The conventional definition of “a species” amongst evolutionary biologists is “a group of organisms whose members interbreed among themselves, but are separated from other groups by genetically-based barriers to gene flow.” Under this view, the origin of a new species is the origin of those reproductive isolating barriers that keep a population distinct from other groups.
Genetic barriers aren’t thought to arise for the purpose of keeping species distinct. Rather, they are usually thought to be evolutionary accidents: geographically isolated populations diverge genetically under natural selection or other evolutionary forces like genetic drift, and that divergence leads to the evolution of genetic barriers (mate discrimination, the sterility of hybrids, ecological differences, etc.) as byproducts of evolutionary change. For example, populations could adapt to different environments (one dry, one wet, for example), leading to them becoming genetically different. When these populations meet each other again, this genetic divergence could result in hybrids that don’t develop properly because the parental genomes are sufficiently diverged that they can’t cooperate in building a single individual.
Under some conditions, however, natural selection might directly favor increasing the genetic barriers between newly-forming species. One of these processes is called “reinforcement,” and it works like this. Suppose two populations have begun to differentiate when they are geographically isolated. They differentiate to the point that there are some problems with the hybrids: hybrids might be partly sterile, for example, or only partly viable. Because these problems might not completely block gene flow (say, only 50% of the hybrids are sterile), the populations aren’t yet regarded as having become completely different species.
But suppose these isolated populations come back into contact with one another. Individuals who mate with members of the “wrong” population produce some maladaptive hybrids. Any individual that could discriminate, and mate only with members of its own population, would leave more copies of its genes than individuals who mate wrongly.
Under these conditions, natural selection could favor the evolution of mate discrimination, promoting those adaptations that allow you to selectively mate only with others of your type. In this way genetic barriers could arise as the direct object of natural selection, and speciation might be completed. This process—the evolution of reproductive barriers to prevent maldaptive hybridization between two populations that attained secondary contact—is called reinforcement.
Reinforcement was once a popular idea in evolution, for it gave natural selection a way to finish off the speciation process. But does it work? One problem is that if two populations come back together again, and can interbreed to some extent, then the evolution of high genetic barriers will be countered by the fact that hybrids keep forming, driving the populations to fuse at the same time selection “wants” them to separate. Which will win? This depends on the balance between selection, hybridization, and also migration of individuals into the “contact zone” from outside. While there is some evidence of reinforcement in nature—seen in patterns of higher mate discrimination between species in areas where their ranges overlap than elsewhere—there hasn’t been much evidence from the lab that natural selection can increase barriers between “incipient” species that are allowed to hybridize.
In a new paper in Current Biology, my hotshot student Daniel Matute modeled the evolution of reinforcement in two species of Drosophila, and found that reproductive barriers could indeed arise—and arise very quickly—when populations were forced to coexist and hybridize, even if migration were allowed from the outside.
He used two of the groups we work on: Drosophila santomea and D. yakuba, two closely related species that coexist on the African island of São Tomé. While these are designated as different species, they can still hybridize in the lab, and half of the hybrids (the females) are fertile, so gene exchange is possible between them. (They also hybridize a bit in the wild where their ranges overlap on the island.) But because there is a penalty associated with hybridization (half of hybrid offspring—all the males—are sterile), natural selection might be able to increase their genetic isolation if the species were forced to coexist.
Daniel produced this coexistence in the laboratory, forcing the species to cohabit in bottles where they could mate either with their own kind or with the other. Further, he allowed different amounts of this hybridization by removing different numbers of the hybrids (easily distinguished by their intermediate pigmentation) from the bottles each generation. Finally, he allowed different amounts of “migration” from the outside by introducing different numbers of flies into the “mixed” bottles. This corresponds to individuals moving into an area of geographic overlap from the outside, a factor that works to overcome reinforcement.
What he found is that, under many conditions, reinforcement did work: the species became more genetically isolated when forced to coexist. And this evolutionary change happened quickly: within five to ten generations (a generation in the lab is about two weeks). Two types of barriers were strengthened: sexual isolation (the species forced to coexist became less willing to mate with each other) and gametic isolation (females who mated with the “wrong” males evolved the ability to get rid of the foreign sperm more quickly, giving them a chance to mate with the “right” males again). Predictably, when hybridization was too strong, or migration from the outside too pervasive, these forms of reinforcement did not evolve. But what is surprising is that under “reasonable” levels of hybridization—meaning conditions likely to be met in wild populations—reinforcement evolved fairly quickly.
The upshot is that these experiments establish reinforcement as a viable process that can “polish off” speciation in the wild. And I should add that in populations of these species on São Tomé, reproductive isolation is indeed higher between populations taken from areas where they coexist than from areas where the species live separately, so perhaps reinforcement in nature explains this.
Curiously, though, the “reinforcement” seen in the wild applies to gametic isolation but not sexual isolation. While sexual isolation (mate discrimination) quickly became stronger in forcibly-coexisting lab populations, it’s no stronger in nature in areas where the species coexist than elsewhere. It’s a mystery to us why both forms of isolation evolve so quickly in the lab but only one is seen in co-occurring populations in nature.
Matute, D. R. 2010. Reinforcement can overcome gene flow during speciation in Drosophila. Curr. Biol. 20:doi:10.1016/j.cub.2010.11.036.