The Evolution of Phenotypic Life-History Trade-Offs: An Experimental Study Using Drosophila melanogaster

Experimental manipulation of an organism's environment can reveal trade-offs among life-history and physiological traits such as fecundity and energy reserves. Such phenotypic trade-offs have been suggested to reflect evolutionary constraints. We test this idea by asking whether the selection responses of laboratory populations of Drosophila melanogaster can be predicted from a trade-off between fecundity and starvation resistance revealed by manipulating levels of dietary yeast. Fecundity and starvation resistance vary inversely over levels of dietary yeast: at high yeast levels, females have a high fecundity and low starvation resistance; at low yeast levels, females have a low fecundity and high starvation resistance. We examine the role of this trade-off in two independent sets of populations that have been selected for increased starvation resistance. We find that the joint selection responses of mean fecundity and starvation resistance cannot be predicted from the phenotypic trade-off in either the ancestral or control populations. This result implies that at least some of the genetic variation and covariation on which selection acted originated in physiological pathways not involved in the phenotypic trade-off. We also find that selection has altered the slope of the phenotypic trade-off such that, across yeast levels, the reproductive cost of a unit gain in starvation resistance is less in the starvation-resistant populations than in the controls.