There's a basic evolutionary principle regarding speciation: Smaller populations create new species faster than large ones. Yet just the opposite may be happening with dung flies, a new study shows. Female dung flies in large groups tend to shun outside suitors much more than do females in small groups. This aloof behavior results in sexually isolated populations, which could eventually diverge into unique species. The underlying reason appears to be a war between the sexes.
Dung flies don't see eye to eye about sex. While males have a vigorous libido, females rarely want to copulate. In fact, when a male climbs onto a female, she shakes so vigorously that he becomes a blur, says David Hosken, an evolutionary biologist from the Zoological Museum at the University of Zurich in Switzerland. This lack of enthusiasm for sex arises from the consequences of the act: The male's penis damages the female's reproductive tract, perhaps to prevent them from mating with other flies.
To gain further insight into the flies' sex lives, Hosken and colleague Oliver Martin established several laboratory populations with one of three characteristics: high density, with 250 pairs of flies in one container; low density, with 25 pairs of flies in one container; and monogamous, which consisted of 20 fly pairs, each in separate vials. After letting the insects romp freely for 35 generations, the team checked in on their sex lives. The monogamous females, who hadn't experienced competitive males, were quite willing to mate. But female sexual activity had plummeted far below the norm in the other populations, probably because these females had been so inundated with amorous males that they became less amenable to intercourse, the researchers suggest in the 26 June issue of Nature.
What was more surprising, though, was how the females reacted to males from outside groups. Again, monogamous females didn't blush a bit. In contrast, the females in the larger populations, in which their conflicts with eager males had heated up, turned a cold shoulder to males from other dense groups--even more so than they did to males they'd lived with. The distaste was greatest in the most dense population. This suggests that the females had evolved an ability to discriminate males from other populations, perhaps by smell, the authors say. "We are not talking about speciation yet," Hosken cautions, "because there are still matings between [members of the high-density] populations." But in another 10,000 generations, he calculates, the groups might see full reproductive isolation.
"This is a clearly noteworthy result," says Brett Holland, an evolutionary biologist at Lawrence Berkeley National Laboratory in California, "because it helps us to understand why we have so many species."