The change in allelic frequency due to selection can be determined for any type of genetic trait by using the general selection model.

The results of selection The results of selection depend on the relative fitnesses of the genotypes. If we have three genotypes (AA1, A A2, and A2A2) with fitnesses Wu, W12, and W22, we can identify six different types of natural selection (Table 23.6). In type 1 selection, a dominant allele A1 confers a fitness advantage; in this case, the fitnesses of genotypes A1A1 and A1A2 are equal and higher than the fitness of A2A2 (W11 = W12 > W22). Because the heterozygote and the A1A1 homozygote both have copies of the A1 allele and produce more offspring than the A2A2 homozygote does, the frequency of the A1 allele will increase over time, whereas the frequency of the A2 allele will decrease. This form of selection, in which one allele or trait is favored over another, is termed directional selection.

Type 2 selection (Table 23.6) is directional selection against a dominant allele A1 (W11 = W12 < W22). In this case, the A2 allele increases and the A1 allele decreases. Type

3 and type 4 selection also are directional selection, but in these cases there is incomplete dominance and the heterozygote has a fitness that is intermediate between the two homozygotes (Wn < W12 < W22 for type 3; W11 > W12 > W22 for type 4). When A1A1 has the highest fitness (type 3), over time the A1 allele increases and the A2 allele decreases. When A2A2 has the highest fitness (type 4), over time the A2 allele increases and the A1 allele decreases. Eventually, directional selection leads to fixation of the favored allele and elimination of the other allele, as long as no other evolutionary forces act on the population.

Two types of selection (types 5 and 6) are special situations that lead to equilibrium, where there is no further change in allelic frequency. Type 5 selection is referred to as overdominance or heterozygote advantage. Here, the heterozygote has higher fitness than the fitnesses of the two homozygotes (W11 < W12 > W22). With overdominance, both alleles are favored in the heterozygote, and neither allele is eliminated from the population. Initially, the allelic frequencies may change because one homozygote has higher fitness than the other; the direction of change will depend on the relative fitness values of the two homozygotes. The allelic frequencies change with overdominant selection until a stable equilibrium is reached, at which point there is no further change. The allelic frequency at equilibrium (q) depends on the relative fitnesses (usually expressed as selection coefficients) of the two homozygotes:

where sn represents the selection coefficient of the A A1 homozygote and s22 represents the selection coefficient of the A2A2 homozygote.

The last type of selection (type 6) is underdominance, in which the heterozygote has lower fitness than both

Table 23.5 Formulas for calculating change in allelic frequencies with different types

of selection

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