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W11

> W12 < W22

Underdominance

Unstable equilibrium

Note: Wu, W12, and W22 represent the fitnesses of genotypes A1 A1, A1 A2, and A2A2, respectively.

Note: Wu, W12, and W22 represent the fitnesses of genotypes A1 A1, A1 A2, and A2A2, respectively.

homozygotes (Wn > W12 < W22). Underdominance leads to an unstable equilibrium; here allelic frequencies will not change as long as they are at equilibrium but, if they are disturbed from the equilibrium point by some other evolutionary force, they will move away from equilibrium until one allele eventually becomes fixed.

lower rate than that of dominant alleles. Recessive alleles increase at the lowest rate, because only the homozygote is favored by selection.

The rate at which selection changes allelic frequencies also depends on the allelic frequency itself. If an allele (A2) is lethal and recessive, Wn = W12 = 1, whereas W22 = 0. The

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