Evolutionary biologists believe that various natural selection pressures can help complete the process by strengthening barriers to gene flow between two groups, pushing them toward full reproductive isolation.
And because natural selection favors organisms that successfully reproduce over those that don’t, it is biased against hybrids, which sometimes die before reproducing or are simply incapable of reproducing.
Natural selection tries to block the formation of these “unfit” hybrids. One way to do that is to gradually increase the genetic differences between two groups of organisms — in this case black and mantled howler monkeys — so that it’s more difficult for them to mate and to produce hybrid offspring.
While working to thwart the formation of hybrids in this way, natural selection strengthens reproductive isolation by increasing genetic differences. This process is called reinforcement; while the idea has been around for more than a century, empirical evidence to support it is scarce.
To test for the presence of reinforcement, Baiz and her colleagues compared the DNA of black and mantled howler monkeys living the Tabasco hybrid zone to the DNA of black and mantled howler monkeys living far from the hybrid zone.
If reinforcement is working to thwart hybridization and to strengthen reproductive isolation, then the genetic differences between the two species in the hybrid zone should be greater than the genetic differences between monkeys of these two species living on either side of the hybrid zone.
And that’s exactly what Baiz and her colleagues found when they compared genetic markers that are at or near genes likely associated with reproductive isolation.
“Speciation is a complex process that can be driven by direct and indirect mechanisms that interact to maintain and strengthen the process, and this study is one of the few natural examples that documents this,” Baiz said. Paper. (paywall) – Baiz M, Tucker P, Cortés-Ortiz L. Multiple forms of selection shape reproductive isolation in a primate hybrid zone. Molecular Ecology, 2018 DOI: 10.5061/dryad.5d4mb06 More.
It’s no wonder that biologists have debated whether this “reinforcement”form of natural selection even exists. If it does, it is acting as a purposeful agent. Now, if these researchers have found an instance of it, what does that mean?
Follow UD News at Twitter!
See also: Monkeys More Closely Related To Sister Species Than Same Species In Different Locations?