Sexual dimorphism in relation to current selection in the house finch

Sexual dimorphism is thought to have evolved in response to selection press ures that differ between males and females. Our aim in this study was to de termine the role of current net selection in shaping and maintaining contem porary sexual dimorphism in a recently established population of the house finch (Carpodacus mexicanus) in Montana. We found strong differences betwee n sexes in direction of selection on sexually dimorphic traits, significant heritabilities of these traits, and a close congruence between current sel ection and observed sexual dimorphism in Montana house finches. Strong dire ctional selection on sexually dimorphic traits and similar intensities of s election in each sex suggested that sexual dimorphism arises from adaptive responses in males and females, with both sexes being far from their local fitness optimum. This pattern is expected when a recently established popul ation experiences continuous immigration from ecologically distinct areas o f a species range or as a result of widely fluctuating selection pressures, as found in our study. Strong and sexually dimorphic selection pressures o n heritable morphological traits, in combination with low phenotypic and ge netic covariation among these traits during growth, may have accounted for close congruence between current selection and observed sexual dimorphism i n the house inch. This conclusion is consistent with the profound adaptive population divergence in sexual dimorphism that accompanied very successful colonization of most of the North America by the house finch over the last 50 years.