Over and over again, on island after island, the anoles evolved to fill different niches, gaining characteristic sets of traits to help their survival in their preferred habitat. One species kept long legs — ideal for sprinting — and small, sticky toe pads more often planted on terra firma. Three others scampered up tree trunks: a small-bodied species that preferred the lower half of the trunk, one that ventured into the low canopy on large toe pads, and one that favored the high canopy, evolving short limbs to expertly navigate thin branches.

After that initial burst of evolution, the lizards remained virtually identical over millions of years. And that’s how Losos found them when he began studying the reptiles in the 1980s.

“The different types seem to have evolved a long time ago, and then stuck there,” Losos said. “Presumably they’ve been like that ever since.”

The anoles’ ability to colonize new land made them well suited to becoming invasive species. In Florida, the native North American green anole (Anolis carolinensis) has lived high up on tree trunks, consuming arboreal insects in the low canopy, for millions of years. Over the past century, however, other anoles have arrived in the state from Cuba, Hispaniola and the Bahamas. The brown anole (Anolis sagrei) dwells on lowermost tree trunks, using its long legs to jump onto the ground to hunt insects. The small-bodied bark anole (Anolis distichus) eats ants crawling along trunks, while the larger knight anole (Anolis equestris) pursues insects and fruit in the upper canopy. Each species had already adapted to its specific niche before arriving in Miami. Their ecology persisted in their new home.

As a lizard enthusiast, Stroud wanted to study his adopted city’s herpetological smorgasbord. To conduct a long-term field study, however, he would need to track the anoles over time. The high mobility of the lizards posed a major problem. If he lost track of an individual, he wouldn’t know whether it had moved out of the area or died. Just as frustrating, he wouldn’t be able to tell if new arrivals were the offspring of existing lizards or new immigrants.

After scouring the city for sites, he realized that the location of Miami’s Fairchild Tropical Botanic Garden made it an ideal study spot because the anoles were effectively trapped on the ersatz island. He could be confident that no lizards had arrived or left.

Stroud’s goal was to measure natural selection operating over several generations in multiple species. He wanted to “catch lots of lizards and measure them and see if their survival told us anything about how evolution occurs in the wild,” he said.

He spent three years taking a variety of measures of body shape and size from the four anoles that call the botanic garden home — 1,692 individuals in total. To gather thousands of data points on leg length, head size and overall survival, Stroud had to capture each lizard using a tiny lasso and then set to work with calipers before injecting a tiny microchip under its skin. The microchip ensured that he could keep track of each individual anole. If he couldn’t detect a tracker, he knew the anole had likely died.

“This type of work is hard enough to do in one species. So to execute a project like this in four species is really exceptional,” said Jill Anderson, an evolutionary biologist at the University of Georgia who was not involved in the research.

When Stroud began analyzing his data, however, he ran smack into the paradox of stasis.

Stasis in the Noise

From the beginning of the project, Stroud and his colleagues were interested in stabilizing selection. They wanted to see if the forces of natural selection continually pushed and pulled the lizards’ traits to keep them centered on the same point. That the anoles had shown little evolutionary change over millions of years indicated that they were on some sort of evolutionary peak, and Stroud wanted to see what factors kept them there.

However, his years of data didn’t show stability at all. Instead, he saw evolution constantly shifting the traits that were best adapted to the environment. “If we look at any one period on its own, we very rarely see stabilizing selection,” Stroud said.