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87: Songbirds Have Southern Roots


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Avian evolution was turned upside down in July, when biologists released a comparative survey of songbird genes. Contrary to conventional wisdom, they found that most songbirds first appeared in Australasia—the ancient landmass that included Australia, New Guinea, Tasmania, and parts of Indonesia—not in Eurasia, as was long believed.

Songbirds are divided into two major branches: the Corvida, with more than 1,000 species, including crows and ravens, and the Passerida, with nearly 3,500 species, including cardinals, robins, and finches. The Corvida are most diverse in Australia, so biologists assumed that they had evolved down under. The Passerida are more diverse in the Northern Hemisphere, so they were assumed to have evolved there.

Genetic research has proved otherwise. A team of evolutionary biologists, led by Keith Barker of the University of Minnesota, examined two slowly evolving genes in 144 bird species among 44 families. (It was the most comprehensive survey to date of perching birds.) The comparison revealed a surprising kinship between certain members of the Passerida and the Corvida. “This means, in a sense, that the Passerida are Corvida,” Barker says. And that means all songbirds, not just the Corvida, originated in Australasia.

By analyzing mutations in the genes, Barker and his colleagues determined that the Passerida left Australasia some 45 million years ago, heading north. Some 25 million years after that, the Corvida also began to leave and to diversify. Why the Passerida were so much more successful than the Corvida—eventually colonizing every continent except Antarctica—remains a mystery. Still, according to ornithologist Rick Prum of Yale University, the out-of-Australia hypothesis is “overwhelmingly supported by molecular data.”

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