Howler monkey study examines mechanisms of new species formation

U-M howler monkey study examines mechanisms of new species formation
A young male howler monkey in Tabasco, Mexico. Though it looks like a black howler monkey, this individual is likely a hybrid of two species, the black howler monkey and the mantled howler monkey. Credit: Milagros González

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 can start the speciation process, 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 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."

The two species at the center of the study, mantled howler and black howler monkeys, diverged about 3 million years ago and lived apart until relatively recently when they came into contact again—perhaps within the last 10,000 years—in a roughly 12-mile-wide hybrid zone in the southeastern Mexican state of Tabasco.

A species was once defined as a group of actually or potentially interbreeding individuals that are reproductively isolated from other such groups. The concept of reproductive isolation is key to that definition and means that despite any hybridization, true species maintain their uniqueness.

However, the modern view of what a species is does not require full reproductive isolation, and hybridization has been discovered to be quite common in nature.

At the howler monkey hybrid zone in Mexico where U-M's Cortés-Ortiz and her colleagues have worked for about two decades, analysis of DNA samples has confirmed that black and mantled howler monkeys interbreed and produce hybrid offspring. The fact that hybridization is occurring between the two groups means that reproductive isolation is incomplete.

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 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 between monkeys of these two 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 , and this study is one of the few natural examples that documents this," Baiz said.

The Molecular Ecology paper is titled "Multiple forms of selection shape reproductive in a primate hybrid zone."

Explore further

What howler monkeys can tell us about the role of interbreeding in human evolution

Journal information: Molecular Ecology

Citation: Howler monkey study examines mechanisms of new species formation (2018, December 22) retrieved 18 September 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

Dec 22, 2018
This comment has been removed by a moderator.

Dec 22, 2018
When #PeeBrain 45yuk howls: "Well I'll be a howler monkey's uncls!"....; he just might really MEAN it. Fits for the king of perverted mean. Just LOOK at that face! Nearer Precursor may be Benito Mussoline...'died' the year that the maggot was born...Il Douche >>>> Il Douchebag. Look up the old 'Duce. You can not NOT see this.

Dec 23, 2018
Take a look. Take a GOOD look. The Mussolini family pix are in many places on the net. INcluding 'Count Ciano'.....pronounced 'Chahno' in Italian. The Italian 'ci is the same as the english 'ch'. I did not down rate you as I believe this forum is for those who value ideas and the fee expression of them.

Dec 23, 2018
Intelligent design randomly made that's never been reproduced?

Dec 23, 2018
So allopatry - geographic isolation - is the sole overall cause, but after that sympatry - geographic coexistence - has contributing - reinforcement - factors (such as hybrid lower fitness, sexual selection et cetera). This is something that has been seen since the 90s [ https://en.wikipe...ciation) ].

Notably the divergence between Homo (humans) and Pan (chimps/bonobos) ancestors were likely not this complex and seems to be missing observable hybridization [ https://www.ncbi....2743777/ ].

- tbctd -

Dec 23, 2018
- ctd- From the abstract:

"We detected extensive admixture that was mostly limited to the narrow contact zone. Loci with reduced introgression into the heterospecific genomic background (the pattern expected for loci associated with reproductive isolation due to selection against hybrids) were significantly more differentiated between allopatric parental populations than loci with neutral and increased introgression, supporting the hypothesis that reproductive isolation is a by-product of divergence in allopatry. Further, loci with reduced introgression showed greater differentiation in sympatry than in allopatry, suggesting a role for reinforcement. Thus, our results reflect multiple forms of selection that have shaped reproductive isolation in this system. We conclude that reproductive isolation may have initially been driven by divergence in allopatry, but later reinforced by divergent selection in sympatry."

Dec 25, 2018
Thus, our results reflect multiple forms of selection that have shaped reproductive isolation in this system. We conclude that reproductive isolation may have initially been driven by divergence in allopatry, but later reinforced by divergent selection in sympatry
-IOW there is an 'urge to diverge' when pop pressures reach a certain point and competition for resources transition to group selection.

Speciation is an integral aspect of life. Pop pressure drives groups to inhabit new niches. A considerable amount of 'effort' is expended in occupying these new niches; more individuals lost who couldn't adapt sufficiently to survive to reproduce.

The remaining group members over time will resist outsiders who are less fit and who would weaken the gene pool by causing the loss of this fitness.

In humans this is called tribalism. Our resistance to outsider inclusion in the gene pool is called prejudice and bigotry. Group selection is called war.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more