The Hemidactylus geckos have evolved into several distinct species in the Indian peninsula, some of which often show up as uninvited guests in our houses. Researchers at the Center for Ecological Sciences, Indian Institute of Science (CES-IISc) have shown that differences in morphology among species of ground-dwelling geckos can indicate changes in the past climate of peninsular India.
“You might not appreciate how pretty they are,” says Aparna Lajmi of the geckos she studied over the course of her PhD at the Indian Institute of Science (IISc), Bengaluru. She is the lead author of a new study linking these little critters to changing past climates in the subcontinent. “Beyond the pale ones we see in our house, one can find many related species in the wild which have different body types and patterns, and they’re all pretty,” Lajmi says.
Lajmi and her colleagues found that the evolutionary timescales over which distinct body forms started appearing in different species of geckos correlate with changes in their habitat as a result of the evolving climate in the Indian peninsula millions of years ago.
Fossil records indicate that geckos belonging to the genus Hemidactylus first appeared on the Indian subcontinent about 36 million years ago. From their common ancestor, the geckos today have evolved into many species which can be divided into 2 broad categories: the scansorial kind which has the ability to climb vertical walls and live in rocks, crevices, trees and urban households, and the terrestrial kind which lives mainly on flat ground. The former are generally longer and have enlarged toepads which help them climb.
How did these different lineages develop? Whenever a group of organisms first occupies an entirely new patch of land, they tend to quickly diversify. This leads to a burst in the formation of new species. This is because competition drives individuals to occupy different comfort zones in terms of the place they live, the food they eat and so on. Each comfort zone forms what is known as an ‘ecological niche’. A group of individuals adapting to a particular niche will start accumulating characteristics unique to surviving in that niche and will eventually diverge to form a new species.
Many such niches give rise to many closely related but distinct species evolving from a common ancestor, like rays emanating from a common point. This phenomenon is called ‘adaptive radiation’.
This is what Darwin observed in the famed finches of the tiny Galapagos islands. The tiny islands harboured as many as 15 different species of finches with slight variations in their features, most prominently in the size and shape of their beaks. Darwin realized that this resulted from the fact that each type of finch had uniquely adapted to eat a particular kind of food like seeds, insects and nectar — which formed the niche for that particular species.
Lajmi and her colleagues came across this kind of differentiation while sampling for terrestrial geckos in the arid landscape of the Deccan plateau. They discovered a new species which showed features different from anything seen before. Speaking over the phone with her advisor Praveen Karanth soon after, Lajmi recalls exulting “Wow, this looks like a case of adaptive radiation!”.
Originally working on a different question, the researchers diverted their attention to closely studying the morphological differences between different species of ground-dwelling Hemidactylus geckos. Among these, they observed many different forms that seemed to correlate with different kinds of habitat.Ground-dwelling (terrestrial) geckos underwent rapid diversification when a drier climate converted forests to grasslands on the Indian peninsula (Note: Geological epochs not drawn to scale)
When these geckos originated in India 36 million years ago, they were all of the climbing kind. Interestingly, the researchers found that geckos began to live on the ground and diversified into several different ground-dwelling species much later than this, around 14 million years ago. This is contrary to the expectation of the ‘early-burst’ model of adaptive radiation, which says that most of the species diversification occurs immediately after the first appearance of a population.
“It was expected that the ground-dwelling type of morphology evolved independently from the more climbing variety of geckos, but the timing of when that happened is the most exciting thing here,” says Aaron Bauer, a Professor of Biology at Villanova University, USA, and an expert on geckos who was not involved with this study.
What caused this delayed diversification? The period around 14 million years ago is referred to as the late Miocene era. The climate in the Indian subcontinent at that time morphed into much drier conditions which led to the conversion of forest habitats into dry grasslands. This created new niches for geckos to adapt, which in turn led to the development of distinct morphological features to facilitate their terrestrial life.
By linking the diversification of terrestrial geckos to the late Miocene era, the researchers have placed an important timestamp on when the climatic conditions governing their habitat changed. “The climate change seen during the Miocene era is a global phenomenon, and this is one of the first studies to show that the same is also true for India,” Bauer notes.
Species diversification is a complex process that depends on a multitude of factors. While it is hard to definitively link the evolution of ground-dwelling gecko species to changing climates, there is a strong correlation. The case can be strengthened by observing the same patterns in the evolution of other organisms around the same period, which is an area of active research.
This study serves as an example of how investigating the glorious diversity of life on the planet we now call home can provide snapshots of how it looked like when it was our cradle.
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This is a companion discussion topic for the original entry at https://indiabioscience.org/news/2019/evolution-of-geckos-linked-with-past-climatic-conditions