• The fitnesses of the genotypes can be inferred from the rate of change of gene frequency in real cases of natural selection.

• If a mutation is selected against but keeps on arising repeatedly, the mutation settles at a low frequency in the population. It is called selection-mutation balance.

• Selection can maintain a polymorphism when the heterozygote is fitter than the homozygote, when fitnesses of genotypes are negatively frequency-dependent, and when different genotypes are adapted to different niches (this third process is a special case of the second).

• Sickle cell anemia is an example of a polymorphism maintained by heterozygous advantage.

• Subdivided populations have a higher proportion of homozygotes than an equivalent large, fused population.

• Migration, in the absence of selection, rapidly unifies gene frequencies in different subpopulations; and it can maintain an allele that is selected against in a local subpopulation.