Effects of disruptive selection X. Selective migration

Summary

Two populations of Drosophila melanogaster were exposed to disruptive selection for sternopleural bristle number. There was 25 per cent, gene flow between the halves of the populations selected for high bristle number and the halves selected for low bristle number. In one population the migrant males were selected in the same direction as the flies in the recipient half (D+) and in the other population the migrants were selected in the opposite direction to the flies in the recipient half (D−). Both populations rapidly showed divergence between their halves and for the first three generations the divergences were very similar. The D+ population eventually attained a greater divergence than the D− population. The D− population maintained a cryptic sex-linked polymorphism. The results are compared to those of Millicent and Thoday (1961) and discussed in relation to the effects of selective migration and disruptive selection. It is concluded that divergence under sufficiently intense disruptive selection is possible even when the migration is much less favourable to divergence than it would be under random mating.