There's no confusing male and female mallard ducks. The male's emerald head stands in stark contrast to the female's dull brown coloring. Males evolved their sheen in order to attract the opposite sex. At least that's what Biology 101 tells us. But a new study suggests that, in some species, once-colorful females were the ones to change, sometimes dulling down as a protective measure.
Charles Darwin was the first to propose sexual selection, the idea that evolution could drive males and females to look and act very differently to attract the opposite sex. But his contemporary rival, Alfred Russel Wallace, took a different tack: He argued that such differences could arise because females face different risks than males, being burdened with motherhood. He noticed that in certain swallowtail butterflies, females, but not males, had changed their coloration to mimic a toxic species, presumably because females carrying eggs were less able to make quick escapes and needed a different way to avoid being eaten. For the most part, the idea was ignored.
In the past 5 years, however, a few studies of birds and lizards have suggested Wallace might have been on to something. Now, Krushnamegh Kunte, an evolutionary biologist at the University of Texas, Austin, has taken a comprehensive look at swallowtail butterflies--genus Papilio--to explicitly test Wallace's hypothesis. The 200 species of Papilio include species with bright males and females as well as with dull females and bright males. The group also includes some species in which both sexes mimic a toxic species, as well as some in which only the female depends on mimicry for defense.
Kunte mapped the relevant traits--wing color and pattern differences, mimicry, etc.--onto a family tree showing the ancestral relationships among more than 50 Papilio species. In most species, males and females looked the same and were not mimics of any sort. But in the species in which males and females looked different, females, not males, had evolved the new look. And in these species, the females were mimicking a toxic butterfly, supporting the idea that they changed their color for protection. Female Eastern tiger swallowtails, for example, traded bright yellow wings still evident in males and close relatives for a darker wing more like that of the toxic pipevine swallowtail.
"I hope that the current male-centered view of evolutionary change ... will give way to more balanced research" that recognizes that it's not just males who evolve to look different, says Kunte, who reports his findings today in Proceedings of the Royal Society B.
"This study contributes to a fuller understanding of the diversity in sexual traits," says Alex Badyaev, an evolutionary biologist at the University of Arizona, Tucson. It shows "Wallace got it right after all," adds Felix Sperling, an evolutionary biologist at the University of Alberta, Edmonton.