for National Geographic News
The Bolivian river dolphin is a separate species from the Amazon river dolphin, scientists announced recently.
Thousands of years ago a powerful drought dried up Brazil's Madeira River, causing a "radical separation" as dolphin populations were caught on different sides of the newly created rapids, said researcher Manuel Ruiz-Garcia.
The Madeira split into today's Beni and Mamoré rivers of northeastern Bolivia. (See a Bolivia map.)
"When they separated, [the dolphins] were never again able to return and reproduce," said Ruiz-Garcia, who heads the Molecular Genetics Lab at Javeriana University in Bogotá, Colombia.
"Thus isolated, the Bolivian river dolphin, Inia boliviensis, eventually developed," he said.
The announcement was made at a recent conservation workshop in Santa Cruz de la Sierra in Bolivia.
Ruiz-Garcia took DNA samples from 40 river dolphins from Bolivia and 56 from Colombia by extracting tissue from their tail muscles.
A limited comparison of the DNA revealed significant genetic differences between the two river-dolphin populations.
This led Ruiz-Garcia to initially estimate that the species separated five to six million years ago.
But after comparing 32 more genes from DNA in another 40 Bolivian dolphins and about 60 Colombian and Peruvian dolphins, he concluded that the separation happened much sooner—about 100,000 to 500,000 years ago.
"Bolivian dolphins are totally different molecularly from other dolphins," Ruiz-Garcia said. "After being split up, they accumulated mutations and formed a new species."
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