This paper is concerned with three related aspects of the behaviour of populations under artificial selection for increased scutellar bristle number: (i) the pattern of response on the probit scale; (ii) the homeostatic behaviour of the selection lines on relaxation of artificial selection; and (iii) correlated responses in generation interval, reproductive capacity and competitive ability. The study was designed so that linkage would be a comparatively unimportant factor in promoting correlated responses to selection, and the effects of genetic sampling from generation to generation were also reduced to a low level.Progress from the base mean of 4·05 bristles in females to a level of almost 8 bristles has been shown to involve two distinct phases with realized heritabilities of 0·34 and 0·10 respectively, the zone of transition corresponding closely to the position of the 6/7 threshold on the underlying scale. In addition to an apparent average reduction of about 25% in the additive genetic standard deviation in phase II by comparison with phase I, the loss in response due to the opposition of natural selection has been shown to reach a maximum near the zone of separation of the two phases.The pattern of behaviour of the populations under artificial and natural selection has suggested the presence in the base population of genes of large effect on both bristle number and reproductive fitness. There is also evidence of additional genetic variation in bristle number which is effectively neutral with respect to fitness. Continued selection for increased scutellar bristle number in large populations has been shown to reduce mean competitive ability by more than 80%.
Heat knockdown resistance and chill‐coma recovery as correlated responses to selection on mating success at high temperature in
Drosophila buzzatii
