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Noor is credited with helping to solve that riddle, but like most good science, it's not what he initially set out to do. As a University of Chicago graduate student, he shaped his thesis to test a theoretical notion called "reinforcement," in which two species in the process of separating experience some kind of subtle pressure to discriminate between mating partners. It was theoretical because no one had actually seen it operate in species that were not already irreversibly set.
Having identified two species of fruit flies that appear to share a large area of the western United States, Noor spent a month tramping around the mountains of Utah, Arizona, and California, setting out buckets of mashed bananas and yeast—an irresistible banquet for fruit flies—and then netting and jarring the flies he wanted. He hauled them back to the lab in Chicago and watched them having sex, keeping track of who danced with whom and how their kids turned out. "It was really 1930s science," Noor says, referring to the pioneering work by Thomas Hunt Morgan, who discovered much of what we know about chromosomes and heritable traits through a series of very clever experiments with the fruit fly Drosophila melanogaster.
Old school, maybe, but still full of power. Morgan was a geneticist and embryologist and, like many of his colleagues at the time, thought that Darwin could be proved wrong in the details of species formation. In fact, Morgan wasn't so sure that species were even real distinctions. He set out to prove his point by tracking how a mutation is handed down through generations, discovering along the way that chromosomes, those squiggly X-shaped things at the center of every cell, are the carriers of inheritance. Morgan established Drosophila melanogaster as the tiny workhorse of biology, a species we understand better than any other; he spawned a generation of great geneticists; and in the end, he showed that Darwin had been more correct about species than anyone had a right to expect.
Noor chose to study Drosophila pseudoobscura and Drosophila persimilis instead of the obsessively studied melanogaster. Some colleagues tried to warn him away from working on what they felt would be dead-end species. But unlike the lab flies, these two species had a natural history. They shared a geographic area and are related closely enough to produce hybrid offspring. That's what he needed.
Under controlled conditions in the lab, a female pseudoobscura from an area where persimilis flies are not found will mate with a persimilis male, even though he sings, dances, and smells funny. Her hybrid offspring will be a mixed success: the males sterile; the females fertile. By contrast, a pseudoobscura female from any area where these kinds of matings might actually occur is not the least bit interested in allowing a persimilis to mate: If she's from an area where her ancestors had the opportunity to accept this kind of pairing, the cues that the persimilis proffers are a powerful deterrent.
This is reinforcement, the concept Noor was testing for. There is a factor—mating selectivity—that keeps the two species moving apart, even though they are still genetically similar enough to produce viable offspring. The selectivity isn't something the pseudoobscura females think about, but it has been incorporated into their behavior genes by the subtle accumulation of reduced odds of reproductive success. Where the two species might have been able to interbreed, countless generations of trial-and-error matings created untold numbers of sterile male hybrids. And those, in turn, have slightly reduced the odds of success for pseudoobscura females who are willing to cross species boundaries. The ones who tried it had fewer offspring in the aggregate than the females who stuck to their kind, and so the entire population, slowly and inexorably, shifted toward pickiness. "Sterility is the ultimate barrier to blending species," Noor says, adding a booming laugh all out of proportion to his slim, wiry body.
This part of Noor's dissertation work was published in Nature, and immediately set him on the fast track. "This is pretty classical work," says Allen Orr, the Shirley Cox Kearns Professor of biology at the University of Rochester, who also received the Darwin-Wallace Medal in February. "Now it pays for them to stop mating with everyone because it produces sterile hybrids," says Orr, who shared both his undergraduate and graduate mentors with Noor. The behavioral barrier between them enables their genes to adapt on separate trajectories, just as if they were on separate islands.
In addition to conceiving of geologic time, this is perhaps the hardest part of getting one's mind around evolution. There isn't a flash and boom that makes one species into two; it's a subtle, interminable process. "It takes hundreds, thousands, maybe tens of thousands of generations for speciation to happen," says Greg Wray Ph.D. '87, professor of biology and director of the center for evolutionary genomics in the Duke Institute for Genome Sciences & Policy. "So how do you study that?"
So far, a big part of the answer has been fruit flies. They multiply geometrically, producing a new generation every three weeks, and can live by the tens of thousands in a space the size of a suburban walk-in closet. Thanks to Noor and a few others, complete genetic sequences for a dozen different species of flies are now just a mouse click away. Starting with Morgan, biologists have learned how to make genetic mutations at will by exposing flies to toxic gases and radiation, resulting in flies without eyes, flies with legs on their heads, and myriad other perversions. Thousands of dissertations have sacrificed millions of flies. And nobody has ever complained.
In the fly room on the fourth floor of Duke's gleaming new French Family Science Center, a single one-inch-diameter glass tube with a teaspoon of yeasty glop at the bottom can sustain fifty adult flies. The tube is in a carefully labeled rack with fifty more tubes on a shelf with a dozen racks, on a wall with twenty-five shelves. There are something like 50,000 flies over graduate student Audrey Chang's shoulder as she sits down to one of a dozen microscopes to sort flies. Through the doors, a room chilled to 64 degrees holds at least another 50,000 flies that do everything a bit slower, including dying. In a bank of freezers lining a nearby hallway, another 50,000 lie in permanent repose.
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