A new University of Michigan study of interbreeding between two species of howler monkeys in Mexico is yielding insights into the forces that drive the evolution of new species.
How do new species emerge in nature? One common but overly simplified version of the story goes like this: A population of animals or plants becomes geographically isolated—by a river that changes course or a mountain range that rises up, for example—and the two separated groups accumulate genetic differences over time as they adapt to their environments in isolation.
Eventually, the DNA of the two groups is so different that the two populations are considered distinct species. Voilà, speciation has occurred.
In reality, the process is much more complex than that. While geographic isolation can start the speciation process, evolutionary biologists believe that other forces—including various forms of natural selection—can help to complete it.
The new U-M study provides rare empirical evidence that multiple forms of natural selection, including a contentious one called reinforcement, are helping to complete the speciation process in a natural howler monkey “hybrid zone,” a place where the two species coexist and occasionally interbreed in a process called hybridization.
The study was published online Dec. 24 in the journal Molecular Ecology. In the paper, the researchers use the primate hybrid zone to identify parts of the genome that are likely to contain genes underlying speciation and to test for signals of the selection forces that shaped them.
“We observed patterns in the genetic data suggesting that hybridization is playing a direct role in completing the speciation process by enhancing genetic differences between species,” said U-M doctoral candidate Marcella Baiz, the study’s first author. The other authors are Liliana Cortés-Ortiz and Priscilla Tucker of the U-M Department of Ecology and Evolutionary Biology.
“We found a signal for multiple forms of natural selection driving species differences, including reinforcement, a process that has been highly debated,” Baiz said. “This result is particularly notable because empirical evidence for reinforcement is extremely rare, especially genetic evidence.”
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