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Amphibians: Evolution?s comeback kings

india Updated: Jan 10, 2007 19:32 IST
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A new genetic analysis carried out by a researcher at Vrije University in Brussels and his colleagues has claimed that amphibians have successfully bounced back from a series of mass extinctions during their evolution.

According to Kim Roelants and his colleagues, amphibians have a remarkable capacity to bounce back from environmental changes, as in the view of biologists, they are the sentinels of environmental change.

Roelants and his colleagues further go on to say that the extreme sensitivity of their skin makes amphibians susceptible to the ravages of genetic mutation, and therefore, it comes as no surprise that at least 43 percent of the amphibian population is in decline, but have a tremendous capacity to regenerate.

The researchers came to their conclusion after conducting a 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 today’s 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 percent of all life forms had disappeared from the face of the planet. 

 
Modern-day amphibians

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 percent of frog species alive today, and more than 81 percent 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.

Roelants, however, warns that an ability to bounce back in the past does not necessarily mean amphibians will show the same resilience in the future.

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