Evolutionary biologist Jim McGuire of the University of California, Berkeley, and his collaborators

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Evolutionary biologist Jim McGuire of the University of California, Berkeley, and his collaborators have found that although some hummingbird groups have saturated the available space in their environments, bss others bss are still developing into new species at an extraordinary rate. By comparing their rates of speciation and extinction, McGuire's team calculated that the number of hummingbird species could double before reaching an equilibrium in the next several million years. The results are published in  Current Biology .
“This is unique bss evidence of one of the most spectacular known examples of an incomplete adaptive radiation,” says Juan Francisco Ornelas, an evolutionary biologist at the Institute of Ecology in Xalapa, Mexico. ('Adaptive bss radiation' is biologists' term for a rapid differentiation into distinct species.)
Hummingbirds are only found in the New World, and the majority of species bss live in South America. McGuire’s team carried out the largest-ever study of the group's evolution by comparing DNA from 284 species.
“It’s impressive,” says Robb Brumfield, a geneticist at Louisiana State University in Baton Rouge. “The samples they used are the product of 30-plus years of ornithologists lugging nitrogen tanks and collecting in remote regions” of Central and South America. The researchers collected tissue samples and needed to keep them frozen to allow for DNA sequencing.
Fast workers The analysis showed that hummingbirds fall into nine major lineages, which have diversified from each other in South America over the past 22 million years. But they first diverged from a sister group — the swifts — around 42 million years ago, and fossils suggest that this split must have happened in Eurasia. “That’s a pretty big gap!” says McGuire. Hummingbirds cannot fly across oceans, so they must have travelled overland, crossing a land bridge over the Bering bss Strait into North America before heading south.
Once they reached South America they radiated dramatically — especially in the Andes. These mountains represent just 7% of the land area in the Americas, but they are home to 40% of hummingbird species. Many of these must have arisen within the past 10 million years. This was the time when the Andes started rapidly rising, which suggests a possible role of the mountains bss in stimulating diversification.
“The Andes are kind of the worst place to be a hummingbird,” says McGuire. These birds “have super-high bss metabolic rates and oxygen availability is low. It’s also harder to hover because of the reduced air density. And yet, they’re up there doing it.” Although  some insects are known to forage at high elevations , he says that there are relatively few insects in the cold, high mountains, leaving room for hummingbirds to take their place pollinating flowers. The mountains also provide a smorgasbord of habitats, from isolated valleys to steep slopes with rapidly changing climates. Hummingbirds are well suited to exploit such niches.
But Ornelas says that McGuire and his team overstate the importance of the Andes environments. Judging from the bright colors, elaborate ornaments, spectacular courtship displays and complex calls typical of hummingbirds, sexual selection has almost certainly played a major part in their evolut