Losses of sexual dichromatism involve rapid changes in female plumage colors to match males in New World blackbirds

Differences in coloration between the sexes (sexual dichromatism) can
increase or decrease in a species through evolutionary changes in either
or both sexes diverging or converging in their colors. Few previous
studies, however, have examined the relative rates of such changes,
particularly when dichromatism is lost. Using reflectance data from 37
species of the New World blackbird family (Icteridae), we compared
evolutionary rates of plumage color change in males and females when
dichromatism was either increasing (colors diverging) or decreasing
(colors converging). Increases in dichromatism involved divergent changes
in both sexes at approximately equal rates. Decreases in dichromatism, in
contrast, involved changes in females to match male plumage colors that
were significantly more rapid than any changes in males. Such dramatic
changes in females show how selection can differ between the sexes.
Moreover, these evolutionary patterns support the idea that losses of
dimorphism involve genetic mechanisms that are already largely present in
both sexes, whereas increases in dimorphism tend to involve the appearance
of novel sex-specific traits, which evolve more slowly. Our results have
broad implications for how sexual dimorphisms evolve.