Hybridization between species is aspect of evolution that is only just now becoming recognized as a force in evolution. It is sort of taking a biological app from one species and adapting it another, and most studies on this phenomenon look at the app adaptation aspect of hybridization.
However, hybridization is more often than not less advantageous from a natural selection standpoint. Although these “new apps” and heterosis might be good for hybrids, many hybrids are sterile. Or if they aren’t sterile, one sex will either be absent or sterile.
Species generally have mechanisms that prevent hybridization. Many of these are behavioral. For example, related bird species often won’t exchange genes because the female are simply not attracted to the males’ songs. But there are molecular responses against hybridization as well.
One of the most contentious hypotheses about hybridization between species is that of reinforcement. What this hypothesis contend is that when two species begin to hybridize readily, there will be a strong selection for greater genetic distance between the two hybridizing species. With greater genetic variation, it will be less likely that the two species will be able to produce viable offspring, and over time, there will be fewer hybrids in the population.
This hypothesis has not been tested much. However, a study of two species of howler monkey in the Mexican state of Tabasco revealed that, yes, reinforcement is a thing.
Mantled and Guatemalan black howler monkeys diverged from a common ancestor about 3 million years ago. The two species have only a narrow contact zone, which is thought to have formed only 10,000 years ago in this tiny part of Mexico.
The researchers examined loci of the genomes of specimens of both species, including those in the hybrid zone. They found that the genetic difference between the two species was greater at the hybrid zone than from monkeys that lived in other regions. This discovery supports the hypothesis of reinforcement. The greater genetic difference between the two species at the hybrid zone means that this greater genetic difference likely has evolved as a way of keeping the two species from producing lots of hybrids, which might not be as fit or as good at reproducing in the wild as pure ones.
This discovery of reinforcement means that we have another tool in sorting out whether two species make sense. If we discover that there is greater genetic difference at a hybrid zone between the two species, then we know that they really are quite taxonomically distinct. If we find the opposite, it means that hybrids aren’t deleterious in the population, and hybridization is either advantageous or neutral for the populations.
Yes, I would like to see this hypothesis tested on the various hybridizing canid populations in the gray wolf species complex. My guess is that it doesn’t exist in these animals, because hybridization isn’t that deleterious. And the genetic divergence isn’t that great to start out with.
But this study gives us a good idea of how hybridization operates in populations, and how some populations evolve to restrict gene flow.
