Tiny Frogs Reveal Big Secrets of Plate Tectonics

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Around 55 million years ago, India and China collided. Some time after that — geologists disagree on exactly when this happened — the Himalayan-Tibetan plateau rose skyward, creating the rugged landscape that today attracts tourists and mountain climbers from all over the world. 

Now a new study has constructed the evolution of various frog species found across eastern Asia, giving geologists a genetic clock by which they can time the upheaval of the region known as "the roof of the world."

In a new article in the journal Proceedings of the National Academy of Sciences, researchers led by David Wake of the University of California, Berkeley detailed how four different waves of divergence among frog species — the point when new species are created from a common ancestor — can be explained by four major tectonic events in the region. 

The uplift caused by India slamming into the Eurasia tectonic plate created natural barriers that isolated the frogs and led to a diversification in species over time. Wake explained this process in an interview with UC Berkley News:

"What we have here is a group of very old frogs that are so fixed to their habitats that they just stuck there, sitting on that mountain mass when it got raised up. They were separated by these uplifts and by the rivers between the mountains into different units, and these give us a fix on the timing of geological events."

The first split in frog lineage occurred around 27 million years ago: Nanorana frogs evolved north and northeast of the Himalaya-Tibetan plateau, and Quasipaa amphibians evolved east and southeast of the plateau.

Further diversification of species occurred around 23, 19, and 9 million years ago as new episodes of geologic chaos played out, thrusting up the world's tallest mountain range, changing climate, and reshaping the landscape through much of southeast Asia.

The breaks in frog speciation every couple of million years supports a less-accepted geologic theory: that the Indian plate pushed into the Eurasian plate in fits and starts, rather than continuously. Additionally, this study provides time-markers for when those "pushes" occurred. 

Today the various frog species share two specific characteristics — robust spines and muscular arms — that represent adaptations for living in steep, rushing mountain rivers. Male frogs use their Popeye-like arms to tightly hug their mates during periods of intense water flow.

Because of their similar physical appearance, scientists had no idea the frogs actually belonged to separate species. It wasn't until researchers performed a genetic analysis that researchers discovered that these traits in fact evolved independently. 

Image: Yu Zeng, UC Berkley

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