Multivariate sexual selection of color and morphology in a complex trait: the <i>Drosophila</i> wing

Identification of traits causally linked to fitness allows for direct tests of the adaptive value of traits. In the case of Drosophila and other insects, wing interference patterns-striking structural color variation generated by optical thin-film interference-have been implicated in mate choice and sexual selection. Yet, we expect wing interference patterns may covary with many aspects of wing morphology, such as size and shape. Here, we used a competitive mating assay in Drosophila melanogaster to assess the degree to which sexual selection acts on wing color independently of wing size and shape. We found that 16% of multivariate wing color, and 17% of corresponding estimated wing thickness, can be explained by wing size and shape. Projection pursuit regression identified 3 suites of traits-a linear combination of color variables, wing size, and a linear combination of wing shape variables-as the most important predictors of fitness. Analysis of the corresponding selection gradients revealed a combination of strong directional selection on wing size, combined with multivariate stabilizing selection on wing size and shape, with weak but significant directional selection on wing color. Our results suggest that sexual selection may act on a complex combination of wing color and morphology in flies.