BIRDS鈥� evolutionary tree has just been shaken to its roots by the discovery of surprising relationships that may force ornithologists to re-evaluate much of what they thought they knew about avian evolution.
The evolutionary history of birds has always been something of a mystery, because most modern orders arose in a sudden burst of speciation some time between 100 and 65 million years ago, leaving precious few intermediate forms to help biologists work out where the orders came from and how they are related. 鈥淚t鈥檚 one of the last big mysteries of birds 鈥� trying to figure out how these orders, which look so cohesive in themselves, are grouped together,鈥� says Sushma Reddy, an evolutionary biologist at The Field Museum in Chicago.
The few genetic comparisons among orders so far have tended to focus on one or a few genes, and have yielded inconsistent results.
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To resolve this problem, Reddy and her colleagues sequenced 19 regions of the genome in 169 species of birds and then used the sequences to construct the most robust avian evolutionary tree ever made.
There are several surprises. For example, falcons are more closely related to songbirds than to other hawks and eagles. The closest kin of grebes 鈥� the drab diving birds 鈥� turn out to be the brilliant long-legged flamingos. Tiny, flashy hummingbirds, according to the new tree, are just a specialised form of nighthawk, whose squat, bulky bodies make them an unlikely cousin. And the closest relatives of perching birds 鈥� by far the largest and most successful bird group 鈥� are the parrots. In fact, the new tree ended up making changes to about a third of all the orders in earlier trees of birds (Science, ). 鈥淭hat shows you how inconsistent it鈥檚 been,鈥� says Reddy.
鈥淔alcons are more closely related to songbirds than to other hawks and eagles; hummingbirds are a specialised form of nighthawk鈥�
This study 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.
However, some of the more spectacularly surprising relationships uncovered will require further corroboration from additional studies before ornithologists will accept them. For example, says Cracraft, the 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 tinamous re-evolved their ability to fly from a flightless ancestor. 鈥淎 lot of us actually don鈥檛 believe this result,鈥� he says.
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