In Hardy-Weinberg contexts under selection, how are allele frequencies updated after fitness differences among genotypes?

In Hardy-Weinberg contexts under selection, how are allele frequencies updated after fitness differences among genotypes?

• A. Allele frequencies are unaffected by selection and stay at p and q.
• B. Allele frequencies are reversed so p becomes q and q becomes p.
• C. p and q are adjusted by the relative fitness of each genotype and renormalized.
• D. Only homozygous frequencies change; heterozygote remains constant.

Answer: (C)

When selection acts, different genotypes contribute different numbers of offspring, so allele frequencies shift because those with higher fitness leave more copies of their alleles. After selection, the allele frequency p for one allele is obtained by weighting each genotype’s contribution by its fitness and then renormalizing by the mean fitness. If the genotype fitnesses are w11 for AA, w12 for Aa, and w22 for aa, and p is the frequency of A with q = 1 − p, the updated frequency of A is p' = [p^2 w11 + p q w12] / w̄, where w̄ = p^2 w11 + 2 p q w12 + q^2 w22 is the mean fitness. The corresponding q' is [q^2 w22 + p q w12] / w̄. This shows that allele frequencies move in proportion to the relative fitness of each genotype and then sum back to 1 through renormalization.

When selection acts, different genotypes contribute different numbers of offspring, so allele frequencies shift because those with higher fitness leave more copies of their alleles.

After selection, the allele frequency p for one allele is obtained by weighting each genotype’s contribution by its fitness and then renormalizing by the mean fitness. If the genotype fitnesses are w11 for AA, w12 for Aa, and w22 for aa, and p is the frequency of A with q = 1 − p, the updated frequency of A is

p' = [p^2 w11 + p q w12] / w̄,

where w̄ = p^2 w11 + 2 p q w12 + q^2 w22 is the mean fitness. The corresponding q' is [q^2 w22 + p q w12] / w̄.

This shows that allele frequencies move in proportion to the relative fitness of each genotype and then sum back to 1 through renormalization.