As long as certain conditions are met, allele frequencies should not change over time
Hardy-Weinberg law
Cost of sex, lowered fitness, the burden of generating genotypes of lowered fitness
Recombinational load
Individuals mate with others who are like themselves phenotypically for selected traits
Positive assortative mating
Individuals only select mates who are phenotypically different from themselves for selective traits
Negative assortative mating