Bird evolutionary tree given a shake by DNA study

• 19:00 26 June 2008
• NewScientist.com news service
• Bob Holmes

A new study – the largest analysis of birds to date using modern genetic methods – has turned up numerous surprising relationships that will force biologists to reevaluate much of what they thought they knew about avian evolution.

Until now, this evolutionary history has been something of a mystery, because most modern orders of birds arose in a sudden burst of innovation sometime between 65 and 100 million years ago. This left few intermediate forms to help biologists discern the evolutionary relationships among orders.

"It's one of the last big mysteries of birds – trying to figure out how these orders, which look so cohesive in themselves, are grouped together," says Sushma Reddy, an evolutionary biologist at the Field Museum of Natural History in Chicago, US.

The few genetic comparisons among orders have tended to focus on one or a few genes, and have yielded inconsistent results.

Odd cousins

To resolve this problem, Reddy and her colleagues

sequenced 19 regions of the genome of 169 species of birds – a total of 32,000 DNA "letters" per species – then used the sequences to construct the most robust avian evolutionary tree ever made.

This new tree contains several notable surprises. For example, falcons are more closely related to songbirds than to other hawks and eagles. The closest kin of the diving birds called grebes turn out to be flamingos. And tiny, flashy hummingbirds, according to the new tree, are just a specialised form of nighthawks, whose squat, bulky bodies make them an unlikely cousin.

In fact, the new tree ended up regrouping about a third of all the orders in earlier phylogenies of birds. "That shows you how inconsistent it has been," says Reddy.

Flight of fantasy?

The new tree may have profound implications for our understanding of the major innovations in the evolutionary history of birds, says Joel Cracraft, curator of ornithology at the American Museum of Natural History in New York, US.

For example, the new tree puts an order of flying birds, the tinamous, squarely in the midst of the flightless ostriches, emus and kiwis.

If true, this implies either that flightlessness evolved at least twice in this lineage, or else that the tinamous re-evolved flight from a flightless ancestor. "A lot of us actually don't believe their result," says Cracraft, who says that further studies will be needed to resolve the issue.

The new family tree adds a further novel twist by suggesting that the closest relatives of perching birds, or passerines, are parrots. This needs further corroboration but, if true, evolutionary biologists will at last have a firm starting point for understanding the evolution of the perching birds, which are by far the largest and most successful order of birds.

Journal reference: Science (DOI: 1.1126/science.1157704)