The most ambitious effort yet to trace the evolutionary history and geography of living birds is ruffling the feathers of some old ideas.
For the first time, the new avian family tree places each of the 9,993 known species of living birds on its own twig. Such a broad view allows biologists to look for patterns of sluggish or exuberant formation of new species, a critical part of understanding nature’s quirky patterns of diversity. In the new tree, Western Hemisphere birds seem to have diversified faster than Eastern Hemisphere ones, Walter Jetz of Yale University and his colleagues report in an upcoming Nature.
Bird species abound in the East, but no single lineage drives this hemisphere pattern. Instead it comes from scattered bursts of rapid species evolution on a variety of branches.
Some patterns in the tree were expected, but the hemisphere trend and some other findings are “surprising and challenging,” says Robert Ricklefs of the University of Missouri-St. Louis, who was not part of the study. “I suspect that [this] paper will stimulate a lot of discussion, some of it derisive, but also considerable research activity to delve into these patterns more deeply.”
The hemispheric trend brings a new twist to the discussion about species formation rates, which often focuses on comparisons across latitude. About three-quarters of modern bird species live in the tropics, and some biologists have argued that such feathered splendor comes from new species forming quickly there and outpacing tropical extinctions. According to some influential earlier studies, species formation dwindles as latitude increases.
The big new tree found no overall relationship with latitude gradient, Jetz says. This observation thus supports a long-standing alternative idea for explaining tropical diversity, he says: “Perhaps birds simply have had more time there.”
The new paper also uncovered some bird groups with an overlooked history of quick splits into new species. Birds such as white-eyes are already famous for diversifying quickly, but Jetz and his colleagues also found fast diversification among ducks, woodpeckers and some groups of gulls.
The researchers spent five years tracing the deep history of birds. For more than 6,000 species, they used genetic data to determine birds’ near relatives. The scientists then placed the rest of the species in the tree by finding a species traditionally considered a relative for which genetic data were available.
Dates from the few available fossils of early birds calibrated the timing of evolutionary changes. “One thing we could definitely do with is more fossils,” Jetz says.
This project was “ambitious” but the methods have some shortcomings, cautions Jason Weir of the University of Toronto Scarborough. Placing a third of the species without help from DNA data and without knowing about real extinctions in the lineage, for example, could have distorted the results. “Unfortunately there is no way to know when the birds should join the tree,” he says.
A different kind of caution comes from study coauthor Arne Mooers of Simon Fraser University in Burnaby, Canada. In recent times, evolutionarily speaking, birds have been adapting and forming new species at a brisk pace of a new species perhaps every 700 years. Extinction has been pruning them. But the challenges of sharing a planet with people, Mooers calculates, is now driving birds extinct at a rate 300 times as fast as they’re forming.
W. Jetz et al. The global diversity of birds in space and time. Nature. doi:10.1038/nature11631.
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