Social selection is density dependent but makes little contribution to total selection in New Zealand giraffe weevils

Abstract

Social selection occurs when traits of interaction partners influence an individual's fitness and can fundamentally alter total selection strength. Unlike for direct selection, however, we have little idea of what factors influence the strength of social selection. Further, social selection only contributes to overall selection when there is phenotypic assortment, but simultaneous estimates of social selection and phenotypic assortment are rare. Here we estimated social selection on body size in a wild population of New Zealand giraffe weevils (Lasiorhynchus barbicornis). We did this in a range of contexts and measured phenotypic assortment for both sexes. Social selection was mostly absent and not affected by sex ratio or the body size of the focal individual. However, at high densities selection was negative for both sexes, consistent with competitive interactions based on size for access to mates. Phenotypic assortment was also density dependent, flipping from positive at low densities to negative at high densities. However, it was always close to zero, indicating negative social selection at high densities will not greatly impede the evolution of larger body sizes. Despite its predicted importance, social selection may only influence evolutionary change in specific contexts, leaving direct selection as the dominant driver of evolutionary change.