December
22, 2018, University of Michigan
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
is scheduled for online publication Dec. 22 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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