We may complain about freezing temperatures, but most cold snaps leave us little worse for the wear. That’s not the case for a common lizard living on the Texas-Mexico border, which, in just the span of a few months, underwent a dramatic genetic transformation in response to cold weather. In fact—in one of the most detailed examples of rapid evolution to date—a new study shows that just one cold snap can change the way green anoles’ muscular and nervous systems respond to temperature.
“It’s a very conclusive instance of rapid evolution,” says Charles Brown, a behavioral ecologist at the University of Tulsa in Oklahoma, who was not involved with the work. The study, he says, is “one of the only real examples in which the genetic mechanisms behind these rapid evolutionary events have been shown.” And Michael Logan, an evolutionary ecologist at the Smithsonian Tropical Research Institute in Panama City, calls the work “the most comprehensive demonstration of natural selection to date” on how body temperature is regulated.
Previous studies have documented weather-linked changes in animal shape and size. As far back as 1898, biologists found a population of sparrows that became bigger after a bad snowstorm. Brown’s research into cliff swallows has shown similar shifts in body shape and even behavior. And other studies of green anoles have shown changes, too: Anoles that moved to a new island with more trees than bushes evolved longer legs, perhaps to travel greater distances, whereas anoles living in cities have apparently evolved stickier feet to cling to metal and glass. Just this week, researchers reported that Brazilian geckos isolated on islands have evolved larger heads to eat bigger termites than they had access to before.
But Harvard University graduate student Shane Campbell-Staton was interested in something else—anoles’ ability to adapt to the cold. Green anoles are brightly colored lizards that can easily switch their coloration from green to brown. Their ancestors hail from Cuba, but the slender reptiles long ago spread across the U.S. Southeast, and they’re often sold in pet stores under the name American chameleon.
Wanting to understand how the wandering lizards managed to adapt to cold temperatures on their journey north, Campbell-Staton looked over several years at five populations of Anolis carolinensis living at different latitudes. He tested their temperature response by putting them in a chamber that gradually cooled. When they get too cold, they lose the coordination to right themselves after they’ve been flipped over. The test was an easy way to assess at what temperature the anoles ceased to be able to function right.
He found that lizards near the southernmost end of the range—near Brownsville, Texas—became uncoordinated at about 11°C. Lizards near the northernmost end of the range, however, could right themselves in temperatures as low as 6°C. And he saw differences in gene activity in lizards from those two places.
He thought he and his colleagues were all done with his fieldwork when the winter of 2013–14 hit. It was a very unusual, extreme weather event. A polar vortex caused the lowest temperatures in 15 years in Texas and the rest of the southern United States. The snap lasted just a week in north Texas, but Brownsville, at the Mexican border, was “hammered,” says Ray Huey, an emeritus evolutionary physiologist at the University of Washington in Seattle who was not involved with the work. There, the cold lasted a month. So Campbell-Staton and his team went back in the spring to see how the extreme weather had affected the lizards. Typically, researchers studying the effects of such an event can only indirectly assess what the animals were like beforehand. But Campbell-Staton already had those data in hand. He “was lucky in one sense,” Huey says. “They saw an opportunity and jumped on it.”
When the researchers repeated the chamber experiments, they found the cold tolerance of the surviving Brownsville lizards had increased—the animals could right themselves down to 6°C, just like the northernmost lizards, Campbell-Staton and his colleagues report today in Science. And their DNA reflected this change. The repertoire of active genes in their livers more closely resembled those of the northern Texas lizards, including genes involved in the functioning of the nervous and muscular systems. That could help the chilled lizards maintain their coordination, the researchers write. “They got the whole animal all the way down to genomics,” Huey says. “It’s not often that you see [coverage] of such diverse levels of biological organization in the response to an extreme evolutionary event.”
“I think these unusual weather events may be an underappreciated mechanism underlying rapid evolution,” Brown says. To see this 100 years after the sparrow study, but with additional techniques, is “really pleasing,” Huey adds.
But as Butch Brodie, director of the University of Virginia Mountain Lake Biological Station in Pembroke points out, it's not clear whether evolution will continue to push the lizards to be ever more cold tolerant. “The temptation from results like these is to assume extreme events drive much of evolutionary change, but it may not be that simple,” he says. It could be that cold tolerance comes at a cost to the lizard, he notes, so that in normal or warm winters, the cold-resistant lizards would be at a disadvantage. “I hope they will follow these populations over time to see if the cold tolerance stays at this new level,” Huey says.
Tracking these lizards to see their evolution over the long term could help researchers understand whether evolution runs in fits and starts or proceeds steadily over time, an insight that would help us understand evolution more broadly.
Campbell-Staton, now at the University of Illinois in Urbana, says he plans to continue visiting Texas to do just that.