Directional & Stabilizing Selection
codominant mode of selection (coefficient s)
- fitness of heterozygote is the mean of the fitness(w) of the two homozygotes
AA = 1; Aa = 1 + s; aa = 1 + 2s
- always increase frequency of one allele at expense of the other
overdominant mode
- heterozygote has highest fitness
AA = 1, Aa = 1 + s; aa = 1 + t
where 0 < t < s
- reach equilibrium where two alleles coexist
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H&C 1997 p. 229