Evolution, phylogeny, sexual dimorphism and mating system in the grackles (Quiscalus spp.: Icterinae)

Abstract

Examines the hypothesis that sexual dimorphism in the boat-tailed Quiscalus major and great-tailed grackles Q. mexicanus is caused by the highly polygynous mating system in these species. In relation to an outgroup, both species have increased disproportionately in male tarsus and tail size, creating an increased sexual dimorphism. This has cooccurred with the evolution of their particular mating system, but the variance among species in male tarsus size can be accounted for by drift, and need not be a result of selection for increased size. Variance among species in male tail size was much larger than expected under a null model of drift, indicating directional selection for long tails. Variance in female tail size was not larger than expected by drift, whereas the variance in female tarsus size was in fact lower than expected by drift, indicating stabilizing selection. -from Author