When one fruit fly species meets another, they sometimes interbreed. And yet despite this genetic mixing, distinct species still persist—over 2,000 of them.

Evolutionary biologist Daniel Matute of the University of North Carolina at Chapel Hill is one of many researchers trying to understand how fruit flies, as well as other organisms, maintain such rich biodiversity. “Why don’t all the species in the world fuse into a single gene pool? Why is there not only one species?” he asks.

In their recent paper in Evolution, Matute and colleagues offer one explanation: When fruit flies from different species mate, their offspring are often really bad at searching for food. If hybrids struggle to find food, then they have less energy to devote to reproduction. And if they don’t have energy to reproduce, the mixing of the parent genomes ends with the hybrid, and the parent species remain separate.

Other researchers have shown how some fruit fly hybrids die quickly or can’t reproduce, but no one had studied whether hybrid flies were successful foragers. To test whether hybrids could find food as well as their pure-species counterparts, the researchers first created hybrids by mating 94 different pairs of fruit fly species. They then placed the hybrid offspring from each pair in two vials with males in one and females in the other, and did the same with pure-species flies. Next, they connected each fly vial to two more vials—one empty and another filled with figs. After a few hours, they checked to see which vial the flies ended up in.

Nearly all of the pure-species flies found the fig food. But for the hybrids, the more distantly related their parents, the less likely they were to reach the figs. For hybrids from the most distantly related pairs, only about 20 percent found the food.

The researchers also found that male hybrids had more difficulty finding food than female hybrids. Matute believes this finding may suggest that at least some of the genes that affect foraging behavior are linked to sex chromosomes. His lab is currently testing this idea.

Evolutionary biologist Leonie Moyle of Indiana University, who was not involved in the research, says studies like this one are important. “They contribute to our understanding of how organisms maintain their evolutionary separateness,” she says.

But the results didn’t surprise her. “People think fruit flies are not ecologically diverse,” she says. “That’s absolutely not true. We would expect them to have divergence in foraging traits.” The fact that the hybrids struggled to find food shows that their parents had evolved over time to go about searching for food differently.

University of Oklahoma evolutionary biologist J. P. Masly, also not involved in the study, says the take-home message was that hybrids “don’t have to be either dead or sterile to show some sort of dysfunction that’s going to affect their fitness—even in what may be considered a subtle way.”

Masly and Moyle both believe the study’s findings may also prove true for other fruit fly behaviors, as well as other organisms. Matute agrees. “Food is just one of the behaviors we can look at,” he says. “The importance of the result is that, in all likelihood, other behaviors are also affecting hybrids.”