Effects of disruptive selection XI: gene flow and divergence

Disruptive selection lines using sternopleural chaeta number in Drosophila melanogaster were run using weaker selection than hitherto. In addition, the design allowed some assessment of the degree to which partial isolation arising from distance, or other factors, increases the effects of disruptive selection. In all lines, even those with selection only eliminating 20 per cent of the flies in each generation, variance and the mean difference between extreme cultures increased, and in all lines isolation between the high and low subpopulations reached the maximum allowed by the experimental design. Reducing migration between the subpopulations from 50 per cent to 30 per cent of the parents in each generation had little effect, but further reduction made disruptive selection more effective. At the same time quite low migration reduced the effects of selection markedly.