Question

Evolution by natural selection is initially relatively more rapid when against a common recessive allele than against a common dominant or codominant allele.

(1 mark): Why?

Answer 1

Answer - Natural selection occurs when individuals with certain genotypes are more likely than individuals with other genotypes to survive and reproduce, and thus to pass on their alleles to the next generation.

Explanation - Directional selection leads to increase over time in the frequency of a favored allele. Let us assume three genotypes (AA, Aa and aa) that vary in fitness such that AA individuals produce, on average, more offspring than individuals of the other genotypes. In this case, there is violation of Hardy-Weinberg assumptions, the A allele would become more common in each generation and would eventually become fixed in the population. The rate at which an advantageous allele approaches fixation depends in part on the dominance relationships among alleles at the locus. The initial increase in frequency of a rare, advantageous, dominant allele is more rapid than that of a rare, advantageous, recessive allele because rare alleles are found mostly in heterozygotes. Hence, a new recessive mutation therefore can't be "seen" by natural selection until it reaches a high enough frequency to start appearing in homozygotes. A new dominant mutation, however, is immediately visible to natural selection because its effect on fitness is seen in heterozygotes. Once an advantageous allele has reached a high frequency, deleterious alleles are necessarily rare and thus mostly present in heterozygotes, such that the final approach to fixation is more rapid for an advantageous recessive than for an advantageous dominant allele.Thus, natural selection is not as effective as one might expect it to be at eliminating deleterious recessive alleles from populations.