Evolution by Hurricanes

The western Atlantic Ocean is a hurricane hotspot, but 2017 was one of the worst years, with ten hurricanes. Most of the damage was caused by the three biggest ones – Harvey, which hit Texas and flooded Houston, Irma, and finally Maria, which caused heavy damage in Puerto Rico. Obviously, their damage is not exclusive to mankind, they also kill many of the animals that happen across their paths. But it appears that the storms do not kill indiscriminately: A new study shows that in two hurricane-hit islands, the surviving lizards were blessed with certain anatomical features. This is the first example of natural selection by hurricanes.

When Colin Donihue, a postdoctoral researcher from Harvard University, began his research, he did not have hurricanes or natural selection in mind. Preparing for research on invasive animal species in the Turks and Caicos Islands in the Caribbean, east of Cuba, he and his colleagues captured small lizards of the Anolis scriptus species and documented their anatomy: The length of their limbs, body weight and more.

Four days after they had concluded their measurements and left the area, hurricane Irma made landfall in the islands and two weeks later, so did Maria. The researchers decided to return to the islands and see how the ferocious winds had affected the lizard population. “We weren't sure what we'd find, but when we got to the field and saw a few lizards running around, we were eager to get catching and start measuring,” said Donihue. “We walked exactly the same transects we had the last time. There were definitely fewer lizards. We had to work harder to catch our sample size.”

They captured close to one hundred lizards, and their measurements clearly showed that the survivors possessed special traits. In both islands, the lizards’ front limbs were, on average, longer than those measured before the hurricanes, while their hind limbs were, on average, slightly shorter. Furthermore, the sticky toe pads on both front and hind legs were larger.

How can these changes be explained? One theory suggests that the hurricanes cause more harm to lizards with smaller toe pads, shorter front limbs and longer back limbs. Thus, after the hurricanes passed, the survivors possessed primarily the opposite traits: An example of natural selection, where individuals most adapted to the environment (the fittest) survive. Despite this, as not all the other possibilities have been ruled out, this is still only a theory. “Perhaps the hurricane blew in lizards with bigger toepads and shorter hind legs from another island. Or perhaps the act of clinging to the branches in high winds actually caused their forelegs to get longer.” Suggested Jonathan Losos, Donihue’s advisor.

Not that he thinks that these other explanations are highly probable. “Still, hurricane-induced natural selection seems like the best explanation for these findings,” he said. If the researchers will be able to show that the surviving lizards passed on to the next generation genes different from those of previous generations – for example, that genes for larger toe pads are now much more prevalent – we would be able to say that the population underwent hurricane-induced evolution.

Only because of the wind

The piece missing in the research is what transpired during the actual days of the hurricane – it appears that not too many people on the islands thought that venturing into the forest in the middle of a hurricane to check on the lizards was a good idea. The researchers assumed that the anatomical traits they found helped the lizards to survive, but how?

Anolis scriptus lizards live on trees, and the major danger they face is that the wind will fling them far into the sea. Larger toe pads help them better cling to branches, and a previous study showed that it was more difficult to knock lizards with long limbs off the trees. This explains the toe pads and the front legs, but what about the back legs, which were actually shorter in the surviving lizards?

To examine this, the researchers reenacted a hurricane: They enabled several lizards to cling to a branch, then turned on a strong fan that simulated Irma and Maria’s powerful winds (with a safety net secured on the opposite end to catch any falling lizard). In response, the lizards moved to the side of the branch farthest from the wind source, i.e., the slightly more protected side, and held on tightly. When the wind got stronger, their back limbs lost their grip and they were left clinging to the branch with only their front limbs, until, no longer able hold on, they were flung into the net.

The researchers believe that the longer back legs hindered the lizards’ grasp of small branches and, due to their greater surface area, “caught” more wind and lost their grip more easily. In comparison, larger and stronger front legs with larger adhesion pads increased the lizards’ chances of holding on to the branch. Nevertheless, in the current study, the researchers did not conduct a comprehensive comparison among lizards with different limb sizes to see how they fared against the wind.

This is the first time that research shows an anatomical change in a population caused by a hurricane, and it probably won’t be the last. “We know that hurricanes are getting more frequent and we know that they’re getting more strong,” said Donihue. “So, setting up a network of sites that are actually set up to investigate the question of how hurricanes are changing the evolutionary trajectory of species I think could be really useful.”

Watch the video by the journal Nature on the research (in English):