An experimental check on quantitative genetical theory I. Short-term responses to selection

Summary

• 1. The paper describes the response, in the early generations, of a large random-breeding population ofDrosophila melanogaster to selection for abdominal bristle score. The main purpose of the work was an examination of the adequacy of existing theory in describing these phenomena.

• 2. Estimates of the heritability of the character by parent-offspring, full-sib and half-sib and half-sibs correlations were in. good agreement, the mean value being 0.52.

• 3. As a further 35 % of the variance in total score could be ascribed to causes affecting sternites separately, it seems very probable that the genes affecting this character act additively or nearly so. This is in agreement with the results from crosses between lines.

• 4. The responses to selection over seven generations, based either on individual or family score, were in fair agreement with predictions from these estimates with three reservations: (a) There was considerable divergence between replicate lines selected upwards, even when the degree of inbreeding was low. This may be peculiar toDrosophila, which should therefore be used with care in experiments intended for extrapolation to other species. (b) The responses to downward selection were less than expected, as a consequence of a decline in genetic variation within two or three generations. (c) The agreement with prediction was best at high intensities of selection, the response at lower intensities being below expectation.

• 5. Various of these observations are relevant to the problem of the maintenance of such variation in a population. It seems probable that the forces controlling any genetic equilibrium cannot be strong and that in this character mutation may play an important role.