Scientists are tracking the relationship between two long-separated species of howler monkey that are now interbreeding to produce hybrids. The rare opportunity to study evolution in action in a fellow primate has confirmed a controversial driver of evolution and may help us understand the dynamics of our own increasingly confusing family tree.
Mantled howler monkeys and black howler monkeys have been a distinct species for 3 million years. We can't know the processes that drove this split, but today they inhabit different parts of Mexico. Until an estimated 10,000 years ago, this geographic separation is thought to have been absolute. Today, however, Tabasco state hosts a 20-kilometer-wide (12-mile) hybrid zone where the two have overlapping territories and interbreeding occurs. University of Michigan PhD student Marcella Baiz studies the effects this is having on the monkeys' genetics, and she's revealed how complex the process can be.
The textbook example of species formation has a river an animal cannot cross emerge in the middle of its habitat. Over time, those on each side of the divide take different evolutionary paths, to the point where they can't produce fertile offspring if further changes bring them back together.
Biologists have realized, however, the truth is seldom so simple. Distinct species do interbreed, and while their success rate will be lower when this happens than when mating with their own kind, it can lead to genes from one population becoming incorporated into another or to a new hybrid species emerging.
A possible, but still controversial, aspect of the process is known as reinforcement, where natural selection enhances differences between two species so that they become less likely to mate and therefore also more different from each other. Since hybrids are, on average, less likely to pass on their genes, those members of a species that breed with their own kind are at an evolutionary advantage to those who interbreed.
For example, if two species differ slightly in size, the larger members of the smaller species, and smaller members of the larger one, are more likely to mate across species lines. Those individuals that differ most from the other species will breed with their own and be more likely to pass on their genes, causing the size gap to expand.
Much as this makes sense, confirmed examples of reinforcement occurring are rare, but Baiz and co-authors report in Molecular Ecology that the howler monkeys' genetics verify the idea. “This study is one of the few natural examples that document [reinforcement]," Baiz said in a statement.
Rather than blurring together, specific points on the monkey's genomes have become more distinct within the hybrid zone, indicating that in this small range, the species are evolving so they can't interbreed.
