The evolution of sexual dimorphism in the house finch. I. Population divergence in morphological covariance structure

Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change by limiting the amount and form of genetic variation available to natural selection. We studied evolution of morpholo gical variance-covariance structure among seven populations of house finche s (Carpodacus mexicanus) with a known phylogenetic history. We examined the relationship between within- and among-population covariance structure and , in particular, tested the concordance between hierarchical changes in mor phological variance-covariance structure and phylogenetic history of this s pecies. We found that among-population morphological divergence in either m ales or females did not follow the within-population covariance patterns. H ierarchical patterns of similarity in morphological covariance matrices wer e not congruent with a priori defined historical pattern of population dive rgence. Both of these results point to the lack of proportionality in morph ological covariance structure of finch populations, suggesting that random drift alone is unlikely to account for observed divergence. Furthermore, dr ift alone cannot explain the sex differences in within- and among-populatio n covariance patterns or sex-specific patterns of evolution of covariance s tructure. Our results suggest that extensive among-population variation in sexual dimorphism in morphological covariance structure was produced by pop ulation differences in local selection pressures acting on each sex.