It’s no secret that the area of rain forest between Mexico to the southernmost point of South America (commonly known as the neotropics) is teeming with colorful creatures of all sorts. The area is also known to be packed with different, though closely related, species. A recent article published in the journal Nature suggests that our ideas about how and why those different species evolved might have been all wrong.
Researchers with the American Museum of Natural History and Louisiana State University recently traced the genetics of 27 species of birds in the neotropics, focusing on comparing groups that were found on two different sides of a geographic barrier, like a river or a mountain range. It has long been thought that geologic changes, like the rising of a new mountain, would divide a group of birds and the flocks on each side would gradually branch out from each other genetically.
The scientists were able to pinpoint the era when two groups branched off from one another, and compared those timelines to known geologic changes in the rain forest. What they found was a little surprising: most branching off occurred during the Pleistocence time period, which started around 2.6 million years ago. The Andes Mountains and the Amazon River were already well in place by then. Instead, researchers learned, it’s likely that small groups of birds branched off from each other for some reason and made long, likely difficult journeys across these barriers to start their own species.
"Organisms, all types of plants and animals are constantly moving,” said AMNH assistant curator and lead publication author Brian Smith. "Everything’s always trying to increase the population size, but there are constraints on that … it’s really hard for most species to get across those barriers. These are really large rivers, and once they get across there, they’re not going to be able to exchange with populations on the other side, so this initiates the speciation process.”
It’s impossible to say what might have driven small groups of birds to break off and take on the challenge of making it across a mountain range or river, but Smith believes once they made it into an area that could support their needs, they were unlikely to put the energy into going back.
The researchers chose to examine birds for these purposes with the hope their trends could yield clues about speciation patterns to test for other types of rainforest species, too. In fact, birds were a good option because we have a comparatively large amount of genetic data for them.
"Of all the species, birds are among the best studied,” he said. "It’s just now that we have enough data to ask these really hard questions.”
Smith estimates that scientists have been collecting DNA samples on birds in the neotropics for the past 30 years, literally representing the work of generations of students and researchers from LSU and other institutions. Analysis of all that data is still in its early stages, leaving open the possibility that researchers may discover more subtle differences in DNA that could suggest new bird species.
Identifying speciation patterns can have an impact on conservation efforts in the neotropics, too. Smith believes that this type of branching off of the family tree could demonstrate that there’s so much we still don’t know about the region—which makes its protection that much more important.
"We still don’t even have a good handle on how many bird species there are on earth,” he said. "It seems like a really basic question, but we don’t know. There are still new bird species that need to be discovered.
Smith added that in the next 10 to 20 years, "We’ll have a much better idea of which are the most important areas that we should be preserving.”
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