Populations Exam 6 Practice

Which formula correctly expresses p' under selection given p, q, and fitnesses w_AA, w_Aa, w_aa?

p' = [p^2 w_AA + p q w_Aa] / Wbar where Wbar = p^2 w_AA + 2 p q w_Aa + q^2 w_aa. p' = p^2 w_AA / Wbar p' = p + q p' = p w_AA / (p w_AA + q w_aa)

Correct Answer:

p' = [p^2 w_AA + p q w_Aa] / Wbar where Wbar = p^2 w_AA + 2 p q w_Aa + q^2 w_aa.

Explanation:

Selection changes allele frequencies by weighting each genotype by its fitness and then normalizing by the population’s average fitness. Before selection, genotype frequencies are p^2, 2pq, and q^2. After selection, survivors occur in proportions p^2 w_AA, 2pq w_Aa, and q^2 w_aa, and the mean fitness is Wbar = p^2 w_AA + 2pq w_Aa + q^2 w_aa. The frequency of the A allele among survivors is the weighted count of A alleles: p' = [p^2 w_AA + pq w_Aa] / Wbar, since AA contributes two A alleles and Aa contributes one A allele. This gives the correct expression for p' after selection.

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