Darwin suggested that females' preference for attractive mates speeds up the formation of new species. But empirical evidence to back up this claim has remained elusive. A new study reveals a rare example of sexual selection at work: Its authors find that female jumping spiders' choosiness for ornamented and dancing mates drives the evolution of new spider species.
The dancing spiders strut their stuff on different stages: A climate change that dried out Southeastern Arizona about 10,000 years ago split the population of jumping spiders (Habronattus pugillis) into several populations that now inhabit several mountain ranges separated by inhospitable desert. Today, male jumping spiders look strikingly different in each mountain range: The ornamentation of their frontmost pair of legs, the white stripe patterns of the front eyes, and even the shape of the head is characteristic for each region. Moreover, the rhythm and the pattern of steps during the courtship dance distinguish each population of males. Female spiders are uniform across the region, suggesting that the differences in the males aren't due to natural selection acting on both sexes.
To investigate whether females' mating preferences could be behind these differences, evolutionary biologists Susan Masta and Wayne Maddison of the University of Arizona in Tucson collected male and female jumping spiders in the Arizona mountains. By comparing the male spiders' appearance and behavior, they could get a handle on how much three populations had changed in the last 10,000 years. Then they figured out that this change was faster than the rate of random mutations in the spider's mitochondrial DNA, suggesting that sexual selection had given them a push, the team reports in the 2 April issue of the Proceedings of the National Academy of Sciences.
Another indication that sexual selection was happening is that the jumping spider populations have begun diverging into distinct species. Masta and Maddison determined this by mating males and virgin females from each mountain region. Male spiders from the Galiuro mountain range needed three times longer to seduce females from the Santa Rita mountains. Even when they succeeded in mating, fewer offspring hatched. These results indicate that spiders from those mountain ranges are in the process of diverging into two separate species.
The study provides compelling evidence that sexual selection can drive the formation of new species, says Kelly Zamudio, an evolutionary biologist at Cornell University. Very few studies have looked at the combination of genetics, appearance, and behavior, Zamudio says: "That's the coolest thing."