Long-term lizard study challenges the rules of evolutionary biology
By lassoing lizards, putting tiny chips on their legs, and tracking them for three years, Georgia Tech's James Stroud revealed why species often appear unchanged for millions of years despite Charles Darwin's theory of constant evolution
Peer-Reviewed PublicationIMAGE:
A FEMALE BARK ANOLE (ANOLIS DISTICHUS) LIZARD.
view moreCREDIT: JON SUH
Charles Darwin said that evolution was constantly happening, causing animals to adapt for survival. But many of his contemporaries disagreed. If evolution is always causing things to change, they asked, then how is it that two fossils from the same species, found in the same location, can look identical despite being 50 million years apart in age?
Everything changed in the past 40 years, when an explosion of evolutionary studies proved that evolution can and does occur rapidly — even from one generation to the next. Evolutionary biologists were thrilled, but the findings reinforced the same paradox: If evolution can happen so fast, then why do most species on Earth continue to appear the same for many millions of years?
This is known as the paradox of stasis, and James Stroud, assistant professor in the School of Biological Sciences at the Georgia Institute of Technology, set out to investigate it. He conducted a long-term study in a community of lizards, measuring how evolution unfolds in the wild across multiple species. In doing so, he may have found the answer to one of evolution’s greatest challenges.
His research was published as the cover story in the Proceedings of the Natural Academy of Sciences.
“We call this a paradox because it doesn’t seem to make any sense,” Stroud said. “The most common explanation is that natural selection is working to stabilize a species’ appearance, with the assumption that an average form will help them survive the best. The problem is, when people do field studies, they almost never find that this kind of ‘stabilizing’ selection actually exists.”
Lassoing Lizards
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.
A Picture of Evolution Is Worth a Thousand Lizards
By incorporating data for each time period, Stroud captured the history of every lizard in the community. He then related survival data to the variation in body traits, which allowed him to analyze which body traits were important predictors of survival. Taken together, the analysis painted a picture of how natural selection operated on the community as a whole.
To his surprise, Stroud found that the stabilizing form of natural selection — that which maintains a species’ same, average features — was extremely rare. In fact, natural selection varied massively through time. Some years, lizards with longer legs would survive better, and other years, lizards with shorter legs fared better. For other times, there was no clear pattern at all.
“The most fascinating result is that natural selection was extremely variable through time,” Stroud said. “We often saw that selection would completely flip in direction from one year to the next. When combined into a long-term pattern, however, all this variation effectively canceled itself out: Species remained remarkably similar across the entire time period.”
The findings provided by Stroud’s study had never been seen before. There had never been such insight into how selection works on a community level, and certainly not at this level of detail.
The reason scientists never understood how evolution works on the community level is because long-term studies like Stroud’s are extremely rare. Researchers are unlikely to undertake such projects because of the great amount of work and time required.
“Evolution can and does happen — it’s this ongoing process, but it doesn't necessarily mean things are constantly changing in the long run,” Stroud said. “Now we know that even if animals appear to be staying the same, evolution is still happening.”
According to Stroud, understanding evolution is critical to everything that we want to understand about life on Earth.
“Understanding evolution doesn’t only help us understand the plants and animals around us and how they're distributed across the world,” he said. “It also shows us how life sustains itself in a world dominated by humans.”
There have been very few studies that monitor how evolution unfolds in the wild at long time scales. That, according to Stroud, is why we have a biased view of what evolution is.
“For a very long time, evolutionary biologists have tried to figure out what was behind this paradox of stasis idea,” Stroud said. “What this study shows is that the answer may not be particularly complicated — we just had to conduct a study in the wild for a long enough time to figure it out.”
A green anole lizard (Anolis carolinensis).
Taking high-resolution photographs of lizard feet to measure the size of adhesive sub-digital toepads.
Researchers identified the lizards by harmless blacklight tags that they implanted under the skin of their legs.
James Stroud uses a tiny lasso attached to a fishing pole to catch a lizard.
James Stroud uses a tiny lasso attached to a fishing pole to catch a lizard.
CREDIT
Day's Edge Prods
Day's Edge Prods
Citation: Stroud, J.T., et al. “Fluctuating selection maintains distinct species phenotypes in an ecological community in the wild.” PNAS, Oct. 2023.
DOI: https://doi.org/10.1073/pnas.2222071120
Writer: Catherine Barzler
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The Georgia Institute of Technology, or Georgia Tech, is one of the top public research universities in the U.S., developing leaders who advance technology and improve the human condition. The Institute offers business, computing, design, engineering, liberal arts, and sciences degrees. Its more than 45,000 undergraduate and graduate students, representing 50 states and more than 148 countries, study at the main campus in Atlanta, at campuses in France and China, and through distance and online learning. As a leading technological university, Georgia Tech is an engine of economic development for Georgia, the Southeast, and the nation, conducting more than $1 billion in research annually for government, industry, and society.
JOURNAL
Proceedings of the National Academy of Sciences
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Fluctuating selection maintains distinct species phenotypes in an ecological community in the wild
ARTICLE PUBLICATION DATE
9-Oct-2023
No lizard is an island
Many species experience little to no change over long periods of time. Biologists often fall back on the same explanation for why this is true: that natural selection favors individuals with more moderate characteristics. Individuals with more extreme features — longer limbs, for example — have a disadvantage, while more moderate or average individuals are more likely to survive and reproduce, passing on their common features.
But new research from Washington University in St. Louis and the Georgia Institute of Technology provides a more complete explanation of how evolution plays out among species that live side-by-side. By directly measuring the long-term survival of lizards in the wild, the scientists showed that co-existing species each occupy a distinct “fitness peak” that is best understood as part of a communitywide “fitness surface” or landscape.
The study, led by James Stroud at Georgia Tech and publishing this week in the Proceedings of the National Academy of Sciences, offers a new way of thinking about how species relate to each other over time and how the differences between them reinforce their distinctness.
Jonathan Losos, the William H. Danforth Distinguished University Professor and a professor of biology in Arts & Sciences at Washington University, said: “If species are adapted to their environment, and the environment doesn’t change, then you wouldn’t expect the species to change. However, when scientists have gone out and studied natural selection, they rarely find evidence of such stabilizing selection.
“Given this disconnect, we set out to study natural selection on the organisms we know so well, Anolis lizards, to measure selection over several years and try to understand what’s going on,” Losos said.
Stroud, who was working as a postdoctoral researcher in Losos’ lab at WashU at the time, identified a place where four different species of anoles were living together on a small island in a lake in the Fairchild Tropical Botanical Garden near Miami.
He caught thousands of individual lizards on the island, tagged them and measured their body proportions. Stroud then re-caught all of the lizards on the island every six months for 2 ½ years, a period of time representing two to three generations of lizards.
New lizards that showed up were island babies, obviously. If a lizard disappeared from his census rolls, it was safe for Stroud to assume it had died, because the surrounding lake, filled with predatory fish, didn’t let them leave. By determining which lizards survived from one year to the next, the researchers could evaluate whether survival was related to the body traits they had been measuring, like leg length.
“What is special about this study is that we simultaneously measured natural selection on four co-existing species, something that has rarely been accomplished,” said Losos, who also serves as the director of the Living Earth Collaborative. “By coincidence, just as our paper was published, another group published a similar study on Darwin’s famous finches of the Galapagos Islands.”
In the Florida lizards, Losos and Stroud found that the stabilizing form of natural selection — that which maintains a species’ same, average features — was extremely rare. In fact, natural selection varied massively through time. Some years, lizards with longer legs would survive better, and other years, lizards with shorter legs fared better. For other times, there was no clear pattern at all.
“The most fascinating result is that natural selection was extremely variable through time,” Stroud said. “We often saw that selection would completely flip in direction from one year to the next. When combined into a long-term pattern, however, all this variation effectively canceled itself out: species remained remarkably similar across the entire time period.”
Scientists do not yet fully understand how evolution works on the community level. There are very few long-term studies like this one because of the great amount of work and time required.
“Evolution can and does happen — it’s this ongoing process, but it doesn’t necessarily mean things are constantly changing in the long run,” Stroud said. “Now we know that even if animals appear to be staying the same, evolution is still happening.”
JOURNAL
Proceedings of the National Academy of Sciences
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Fluctuating selection maintains distinct species phenotypes in an ecological community in the wild
ARTICLE PUBLICATION DATE
9-Oct-2023
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