Correlated responses to selection on body size in Drosophila melanogaster

Correlated responses to artificial selection on body size in Drosophila mel anogaster were investigated, to determine how the changes in size were prod uced during development. Selection for increased thorax length was associat ed with an increase in larval development time, an extended growth period, no change in growth rate, and an increased critical larval weight for pupar iation. Selection for reduced thorax length was associated with reduced gro wth rate, no change in duration of larval development and a reduced critica l larval weight for pupariation. In both lines selected for thorax length a nd lines selected for wing area, total body size changed in the same direct ion as the artificially selected trait. In large selection lines of both ty pes, the increase in size was achieved almost entirely by an increase in ce ll number, while in the small lines the decrease in size was achieved predo minantly by reduced cell size, and also by a reduction in cell number. The implications of the results for evolutionary-genetic change in body size in nature are discussed.