Why can't beetles grow bigger? Evolutionary constraints limit response to selection for larger larvae in Tribolium castaneum

Although body size is a trait closely linked to fitness, its evolution is often severely limited. This may be due to a suite of evolutionary constraints, including tradeoffs, lack of genetic variance, or physiological regulations. Holometabolous insects provide a unique study system for investigating such constraints, as they can grow during only one life stage, but constraints on larval size often have lifetime effects. These constraints were explored experimentally through selection for larger larval size in the flour beetle Tribolium castaneum. Though a response to selection was observed in the first generation, there was no difference between control and selected lines in the second through fourth generations. This was due, in part, to increased mortality in the lines that were selected for larger larval size. We conclude that there is a genetic tradeoff between size and survivability, which helps explain one evolutionary constraint on body size.