How are fitness values defined for genotypes in a selection scenario?

• A. Fitness values w_AA, w_Aa, w_aa scale reproductive success relative to the least fit genotype, and genotype frequencies in the next generation depend on p and these w values
• B. Fitness values determine mutation rates
• C. Fitness values determine migration rates
• D. Fitness values are always equal for all genotypes

Answer: (A)

In a selection scenario, fitness values quantify how successful each genotype is at leaving offspring, relative to the least fit genotype. These w values determine how genotype frequencies in the next generation are formed: you weight each genotype’s current frequency by its fitness and then renormalize to sum to one. Concretely, with allele frequency p and q = 1 − p, the next generation’s allele frequency is 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. This means the genotype with higher fitness contributes more to the gene pool, shifting allele frequencies toward the advantageous allele.

Other statements don’t capture this role of fitness in shaping inheritance. Fitness does not set mutation or migration rates, and fitness values need not be equal across genotypes.