W

as derived in Math box 6.1. This equation provides three generalizations that match with our intuitions about natural selection. Allele frequencies do not change when pq = 0 or when there is no genetic variation since one allele or the other has reached loss (p or q = 0). Allele frequencies do not change when all fitness values are identical - meaning there is no natural selection - so the terms inside the square brackets give a value of zero. Lastly, allele frequencies do not change when the fitness differences weighted by an allele frequency (the p(wAA - wAa) and q(wAa - waa) terms) cancel each other out to yield a zero inside the square brackets.

6.2 General results for natural selection on a diallelic locus

• Selection against a recessive phenotype.

• Selection against a dominant phenotype.

• The general effects of dominance.

• Heterozygote disadvantage and advantage.

• The strength of natural selection.

The previous section presented the basic building blocks of a model for natural selection acting through genotype-specific viability on one locus with two alleles. This section will present the general results of natural selection under this very basic model. This task is simpler than it might seem since all the outcomes of the selection model can be represented by five general categories of fitness values for the three genotypes (Table 6.4). Notice that Table 6.4 presents fitness values in terms of selection coefficients rather than relative fitness. Selection coefficients are simply the difference between a relative fitness value and one:

Table 6.4 The general categories of relative fitness values for viability selection at a diallelic locus. The variables s and t are used to represent the decrease in viability of a genotype compared to the maximum fitness of 1 (1 - wxx = s). The degree of dominance of the A allele is represented by h with additive gene action (sometime called codominance) when h = 1/2.

Category Genotype-specific fitness

Table 6.4 The general categories of relative fitness values for viability selection at a diallelic locus. The variables s and t are used to represent the decrease in viability of a genotype compared to the maximum fitness of 1 (1 - wxx = s). The degree of dominance of the A allele is represented by h with additive gene action (sometime called codominance) when h = 1/2.

Category Genotype-specific fitness

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