An evolutionary museum of bush frogs

It's a spectacular 'evolutionary museum' of bush frogs out there, and a vital hub for diversification of the colourful croakers, ecological scientists sampling more than 60 species of the amphibians from the mountain ranges of Western Ghats say¹.

In biodiverse Western Ghats, along the western peninsular stretch of India, the high reaches of the mountains act as the hubs of diversification, and low- and mid-level high regions serve as evolutionary museums, they say in a new report. The reason behind this diversity? The region's geology, ecology, and climate — all these make it conducive for such species to thrive and diversify.

“This is the largest vertebrate radiation (a process through which organisms diversify) in the region,"says lead author S. P. Vijayakumar, a postdoctoral fellow at the Centre for Ecological Sciences at Indian Institute of Science (IISc).

Sampling species from 13 out of 14 group of mountains (or massifs) in the Western Ghats at different heights, they came across an astounding variety of bush frogs from within the same genus Raorchestes — more than 60 species.

Searching if they all had a common ancestor, the researchers found a large radiation splitting into two smaller ones — one in the south and the other in the north of the Ghats. The dividing line — the biogeographic barrier — is the Palghat Gap. Geology at deep time created two clades, with the Palghat Gap acting as dividing line for macro evolutionary patterns.

“We found around 34 species in the south and around 24 in the north," says Vijayakumar. Through sampling and analysis of 19 species of 'sister pairs' — the most closely-related lineages — they confirmed that these sister species occurred on adjoining mountains due to recent geology, with formation of valleys driving the diversification. But they also found sister species on the same mountain tops, across grasslands and shola forests.

The researchers also established that most of the diversification occurs within ancestral lineages in the elevation zone between 1200-1800 meters, which they call the “centres of diversification”.

Low- and mid-level elevations are home to older lineages. All these ancient lineages persisted here as if part of an open museum.

The study details the signatures of multiple processes acting on radiation within a small geographical region of the Ghats. These act at varying points in time.

“This is one of the few studies that examines the relative role of different processes. And we find that geography, climate and environmental gradients all play a role, but at different points in space and time,” says Kartik Shanker, a co-author of the study and an associate professor at the Center for Ecological Sciences at IISc.

So, geology in deep time through the Palghat Gap created two clades. Geology in recent times resulted in sister species across mountain tops. Ecological gradients along slopes resulted in speciation. And finally, climate triggered in-situ diversification within mountain tops.

“The study lays the foundation for understanding spatial patterns of diversity in this hotspot," says Vijayakumar.

Bush frogs are interesting creatures. They lay eggs upon vegetation, they are even known to dig burrows. They don’t depend on water for reproduction and don’t have a tadpole stage. Their calls range from insect-like to metallic tunes, from drip-drip of a leaky tap to a staccato tuk-tuk-tuk. With males around 20-25 mm in size, they can comfortably perch on human thumbnails and call.

“Evolutionary and biogeographic inferences can be really exciting when you combine field ecology, distribution modeling and genetics,” says Shanker. “For example, intensive sampling across all massifs, elevations and habitats allowed us to identify sister species with such confidence, enabling all of the remaining analysis,” he adds. The region — a gold mine for evolutionary ecologists — needs more exploration more to make more robust analyses.

(Pictures Credits: Vijayakumar, S. P.)