Genome-Wide Patterns of Genetic Variation within and among Alternative Selective Regimes

Environmental heterogeneity has been hypothesized to influence levels of genetic variation but the effect of heterogeneity depends on (i) the form of heterogeneity, (ii) whether ecologically relevant or neutral loci are being considered, and (iii) the genetic basis of ecological adaptation. We surveyed genome-wide SNP diversity in replicate experimental Drosophila melanogaster populations with equal census sizes that evolved for 42 generations under one of four selection regimes: (i) salt-enriched environment (Salt), (ii) cadmium-enriched environment (Cad), (iii) temporally (Temp) or (iv) spatially (Spatial) variable environments. There was significant differentiation between all pairs of treatments but the greatest differentiation occurred between the two homogenous treatments (Cad and Salt). For sites likely under differential ecological selection (and those closely linked to them), the pattern of within-population diversity π followed the expectation from classic antagonistic selection theory: Spatial>Temp>Salt≈Cad. However, neutral diversity unlinked to selected sites followed a different pattern: Spatial>Salt≈Cad>Temp. As implicated by the latter result, measures of F(ST) among replicate populations within treatments are consistent with differences in effective population sizes among selective regimes despite equal census sizes. Though there are clear changes in the rank order of treatments when contrasting selected and neutral sites with respect to π, the rank ordering of treatments with respect to F(ST) appears reasonably consistent between site categories. These results demonstrate that alternative selective regimes affect within- and among-population diversity differently for different site types.