Bird family tree compiled

Birds_1
Photo by Annelisa Leinbach.

For the first time, all 9,993 living bird species can roost on one family tree.

In a five-year study titled “The global diversity of birds in space and time,” published Oct. 31 in the journal Nature, a research team led by Yale evolutionary biologist Walter Jetz compiled the most complete version of the avian family tree to date using data from DNA sequencing and fossil records. Contrary to expectations, the tree suggests that birds’ overall rate of speciation, or formation of new species, has increased over time.

Family trees allow scientists to determine diversification rates, or the calculated rate at which new species are formed and maintained, said Jacob Musser GRD ’15, a Ph.D. candidate in ecology and evolutionary biology at Yale. While previous studies have focused primarily on diversification within one or two small groups of birds, the new study uses a complete tree to chart patterns of diversification across all bird life and map them geographically, Musser said. The approach allowed the researchers to determine, to their surprise, that both rapidly and slowly diversifying groups are interspersed throughout the tree and therefore a number of different bird species are responsible for increased bird diversity, he added.

The study also reveals that these diversification rates do not vary with latitude. Scientists have long debated why there are more bird species in the tropics than in higher latitudes and had attributed the difference to higher diversification rates, said Jetz, lead author of the study. But the researchers found rapidly and slowly diversifying groups in both the tropics and high latitudes, indicating that geography was not the sole contributing factor to bird diversity.

“When you get closer to the equator, you have more species,” Musser said. “But that doesn’t mean you’re producing species faster.”

The tropic regions of the Earth have existed for a greater amount of time than its more temperate regions, which may have given tropical bird species more time to diversify, researchers said.

After determining diversification rates, the researchers began considering how ecological factors such as global warming might have caused differences in the rates.

“Special innovations in evolving birds may have seized upon certain environmental niches that allowed them to diversify faster,” Jetz said. Gulls and woodpeckers, for instance, are high latitude species groups whose diversification may have been aided by climate change.

Of the 9,993 species included in the family tree, 3,330 had no documented DNA. Rather than leaving these undocumented species off the tree, the researchers chose to make assumptions about their placements within certain families, said Arne Mooers, co-author of the study and professor of biodiversity at Simon Fraser University. To ensure accuracy, the team generated thousands of trees allowing for alternate placements of these birds, then averaged the results.

“It required a bit of chutzpah to do that,” Mooers said. But the step, he added, was necessary. “There are lots of questions you can’t ask about speciation if you only have two-thirds of the total species [on the tree]. It’s better to include them all [than] be led astray by species that are missing.”

For Jetz, the most significant part of the research was being able to tie the evolution of birds over time to their current geographic distribution.

“I think that’s conceptually really the most novel and exciting bit, that we are able to connect species alive today to their position and history in the family tree of birds,” Jetz said.

Other members of the research team included J. Joy of Simon Fraser University, G. H. Thomas of the University of Bristol and K. Hartmann of the University of Tasmania in Australia.

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