— Amphibians bounced back spectacularly from a series of mass extinctions during their evolution, according to a new genetic analysis.
In the present day, biologists see amphibians as sentinels of environmental change. The extreme sensitivity of their skin makes them more prone than other organisms to soaking up toxins and suffering from fungal infection, and provides little protection from ultraviolet radiation, which causes genetic mutation.
Many amphibian populations have declined in recent years. At least 43% of populations of amphibian species are declining, some research suggests (see, for example, Global frog crisis defies explanation).
That said, a look at their latest evolutionary tree reveals that amphibians have a remarkable capacity to bounce back from environmental changes, says Kim Roelants of Vrije University in Brussels, and colleagues.
Their new research suggests that the amphibian class as a whole has recovered spectacularly from a series of mass extinction events that affected all life on earth, with explosive booms of speciation. "For instance, right after dinosaurs went extinct 65 million years ago, at the Cretaceous-Tertiary border, there was a huge explosion of frog species," he says.
Until now, biologists have lacked a detailed evolutionary tree of amphibians: fossil records of amphibians are scarce. So the researchers used genetic analysis of amphibian DNA to reverse-engineer an evolutionary tree.
They did this by taking equivalent fragments of DNA from 171 existing species. By looking at the similarity between corresponding fragments of DNA from two species, for example, they were able to estimate when each common ancestor diverged into two new species. Generally, very similar fragments indicate a more recent common ancestor than dissimilar fragments.
The work revealed that todays amphibians have three common ancestors, which arose around 350 million years ago. That trio suddenly branched out between 250 million and 225 million years ago around the time of a global mass extinction at the end of the Permian Period, when some 95% of all life forms had disappeared from the face of the planet. The three eventually gave rise to frogs and toads, salamanders, and caecilians (snake-like animals that live underground), respectively. This boom, called explosive radiation, gave rise to the gamut of amphibians we know today.
Another two booms happened around the Cretaceous-Tertiary extinction, about 65 million years ago. In total, approximately 86% of frog species alive today and more than 81% of salamander species descend from just five amphibian species that survived this mass extinction 65 million years ago.
Explosive radiations happen when a biodiversity crisis, such as a mass extinction, opens up a lot of new habitats and ecological niches for the few surviving species to expand into. Fossil records and DNA studies suggest that there was huge change in vegetation at the Cretaceous-Tertiary border, giving rise to modern flowering plants. This change could have given amphibians a range of new unexploited habitats. For example, the first known tree-frogs and tree-dwelling salamanders first appeared around this time, says Roelants.
Roelants warns that an ability to bounce back in the past does not necessarily mean amphibians will show the same resilience in the future. "It is unfortunate, but you can't talk of super-amphibians," he says.
Falling populations of amphibians today have concerned many conservation biologists (see Global frog crisis defies explanation).
"It is likely that survivors of [todays] extinction will re-diversify but this will probably take thousands or million of years," says Roelants. Which species survive will simply be down to chance, he says. Some may have characteristics that allow them to survive in a new environment, others not.
Toads, for instance, are among the hardier amphibians Roelants has seen some thriving in the sewers of Asian cities. Yet they are suffering more than many other families from the current decline in amphibians (see Global warming fuels fungal toad-killer).
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073 pnas.0608378104)