Extremely divergent skinks offer insight into how evolution works

A diverse group of lizards known as social skinks is helping scientists answer questions about some fundamental mechanisms of evolution.

Skinks are a large group of lizards that generally follow a similar body plan.

But one group, known as the social skinks of the tribe Tiliquini, exhibits a wide variety of forms, from large-bodied, short-legged species to long-tailed, long-legged animals. These wildly divergent skinks give biologists an opportunity to study the very foundations of evolution.

They all evolved over the past 20 million years, allowing researchers to track how different species developed different traits.

Dr Ian Brennan is a researcher at the Natural History Museum who studies reptiles. He was part of the team that built a new family tree for these skinks and then used it to answer questions about their evolution.

“This particular group has become really strange,” Brennan says. “And that amount of morphological variation over a relatively short period of time is quite interesting.”

“Our first goal was to build an evolutionary tree so that the group could understand their relationships and appreciate how these species are related to each other. Once we had that tree, our second goal was to look at how they evolved from a standard skink form, which they look like at the base of the tree, to these really derived, really modified forms that are deeply embedded in the tree.”

They found that lizards appear to have gone through periods of relatively slow and steady evolution, punctuated by periods of rapid change. The findings are published in the journal Current Biology.

Let’s talk about skinks

There are about 1,500 species of skinks, distributed in almost every environment except the polar and subarctic regions. Among them are species that dig burrows, others that climb, and still others that swim.

Despite this wide geographic distribution and diversity of behavior, most skinks look very similar. Their basic shape is that of a smooth, tubular body, well-developed legs, and a poorly developed neck.

“Skinks are a group that is generally quite well preserved morphologically,” Brennan says. “For example, when someone who knows about lizards sees a skink, they think, ‘That’s a skink!'”

“You just know what they look like.”

Social skinks are a little different. Named so because, unusually for lizards, many species in this group live in highly gregarious family groups, they also push the boundaries of what skinks look like.

Some species have shrunk, developed short tails and legs, and are covered in spines. This allows them to fit into crevices in trees and bark. Others have developed incredibly long, prehensile tails and long legs that allow them to climb through the treetops. While blue-tongued skinks have increased in size, become larger, and cover themselves with bone-encrusted skin as they move through the Australian outback.

It is this diversity of characteristics that has allowed Brennan and his colleagues to study some of the foundations of evolution.

Evolutionary responses

One evolutionary theory suggests that the evolution of different traits, such as large body size or elongated limbs, occurs slowly over time. This would mean that species like the blue-tongued skink would have developed each distinctive feature gradually, in a process known as Darwinian gradualism.

Another theory, called the Simpsonian leap, argues that these features can appear suddenly. This would mean that blue-tongued skinks developed their characteristic short limbs and elongated bodies in a very short time.

The researchers found a mix of both phenomena: evolution was gradual at times, but it was punctuated by rapid bursts of inventiveness.

“We tested this idea by using comparative methods to look at the evolution of these traits,” Brennan says. “And we found that for most traits, we see this heterogeneous pattern.”

“Evolution can be gradual for long periods of time, but then it is punctuated by periods when enormous changes occur.”

It seems likely that these rapid bursts of evolution are related, for example, to species arriving on a new island or exploring a new lifestyle, such as becoming nocturnal. And it’s likely that this pattern is much more widespread in nature than previously thought.

“Where we’ve studied things, jumping patterns are more common than we would have thought,” Brennan says. “But we haven’t really studied enough groups to be sure.”

This relatively small group of lizards helps researchers answer some key questions about evolution, but it could also shed light on other interesting topics. Brennan hopes the new evolutionary tree could in the future shed light on how and why sociality evolved in reptiles.

This article is republished with the kind permission of the Natural History Museum. Read the original article here.