Direct and correlated responses to artificial selection on developmental time and wing length in Drosophila buzzatii.

Developmental time and body size are two positively correlated traits closely related to fitness in many organisms including Drosophila. Previous work suggested that these two traits are involved in a trade-off that may result from a negative genetic correlation between their effects on pre-adult and adult fitness. Here, we examine the evolution of developmental time and body size (indexed by wing length) under artificial selection applied to one or both traits in replicated D. buzzatii populations. Directional changes in both developmental time and wing length indicate the presence of substantial additive genetic variance for both traits. The strongest response to selection for fast development was found in lines selected simultaneously to reduce both developmental time and wing length, probably as an expected consequence of a synergistic effect of indirect selection. When selection was applied in the direction opposite to the putative genetic correlation, that is, large wing length but fast development, no responses were observed for developmental time. Lines selected to reduce both wing length and developmental time diverged slightly faster from the control than lines selected to increase wing length and reduce developmental time. However, wing length did not diverge from the control in lines selected only for fast development. These results suggest a complex genetic basis of the correlation between developmental time and wing length, but are generally consistent with the hypothesis that both traits are related in a trade-off. However, we found that this trade-off may disappear under uncrowded conditions, with fast-developing lines exhibiting a higher pre-adult viability than other lines when tested at high larval density.