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

Social selection occurs when traits of interaction partners influence an
individual’s fitness and can alter total selection strength. However, we
have little idea of what factors influence social selection’s strength.
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 measured phenotypic assortment by body
size and tested whether social selection varied with sex-ratio, density,
and interacted with the body size of the focal individual. Social
selection was limited and unaffected by sex ratio or the size of the focal
individual. However, at high densities social selection was negative for
both sexes, consistent with size-based competitive interactions for access
to mates. Phenotypic assortment was always close to zero, indicating
negative social selection at high densities will not impede the evolution
of larger body sizes. Despite its predicted importance, social selection
may only influence evolutionary change in specific contexts, leaving
direct selection to drive evolutionary change.