Species need not sit around waiting for natural selection to shape them. According to a new study, a creature's personality can also be an important evolutionary driving force--one that may give the species some control over its own destiny.
Famed evolutionary theorist Ernst Mayr championed the idea that behavior could guide evolution; he reasoned that an animal's behavior determines how it interacts with its environment. Duke University evolutionary ecologist Renee Duckworth saw a good opportunity to test Mayr's theory while studying the western bluebird, Sialia mexicana, in Montana. The species has a bit of a split personality: Some members are consistently more aggressive than others. Could this behavior difference eventually lead to different physical characteristics?
The first step was figuring out whether aggressive birds choose different habitats than nonaggressive ones. In a series of field trials, Duckworth found that more aggressive birds favored open meadows, which are rife with potential nesting sites. Less aggressive birds, who didn’t compete as well for these prime locations, wound up settling in closed forest areas with fewer nesting sites.
The habitats favor different foraging strategies: Birds must hover and hop to get food in open areas, while forest-dwelling birds can simply stay in trees to find tasty insects and berries. Accordingly, Duckworth found that birds in open areas with longer tails and legs--traits that enabled them to forage more effectively--had more offspring survive to independence than did those with shorter tails and legs. Neither trait seemed to matter for birds nesting in forest areas.
Over time, such a difference in selection pressure could split a species, says Duckworth, who reported her findings online 11 April in Proceedings of the Royal Society B. That scenario is unlikely to happen with the bluebirds, however. Duckworth says that forest fires prevent either of the habitats from being stable enough to keep the two personality types genetically separated for long--a requirement for new species to arise.
The idea that a behavioral trait can influence a species' evolution is "very intuitive," says evolutionary physiologist Raymond Huey of the University of Washington in Seattle. He believes the study will provide evolutionary researchers with a new way of looking at the forces that drive evolution. "There's a debate going back to Darwin about what drives evolutionary change," Huey says. The study, he believes, is "one that will resonate."