Long before DNA was discovered, Darwin proposed that sex is a powerful evolutionary force. As well as needing to stay fit against the perils of the environment, animals must also compete against one another for mates. Now, researchers have turned up the first direct evidence that sexual behavior has shaped the DNA sequence of a gene that confers an advantage in the race for mates.
The consequences of such sexual selection are everywhere: Consider the peacock's showy tail. But documenting the process at the level of genes has been tough. To catch sexual selection in action at that level, a team led by Bruce Lahn, an evolutionary biologist at the University of Chicago, focused on a single gene known to be crucial for reproductive success in male primates. In many primate species, females maintain multiple sexual relationships, putting males under pressure to ensure that their sperm reaches the egg first. A gene expressed by males and thought to be important for sperm's success is SEMG2, which codes for a protein in semen that links up into chains to cause coagulation. Inside the female, a different protein breaks the chains apart, releasing the sperm to go egg-hunting. Tailoring semen viscosity to better deliver sperm to the inner sanctum is thought to be one strategy for males to get an edge over one another in the race to fertilize eggs.
Lahn's team examined the number of mutations that had accumulated in SEMG2 among a range of primates, from great apes like chimpanzees and gorillas, to monkeys such as the colobus and macaque. If sexual selection really does drive evolution at the genetic level, Lahn hypothesized, then changes in the sequence of SEMG2 should accumulate faster in primate species with more promiscuous females.
Sure enough, SEMG2 evolved faster in promiscuous species than in prudish ones, the team reports in the December issue of Nature Genetics. Chimpanzees, with females engaging an average of eight males per ovulation period, have the fastest rate of SEMG2 evolution, whereas the monogamous gibbons have the slowest. Human SEMG2 evolution fell in the middle of the spectrum, but Lahn cautions that our long cultural evolution makes direct comparison difficult. He says this is the first demonstration that different degrees of sexual selection influence the DNA sequence of a reproductive gene.
Willie Swanson, an evolutionary biologist at the University of Washington, Seattle, finds the results convincing but says the story isn't complete. Now the researchers need to find the "driving force" that makes one version of the semen protein more advantageous than another, he says.