Figure 1

The effect of selection on the mean and variance components in the presence of epistasis. Directional selection, β=0.2 (solid line) is contrasted with the neutral case (dashed line); shaded areas indicate ±1 s.d. The left panel shows the change in mean from its initial value and the right panel shows the additive variance, V_A, and the additive × additive variance, V_{A A} (lower pair of curves). Only the genic components of variance are shown; random linkage disequilibria make no appreciable difference on average. There are M=3000 loci, and N=100 haploid individuals. Alleles are given equal main effects but random sign . Sparse pairwise epistasis is represented by choosing a fraction 1/M of pairwise interactions, ω_{ι j}, from a normal distribution with s.d. . The trait is now defined as z=δ.γ+δ.ω.δ^T, where δ=±(1/2). Initial allele frequencies are drawn from a U-shaped β-distribution, mean p̄=0.2 and variance 0.2 p̄q̄. Individuals are produced by Wright–Fisher sampling from parents chosen with probability proportional to W=e^{β z}. For each example, three sets of allelic and epistatic effects are drawn and for each of those, three populations are evolved; this gives 9 replicates in all.