Figure 9.12 Simulations of directional selection on a quantitative trait with strictly additive genetic variance under three scenarios (parts (a), (b), and (c)) for the genetic architecture of the trait. From top to bottom in each part, the graphs show the plus allele frequencies (in b and c 10 loci are shown of the total 100 loci that influence the trait), the genotypic and phenotypic mean values for the trait, genotypic and phenotypic trait variance (VP and VG), and the narrow-sense heritability. In (a), there are 10 loci each with equal and large effects on the trait (10% of VG). In (b), 100 loci have equally small effects (each 1% of VG) on the trait. In (c), there are two loci with large effects (20% of VG each) and 98 loci with small effects, as well as recurrent mutation between plus and minus alleles (|i = 0.001 mutations gamete-1 generation-1). In (c), the initial allele frequencies for the two loci of large effect (dashed allele frequency lines) and eight loci of small effect are initially 0.1 while the remaining 90 loci are fixed for the minus allele. A selection plateau is reached in (a) when all of the loci causing genotypic variation in the trait reach fixation (VG and h2 drop to 0). Simulations in (b) and (c) never reach selection plateaus nor exhaust all trait variation because at least some loci that influence trait variation remain segregating. Genetic variation is maintained in (b) because selection in finite populations is not able to fix alleles at all loci with small effects. Even though the two loci with major effects fix rapidly in (c), recurrent mutation at the many loci with small effects maintains some additive genetic variation for continued response to selection. Selection was accomplished by forming the next generation with the 50 individuals with the largest phenotypic values. In all simulations the maximum genotypic value was 10.0, VE = 0.1, and the truncation point for natural selection each generation was the 50th percentile of phenotypic value. QTL, quantitative trait loci.

to reduce genetic variation balanced by mutation introducing novel genetic variation. Depending on the number of loci influencing a trait and the mutation rate, depletion of additive genotypic variance in quantitative traits caused by long-term selection can be counteracted by the addition of new alleles through mutation. Figure 9.12c shows an example of the impact of recurrent mutation on a quantitative

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