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Hummingbird diversity still booming

April 7, 2014 This article courtesy of Nature News.

The group has branched into hundreds of species and some lineages are still going.

In just 22 million years, hummingbirds have diversified from a single common ancestor into around 338 tiny, colourful species. They aren’t finished either.

Ornithologist Jimmy McGuire from the University of California, Berkeley, has found that while some hummingbird groups have saturated the available space in their environment, others are still diversifying at an extraordinary rate. By comparing their rates of speciation and extinction, McGuire calculated that there could be as many as 767 species of hummingbird in the future — more than twice the number that currently exist. The results are published in Current Biology.

“This is unique evidence of one of the most spectacular known examples of an incomplete adaptive radiation,” says Juan Francisco Ornelas, an evolutionary biologist from the Institute of Ecology in Xalapa, Mexico.

McGuire’s team carried out the largest-ever study of hummingbird evolution by comparing DNA from 284 species. “It’s impressive,” says Robb Brumfield, a geneticist from Louisiana State University in Baton Rouge. “The samples they used are the product of thirty-plus years of ornithologists lugging nitrogen tanks and collecting in remote regions” of Central and South America.

The analysis showed that hummingbirds fall into nine major lineages, which diversified from each other in South America over the last 22 million years. But they first diverged from a sister group — the swifts — around 42 million years ago, and recently-discovered fossils suggest that this split must have happened in Eurasia. “That’s a pretty big gap!” says McGuire. Hummingbirds are not strong enough to fly across the Pacific, so they must have travelled overland, crossing the Bering Strait into North America before heading south.

In South America, they radiated dramatically, and especially so 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 last 10 million years, when the Andes started rapidly rising.

“The Andes are kind of the worst place to be a hummingbird,” says McGuire. These birds “have super-high 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”.

He says that the cold, high mountains excluded insect pollinators, leaving room for hummingbirds to take their place. They also provided a smorgasbord of habitats, from isolated valleys to steep slopes with rapidly changing climates. Hummingbirds were well-suited to exploit these niches since they have tiny ranges—and McGuire found that Andean species have 4-fold smaller ranges than non-Andean ones.

But Ornelas says that the team overstates the importance of the Andes. With their bright colours, elaborate ornaments, spectacular courtship displays, and complex calls, sexual selection has almost certainly played a major part in hummingbird evolution.

Either way, hummingbirds as a group are producing new species at a decelerating pace, probably because they are running out of space or ecological niches to fill. But on closer inspection, McGuire found that some lineages are diversifying 15 times faster than others, and still speciating at a high rate. “The rates are all over the place,” he says. “Even though ecological space is starting to run out, there’s still room for more species.”


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