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A new Uni­ver­sity of Michi­gan (U-​M) study of inter­breed­ing between two species of howler mon­keys in Mex­ico is yield­ing insights into the forces that drive the evo­lu­tion of new species.

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.
Image credit: Mila­gros González

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 was sched­uled for online pub­li­ca­tion on 22 Decem­ber 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.

Mar­cella Baiz, lead author, doc­toral can­di­date, Uni­ver­sity of Michi­gan Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy, USA

“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 cen­tre of the study, man­tled howler mon­keys (Alouatta pal­li­ata) and black howler mon­keys (Alouatta niger­rima), 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 20-​km-​wide hybrid zone in the south-​eastern 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 of spe­ci­a­tion by strength­en­ing bar­ri­ers to gene flow between two groups, push­ing them toward full repro­duc­tive isolation.

And because nat­ural selec­tion favours 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.

(Source: Uni­ver­sity of Michi­gan news release, 22.12.2018)