Evolution of Lizards: A New Study Reveals the Patterns and Drivers of Adaptation and Speciation

Charles Darwin proposed that evolution is a constant process that causes animals to adapt for survival. However, many fossils show that some species have remained unchanged for millions of years, despite the changing environment.

This is known as the paradox of stasis, and it has puzzled evolutionary biologists for a long time. How can evolution be both fast and slow at the same time?

A recent study by James Stroud, assistant professor in the School of Biological Sciences at the Georgia Institute of Technology, may have found the answer to this question.

He conducted a long-term study in a community of lizards, measuring how natural selection operates in the wild across multiple species.

His findings suggested that evolution is not always driven by natural selection, but also by random fluctuations in traits and survival.

His research was published as the cover story in the Proceedings of the National Academy of Sciences.

Lassoing lizards

(Photo : Ed Ram/Getty Images)

Stroud set up a field study with four different species of Anolis lizards (anoles) on a small island at the Fairchild Tropical Botanic Gardens in Coral Gables, Florida.

He measured natural selection in all four lizard species over five consecutive time periods by catching and monitoring the survival of every lizard on the island.

Stroud and his colleagues searched day and night for lizards.

Using long fishing poles with tiny lassos at their tips, they gently captured them by their strong necks, placed them in coolers, and documented the exact branch or stump where they found each lizard.

Back in the lab, Stroud measured the lizards' heads, legs, feet, weight, and even the stickiness of their toes.

After assigning an identifying number to each lizard and marking them with a tiny tag under the skin, the team released the lizards to the same branches where they'd found them.

They went out in the following days and weeks to catch the rest of them. Every six months for three years, Stroud and his team started the process over again.

Catching the same lizards, taking measurements, releasing them, and making notes of which lizards survived and which didn't.

Also Read: Climate Change and Lizards: Reproduction Strategy May Need to Change, Researchers Say

Evolution by chance

Stroud expected to find that natural selection was favoring certain traits that helped the lizards survive better in their habitats.

For example, he thought that lizards with bigger heads would have an advantage in eating larger prey, or that lizards with stickier toes would be able to cling better to smooth surfaces.

However, he was surprised to find that natural selection was not consistent or predictable across time or species.

Instead, he found that natural selection was often weak or absent, meaning that there was no clear relationship between traits and survival.

Moreover, he found that natural selection was often fluctuating or reversing, meaning that traits that were favored in one time period were disfavored in another, or vice versa.

For example, bigger-headed lizards had higher survival in one time period, but lower survival in another.

Stroud concluded that natural selection was not the only factor influencing evolution in these lizards. He suggested that random fluctuations in traits and survival, also known as genetic drift, were also important.

Genetic drift occurs when some individuals survive or reproduce more than others by chance, not because they have better traits. This can cause traits to change over time without any adaptive advantage.

Stroud's study shows that evolution can be both fast and slow depending on the balance between natural selection and genetic drift.

When natural selection is strong and consistent, evolution can be fast and adaptive. When natural selection is weak or fluctuating, evolution can be slow and random.

This may explain why some species appear unchanged for millions of years despite environmental changes.

Stroud's study also highlights the importance of long-term studies in evolutionary biology.

By tracking individual lizards over multiple generations, he was able to measure natural selection more accurately and realistically than previous studies that relied on snapshots or averages.

He hopes that his study will inspire more researchers to conduct long-term studies in other organisms and environments.