Researchers have determined that monkey populations and species living in close proximity to each other look so different in order to "strengthen reproductive isolation between populations."

Facial features of Old World monkeys evolved to separate themselves from other geographically proximate species to prevent crossbreeding.

A new scientific study was published this week in the journal Nature Communications.

"Evolution produces adaptations that help animals thrive in a particular environment, and over time these adaptations lead to the evolution of new species," said study author James Higham, an assistant professor in New York University's Department of Anthropology, according to a press release issued by the university. "A key question is what mechanisms keep closely related species that overlap geographically from inter-breeding, so that they are maintained as separate species?"

Researchers from NYU and Exeter University, in the United Kingdom, figured out that that the answer to Higham's question is precise facial feature differentiation.

Features include colorful eyebrow patches, ear tufts, various face markings, nose spots, mouth patches, and more.

The researchers came to their conclusion after taking pictures and analyzing the facial features of 22 types of guenons, or Cercopithecini, the first monkeys that evolved in the forests of Central and West Africa.

By plotting and comparing specific facials features, the researchers were able to show that each species' markings and features became more distinct over time, according to the release.

They were also able to prove that species that spent time in close proximity were more likely to stand out from each other than those who spent less time together.

"These results strongly suggest that the extraordinary appearance of these monkeys has been due to selection for visual signals that discourage hybridization," said lead author William Allen, who has since left NYU's Department of Anthropology for the University of Hull in England, according to the release. "This is perhaps the strongest evidence to date for a role for visual signals in the key evolutionary processes by which species are formed and maintained, and it is particularly exciting that we have found it in part of our own lineage."

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