The bird tree
Nature reports a big flutter in certain circles:
Scientists have mapped the evolutionary relationships among all 9,993 of the world’s known living bird species. The study, published today in Nature, is an ambitious project that uses DNA-sequence data to create a phylogenetic tree — a branching map of evolutionary relationships among species — that also links global bird speciation rates across space and time.
“This is the first dated tree of life for a class of species this size to be put on a global map,” says study co-author Walter Jetz, an evolutionary biologist at Yale University in New Haven, Connecticut.
But the endeavour is also controversial, owing to the large number of species for which no sequence data are available. “This is a conceptually brilliant attempt to link space with time while crafting a complete phylogeny,” says Trevor Price, an evolutionary biologist at the University of Chicago in Illinois. “But there are almost certainly introduced artefacts by lacking one-third of the sequences used to create it.”
Jetz and his colleagues built on an extensive phylogenomic study, published in 2008, to divide bird species into 158 clades, well-established groups believed to have evolved from a common ancestor. Using ten fossils, the researchers dated and anchored that backbone, and placed all the living species on the tree, starting with the roughly 6,600 for which genetic information was available.
The researchers found that although rapid radiations have occurred throughout time and space, the rate of speciation has sharply increased over the past 40 million years.
“This paper makes an attempt to account for how species disperse, but we aren’t quite there yet,” says Price. More fossils will be needed to verify past movements.
Nonetheless, Jetz says that he expects the tree will stand the test of time. “As more sequence data are added, some details will change, but I can’t really imagine a case where any of the core findings are turned upside down,” he says. “This is certainly not the last word on phylogeny of birds. We hope it will trigger additional efforts to continue improving our understanding of the avian tree of life.”
It had been assumed earlier that new species arise when one parent species stumbles into a relatively empty ecological niche. This allows many daughter species to radiate out to fill the niche. The rapid diversification is followed by a culling due to competition. Remarkably, this new study seems to find a completely different picture of evolution. It seems that certain clades, such as hummingbirds, parrots and a number of songbird lineages, diversify faster than others irrespective of geography, or even their position in the family tree of birds. No wonder there is such a fuss about this paper.