STABILIZING SELECTION DETECTED FOR BRISTLE NUMBER IN DROSOPHILA-MELANOGASTER

Stabilizing selection, which favors intermediate phenotypes, is freque ntly invoked as the selective force maintaining a population's status quo. Two main alternative reasons for stabilizing selection on a quant itative trait are possible: (1) intermediate trait values can be favor ed through the causal effect of the trait on fitness (direct stabilizi ng selection); or (2) through a pleiotropic, deleterious side effect o n fitness of mutants affecting the trait (apparent stabilizing selecti on). Up to now, these alternatives have never been experimentally dise ntangled. Here we measure fitness as a function of the number of abdom inal bristles within four Drosophila melanogaster lines, one with high , one with low, and two with intermediate average bristle number. The four were inbred nonsegregating lines, so that apparent selection due to pleiotropy is not possible. Individual fitness significantly increa sed (decreased) with bristles number in the low (high) line. No signif icant fitness-trait association was detected within each intermediate line. These results reveal substantial direct stabilizing selection on the trait.