Frogs Illustrate the Creative Destruction of Mass Extinctions

Until now, biologists have struggled to reconstruct an accurate family tree for frogs. Based on fossils and limited genetic data, it appeared that most modern frog species popped up at a slow and steady pace from about 150 million to 66 million years ago. New research shows that a mass extinction 66 million years ago sparked an explosion of new frog species.

Scientists from The University of Texas at Austin, the University of California, Berkeley, the Florida Museum of Natural History and Sun Yat-Sen University in China compiled the largest set of frog genetic data ever evaluated for evolutionary relationships to draw the most accurate frog family tree to date. They find evidence of not one but three explosions of new frog species, on different continents, and all concentrated in the aftermath of the mass die-off of most dinosaurs and many other species about 66 million years ago.

It suggests that the mass extinction event at the end of the Cretaceous—largely caused by an asteroid striking the Earth—also wiped out most of the frog species that were alive then, leaving behind only a few survivors. With many other species also wiped out, the slate was wiped clean. There were many new ecological niches that the remaining frogs could spread into. That provided a kind of evolutionary rocket fuel for their rapid diversification.

"We know that the mass extinction event wiped out most of the dinosaurs, except for a few bird species, which then exploded in diversity and became one of the dominant groups of land animals," says David Hillis, professor of integrative biology at UT Austin and co-author of a paper appearing this week in the journal Proceedings of the National Academy of Sciences. "As we look at more and more groups of life, we see the same pattern, and that turns out to be the case for frogs as well."

A tree frog from North America (Hyla chrysocelis). Photo: David Hillis

The new paper's corresponding authors are David Cannatella at UT Austin, David Wake of the University of California, Berkeley, and Peng Zhang of Sun Yat-Sen University. Other authors include Yan-Jie Feng and Dan Liang of Sun Yat-Sen; and David Blackburn of the Florida Museum of Natural History.

The researchers gathered genetic samples from 156 frog species and combined it with previously published data on 145 more species to build the most complete frog family tree yet. In the new samples, they studied variations in 95 genes—compared with past studies that used only 5 to 12 genes—to get a much higher-resolution view of how individual species relate to one another.

Today, there are more than 6,700 known frog species, representing 55 families and living in a wide range of habitats from trees to aquatic environments to underground. This study significantly tightens the time frame in which this diversity came about.

Support for the research was provided by the U.S. National Science Foundation, the National Natural Science Foundation of China and the National Youth Talent Support Program and National Science Fund for Excellent Young Scholars of China.