The evolution of sexual size dimorphism in the house finch. II. Populationdivergence in relation to local selection

Recent colonization of ecologically distinct areas in North America by the house finch (Carpodacus mexicanus) was accompanied by strong population div ergence in sexual size dimorphism. Here we examined whether this divergence was produced by population differences in local selection pressures acting on each sex. In a long-term study of recently established populations in A labama, Michigan, and Montana, we examined three selection episodes for eac h sex: selection for pairing success, overwinter survival, and within-seaso n fecundity. Populations varied in intensity of these selection episodes, t he contribution of each episode to the net selection, and in the targets of selection. Direction and intensity of selection strongly differed between sexes, and different selection episodes often favored opposite changes in m orphological traits. In each population, current net selection for sexual d imorphism was highly concordant with observed sexual dimorphism-in each pop ulation, selection for dimorphism was the strongest on the most dimorphic t raits. Strong directional selection on sexually dimorphic traits, and simil ar intensities of selection in both sexes, suggest that in each of the rece ntly established populations, both males and females are far from their loc al fitness optimum, and that sexual dimorphism has arisen from adaptive res ponses in both sexes. Population differences in patterns of selection on di morphism, combined with both low levels of ontogenetic integration in herit able sexually dimorphic traits and sexual dimorphism in growth patterns, ma y account for the close correspondence between dimorphism in selection and observed dimorphism in morphology across house finch populations.