Differential density-dependent selection on resident and immigrant birds drives the evolutionary dynamics of secondary sexual traits
Evolutionary changes are expected to occur when selection acts on the genetic section of a phenotype. However, the evolutionary trajectories of phenotypes in a population are not only due to natural selection. Alternatively, gene flow can boost or attenuate evolutionary change, although our understanding of the relative roles of these two mechanisms in shaping eco-evolutionary dynamics of phenotypes is still poor. We describe a dramatic increase of a sexual trait, the white forehead patch size (FPS) in pied flycatcher males over a period of 29 years. At the phenotypic level, population density coerces a strong selection on the expression of FPSs, but the strength of selection diverges between residents and immigrants as population density increases. Specifically, the intensity of selection increases with population density only for resident males, favouring larger FPSs. Using quantitative genetic models, we also confirm at the genetic level the same diverging pattern between resident and immigrants as population density increases. Our results show a differential density-dependent selection acting on the expression of sexual traits mediated by the resident status of individuals. The shift of the mean size of a sexual trait can be explained by a true positive selection on large forehead patches of resident males and a negative selection on the same trait for immigrants that operate a density-dependent manner. We discuss the implications that our results may have on the mediating role of gene flow in the eco-evolutionary dynamics of secondary sexual traits and how these results may fit with the so-called lek paradox.