Biologist may have solved one of evolution’s greatest challenges

Charles Darwin proposed that evolution was a continuous process driving species to adapt for survival, a notion that was met with skepticism by some of his contemporaries. They posed a puzzling question: If evolution is ongoing, how can fossils from the same species, discovered in the same location, appear identical despite a vast age gap of 50 million years?

In recent decades, a surge in evolutionary research has unveiled rapid evolutionary changes, sometimes occurring within a single generation. While this excited evolutionary biologists, it also deepened the paradox: If evolution can be swift, why do many species on Earth remain virtually unchanged for millions of years? This enigma is known as the paradox of stasis.

James Stroud, an assistant professor at the Georgia Institute of Technology’s School of Biological Sciences, embarked on a comprehensive study to investigate this conundrum. He conducted long-term research in a community of lizards, observing how evolution unfolds naturally across multiple species. In this pursuit, Stroud may have unraveled one of evolution’s most perplexing challenges.

His groundbreaking research, featured as the cover story in the Proceedings of the National Academy of Sciences, tackles this paradox. Stroud remarked, “We refer to this as a paradox because it defies conventional logic. The prevailing explanation suggests that natural selection strives to stabilize a species’ appearance, assuming that an average form provides the best chances of survival. The hitch is that field studies rarely support the existence of this presumed ‘stabilizing’ selection.”

A green anole lizard (Anolis carolinensis). Credit: Day’s Edge Prods

Lassoing lizards

In pursuit of answers, Stroud initiated a comprehensive field study centered around four distinct species of Anolis lizards, commonly known as anoles. This research unfolded on a small island nestled within the Fairchild Tropical Botanic Gardens in Coral Gables, Florida. Over the course of five consecutive time periods, Stroud meticulously assessed natural selection by meticulously tracking every lizard’s survival on the island.

Determined to collect data, Stroud and his colleagues tirelessly scoured the island day and night, employing specialized tools—a set of elongated fishing poles equipped with small lassos at their tips. With utmost care, they captured the lizards by their robust necks, placing them into coolers. Notably, each lizard’s precise location, whether perched on a branch or nestled on a stump, was meticulously recorded.

Once back in the laboratory, Stroud embarked on an exhaustive examination of these lizards. He measured various attributes, including their head dimensions, leg and foot characteristics, weight, and even the adhesive properties of their toes. To facilitate individual identification, each lizard received a unique numerical assignment and was discreetly marked with a minuscule tag beneath the skin. Subsequently, these lizards were returned to their original branches, mirroring the locations of their initial discovery. The research team continued their work in the ensuing days and weeks to capture and assess the remaining lizards.

Every six months, over a span of three years, Stroud and his dedicated team repeated this process diligently. Their ongoing efforts included capturing the same lizards, conducting precise measurements, releasing them into their familiar habitats, and meticulously documenting which lizards thrived and which faced different outcomes.

James Stroud uses a tiny lasso attached to a fishing pole to catch a lizard. Credit: Day’s Edge Prods

A picture of evolution is worth a thousand lizards

By meticulously integrating data from each time period, Stroud effectively chronicled the life history of every lizard within the community. He then established correlations between survival data and the diversity of body traits, enabling a comprehensive analysis of which physical characteristics proved to be pivotal predictors of survival. Collectively, this analytical approach provided insights into how natural selection operated within the entire community.

Much to his astonishment, Stroud’s findings challenged the prevailing notion of stabilizing natural selection, which typically maintains a species’ consistent, average characteristics. In reality, he discovered that natural selection exhibited substantial variation over time. Some years favored lizards with longer legs, while in other years, those with shorter legs thrived. At times, no clear patterns emerged at all.

Stroud remarked, “The most intriguing revelation is the remarkable variability in natural selection across time. We frequently observed that selection would dramatically shift direction from one year to the next. However, when this variation was examined over a long-term perspective, it effectively neutralized itself: species remained remarkably consistent throughout the entire timeframe.”

Stroud’s study unveiled unprecedented insights into how selection operates at the community level, offering a level of detail previously unattained. This newfound understanding was a result of the scarcity of long-term studies like Stroud’s, which demand substantial effort and time.

“Evolution is an ongoing process—it happens continuously, but it doesn’t necessarily translate into constant change over the long haul,” Stroud emphasized. “Now, we comprehend that even when it appears that animals remain unchanged, evolution is still at play.”

Taking high-resolution photographs of lizard feet to measure the size of adhesive sub-digital toepads. Credit: Day’s Edge Prod
Researchers identified the lizards by harmless blacklight tags that they implanted under the skin of their legs. Credit: Day’s Edge Prods

According to Stroud, grasping the mechanisms of evolution is pivotal for comprehending various facets of life on Earth. He explained, “Understanding evolution not only enhances our knowledge of the plant and animal kingdoms and their global distribution but also offers insights into how life adapts and endures in a world increasingly influenced by human activity.”

The scarcity of studies monitoring the evolution of species in the wild over extended timeframes, as per Stroud, has skewed our perception of what evolution truly entails. “For a considerable duration, evolutionary biologists grappled with unraveling the mystery behind the paradox of stasis,” Stroud noted. “What this study underscores is that the solution may not be excessively intricate—it merely necessitated a protracted, real-world study to unveil it.”

Source: Georgia Institute of Technology

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