When Charles Darwin first saw a Madagascar star orchid —sent to him by an enthusiast — he predicted the existence of a long-tongued pollinator that could reach for nectar inside the flowers’ long tubes. The discovery of Morgan’s sphinx moth, which had a tongue just long enough (and no longer), proved Darwin right — some two decades after his death.
It’s one of the great demonstrations of evolution by natural selection. But what naturalists really want is to catch natural selection in the act, as it’s doing the selecting. That’s much harder, and a bit of luck is needed.
Opportunity knocked at the door of biologist Colin Donihue at Harvard University in Cambridge, Massachusetts, a year ago, just after he and his colleagues had returned from Pine Cay and Water Cay, two small islands in the Turks and Caicos archipelago to the north of the Caribbean, where they had been studying the lives and times of its resident anole lizards (Anolis scriptus).
On 8 September last year, Hurricane Irma struck the islands, battering them with sustained winds of up to 265 kilometres per hour. Irma was the first category-5 hurricane on record to strike the region, but more was to come: two weeks later, Hurricane Maria swept across the islands with winds of up to 200 kilometres per hour. Dozens of people in the region died. Reconstruction and rebuilding efforts continue.
Three weeks after the winds had died down, the researchers were back on Pine Cay and Water Cay to assess the damage, and to see how (or even if) their lizards had survived. Their study, published this week in Nature, is the first to use an immediate before-and-after comparison to assess the impacts of hurricanes on evolutionary selection (C. M. Donihue et al. Nature http://doi.org/csgp; 2018).
Serendipity can go only so far. The researchers hadn’t marked the lizards, so they couldn’t identify and track the fate of individuals. But they found clear trends of natural selection in action. In general, anoles found after the storms had bigger toepads, longer forelimbs and shorter hindlimbs than did lizards collected before the storm.
What have these traits to do with hurricanes? The lizards live in bushes and other low-growing vegetation. Toepads allow them purchase on the branches as they move, and it’s a fair bet that limb proportions also play a part in helping a lizard to keep in contact with a branch, resisting moves by predators, other lizards — or, as it turned out, hurricanes — to knock them off.
The researchers took their idea further with a simple laboratory experiment, in which they allowed lizards to get settled on a perch and then blew them off using a commercial leaf blower, recording the speed at which the lizards were dislodged. (The lizards flew into comfy padding, and were not injured in the experiments.)
This experiment supported the researchers’ hypothesis. In particular, it showed that when lizards are subjected to a stiff breeze, they hang on tightly with their forelimbs and let their hindlimbs hang loose. Longer hindlimbs, then, offer more purchase to the winds, explaining why lizards found after the storms tended to have shorter hindlimbs, but longer forelimbs.
These bigger toepads, shorter hindlimbs and longer forelimbs did not evolve as a direct response to the hurricanes, but they show how such evolution occurs. Natural selection interfered with the way in which these traits were spread across the population. Specifically, those lizards unable to hang on when the storms blew up — those with smaller toepads, longer hindlimbs and shorter forelimbs — were (presumably) blown away and perished. Those lizards better able to hang on tightly would have survived to weather another day. In technical terms, the mean values of the crucial traits measured before the storms had shifted. These changes are phenotypic — merely observable characteristics. They say nothing about the genetic assimilation of such changes, which will presumably happen when the surviving lizards breed and new lizards are recruited to the population. Such changes are unlikely to be the last, given the expected increases in extreme weather that the future will bring.
Nature 560, 5 (2018)