There are still people out there who don’t believe in evolution, and frankly, we don’t want to start that argument. Instead, we’d prefer to just point to those moments when natural selection made itself suddenly, dramatically known. Like a couple of years ago, when a single winter storm changed the genetic makeup of an entire species of Texan reptile. Meet the green anole lizard.
In the summer of 2013, biologist Shane Campbell-Staton was elbow-deep in a DNA study of several dozen anole lizards ranging from the southern tip of Texas all the way up to Oklahoma. The purpose of his research? To discover how this subtropical reptile could survive in the (relatively) cold environment of the southern United States. And then the 2014 cold snap hit.
On January 2, an arctic front cut across the entire continent, resulting in record lows from coast to coast. And when Campbell-Staton saw a picture of an anole lizard lying dead in the snow, he realized that he had the chance to see exactly how the genes that he was studying reacted to their greatest challenge yet.
What he found was a perfect example of evolution in action. Not all of the anole lizards had perished in the cold, and the offspring of those that had turned out to be much better suited to less-than-tropical temperatures. Most interestingly, it was the lizards from southern Texas who had changed the most — their northern cousins hadn’t had to make as extreme of an adjustment. Those results were borne out not only in lab tests that actually pitted the lizards against low temps, but were also reflected in DNA markers that clearly separated them from their heat-loving ancestors. And it would never have happened if it weren’t for that January storm.
These kinds of easily traceable, all-at-once evolutions don’t happen a lot, but when they do, they can usually be traced to a single, cataclysmic event. In 1898, the wonderfully named Hermon Bumpus took note of how another sudden cold spell altered Rhode Island’s population of house sparrows. After the storm, he and his assistants collected a total of 136 sparrows that had been driven to the ground. 72 of these eventually recovered; the other 64 did not survive their ordeal. Bumpus took careful note of the survivors and documented a certain set of physical characteristics that set them apart from their unlucky companions. The more successful traits weren’t particularly useful before the storm hit, but they ended being the only ones to be passed on to the next generation afterward.
There’s another example, and this one is perhaps even more dramatic even if it’s not quite as sudden. The peppered moth is a common insect in Ireland and Great Britain, and gets its name from its white wings spattered with black speckles. But every once in awhile, one of the moths would be born with almost entirely black wings. The dark-colored moths stood out against the pale trees, and could be easily seen by predatory birds. But then the Industrial Revolution struck. The trees in the cities began to be coated in dark soot, and suddenly only the dark-colored peppered moths — which had previously been genetic anomalies — became the most common variety. If that’s not evidence enough, then just consider this: when cleaning efforts left the trees pale once again, the original white-with-black-spots breed got back on top once more. Bam. In evolutionary biology circles, that’s what’s known as a mic drop.
References: (1) https://gizmodo.com/a-single-winter-storm-forced-these-lizards-to-start-evo-1797537297 (2) https://science.sciencemag.org/content/357/6350/495 (3) https://www.fieldmuseum.org/blog/hermon-bumpus-and-house-sparrows (4) https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.1136