Loss / Fixation of an allele with "Underdominant" selection

    In Underdominant selection, either homozygote has greater fitness than the heterozygote A₁A₂. [As with "overdominant", "underdominant" is a misnomer, because genotypes are not dominant to other genotypes]. Calculation of the equilibrium frequency  is the same as for overdominant selection* (where the heterozygote is favored), but the equilibrium is unstable. Only if the allele frequency remains precisely at will both alleles be maintained, which can occur only in an ideal, infinite population. In any finite population, the initial p = f(A₁) will either start away or drift from , and p will go rapidly to loss or fixation, depending on the direction and magnitude of the initial deviation.

*Recall that  = (1 - W₂₂) / (2 - ( W₁₁ + W₂₂)) provided  W₁₁ is normalized to W₁₁ = 1.