Fig. 1

A graphic summary of hypotheses regarding how functional roles and selection pressures may influence trait variation. Thin arrows represent direction of selection and thick arrows represent direction of evolution for each column. Flat-headed lines indicate the relative spread of trait variation. The orange line in (a) represents trait optimum. The blue-shaded area in (b) represents the direction in which selection shifts the trait distribution. Stabilising selection on naturally or sexually selected traits (a) acting on one or both sexes may decrease trait variation while directional selection on naturally or sexually selected traits (b) may increase trait variation. In the panels for sexually selected traits, the effect of selection on trait variation has been shown separately to highlight how variation between the sexes may respond differentially. Additionally, natural selection can also affect sexes differently, potentially resulting in distinct patterns of variation between males and females, similar to what is shown for sexually selected traits. However, even in such cases, the variation in naturally selected traits is generally expected to be less pronounced than in sexually selected traits or traits not under selection