Artificial selection on larval growth curves in Tribolium: correlated responses and constraints

Body size is often constrained from evolving. Although artificial selection on body size in insects frequently results in a sizable response, these responses usually bear fitness costs. Further, these experiments tend to select only on size at one landmark age, rather than selecting for patterns of growth over the whole larval life stage. To address whether constraints may be caused by larval growth patterns rather than final size, we implemented a function-valued (FV) trait method of selection, in which entire larval growth curves from Tribolium were artificially selected. The selection gradient function used was previously predicted to give the maximal response and was implemented using a novel selection index in the FV framework. Results indicated a significant response after one generation of selection, but no response in subsequent generations. Correlated responses included increased mortality, increased critical weight, and decreased development time (DT). The lack of response in size and development time after the first generation was likely caused by increased mortality suffered in selected lines; we demonstrated that the selection criterion caused both increased body size and increased mortality. We conclude that artificial selection on continuous traits using FV methods is very efficient and that the constraint of body size evolution is likely caused by a suite of trade-offs with other traits.