Scientists say a single unusually cold season altered the course of history for one American reptile. The green anoles who survived the winter of 2014 were those who could stand the cold—a trait they passed on to their offspring. The researchers published their findings in the journal Science.

The green anole, Anolis carolensis, also known erroneously as the American chameleon, is a vibrant little lizard that makes its home in the southeastern U.S. and Caribbean. Its range extends from Texas as far north as Oklahoma. This is unusual for reptiles, whose cold-blooded bodies typically restrict them to balmier climates.

Graphic by Julie McMahon

To find out how the anoles were managing it, Shane Campbell-Staton, now of the University of Illinois, paid visits to five scattered populations in 2013. He collected samples and a few live lizards from each group to test their DNA, gene expression, and tolerance for low temperatures.

He found a fair amount of variation between lizard communities. Those in Oklahoma had clearly evolved to handle the weather there, while specimens from further south couldn't take the cold.

Satisfied with his data and findings, Campbell-Staton prepared to wrap up the project.

Then winter came. You may remember the winter of 2014, when a polar vortex created record-breaking low temperatures and wrought terrible storms across the U.S., including in anole territory. Campbell-Staton couldn't help but wonder how—or if—the cold-intolerant lizards had survived.

The next spring and summer, he and his colleagues made another circuit through anole country and collected more samples. The Oklahoma families hadn't fared too poorly. But down south, things had clearly changed. The genetic code of Texan lizards looked more like their northern cousins, and individuals were far better at handling a chill. 

The research team realized that the brutal winter had killed off most of the cold-intolerant lizards, leaving behind only those who happened to have genes more like their northern cousins'. Those lizards reproduced, creating new generations of cold-ready individuals.

But that's not necessarily a good thing.

"One might think, 'Oh, they responded! They're better now,'" Campbell-Staton said in a statement. "But selection always comes at a cost, which is death, basically. It may be that the animals that did not survive this storm had the genetic variants to survive a heat wave, or a drought, or some other extreme event. And now those lineages are essentially gone."