scient if icamerican.co m http://www.scientificamerican.com/article/hummingbird-evo lution-is-booming/?print=true

Hummingbird Evolution Is Booming

This hummingbird is one of many species adapted to theecosystems of the Andes.

The successf ul, 22 million-year-old group could double in itsnumber of species bef ore leveling of f

Hummingbirds took just 22 million years to diversif y f rom asingle common ancestor into 338 tiny, colorf ul species. Andthey have not f inished yet.

Evolutionary biologist Jim McGuire of the University ofCalif ornia, Berkeley, and his collaborators have f ound thatalthough some hummingbird groups have saturated theavailable space in their environments, others are stilldeveloping into new species at an extraordinary rate. Bycomparing their rates of speciation and extinction, McGuire'steam calculated that the number of hummingbird speciescould double bef ore reaching an equilibrium in the next several million years. Theresults are published in Current Biology.

“This is unique evidence of one of the most spectacular known examples of an incomplete adaptiveradiation,” says Juan Francisco Ornelas, an evolutionary biologist at the Institute of Ecology in Xalapa,Mexico. ('Adaptive radiation' is biologists' term f or a rapid dif f erentiation into distinct species.)

Hummingbirds are only f ound in the New World, and the majority of species live in South America. McGuire’steam carried out the largest-ever study of the group's evolution by comparing DNA f rom 284 species.

“It ’s impressive,” says Robb Brumf ield, a geneticist at Louisiana State University in Baton Rouge. “Thesamples they used are the product of 30-plus years of ornithologists lugging nitrogen tanks and collectingin remote regions” of Central and South America. The researchers collected tissue samples and needed tokeep them f rozen to allow f or DNA sequencing.

Fast workersThe analysis showed that hummingbirds f all into nine major lineages, which have diversif ied f rom eachother in South America over the past 22 million years. But they f irst diverged f rom a sister group — theswif ts — around 42 million years ago, and f ossils suggest that this split must have happened in Eurasia.“That’s a pretty big gap!” says McGuire. Hummingbirds cannot f ly across oceans, so they must havetravelled overland, crossing a land bridge over the Bering Strait into North America bef ore heading south.

Once they reached South America they radiated dramatically — especially in the Andes. These mountainsrepresent 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 startedrapidly rising, which suggests a possible role of the mountains in stimulating diversif ication.

“The Andes are kind of the worst place to be a hummingbird,” says McGuire. These birds “have super-highmetabolic 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 f orage at high elevations, he saysthat there are relatively f ew insects in the cold, high mountains, leaving room f or hummingbirds to take theirplace pollinating f lowers. The mountains also provide a smorgasbord of habitats, f rom isolated valleys tosteep 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. Judgingf rom the bright colors, elaborate ornaments, spectacular courtship displays and complex calls typical ofhummingbirds, sexual selection has almost certainly played a major part in their evolution.

Either way, hummingbirds are producing new species at a decelerating pace, probably because they arerunning out of space or ecological niches to f ill. But McGuire f ound that some lineages have diversif ied 15times f aster than others, and and still maintain an elevated pace. “The rates are all over the place,” he says.“Even though ecological space is starting to run out, there’s still room f or more species.”

This article is reproduced with permission f rom the magazine Nature. The article was f irst published on April3, 2014.