A is maintained by migration even though it is locally disadvantageous.

Two points may be made here:

1. A balance of migration and selection is another process to add to the list of processes that can maintain polymorphism.

2. We have seen how migration can be strong enough to unify gene frequencies between subpopulations, or if migration is weaker the gene frequencies of different subpopulations can diverge under selection.