Caught in a race against climate change, lizards hit an evolutionary dead end
The brown anole has little room left to evolve.
Brown anoles are one of the most successful species on the planet. These resilient creatures have settled throughout a large portion of the Western Hemisphere, even landing in such distant places as Hawaii and Singapore by hitching rides across the Pacific in shipments of ornamental plants. In the southeastern United States, they are actually displacing native green anoles, driving them higher into the trees. These cold-blooded creatures are happy almost anywhere, from shady forests to sun-drenched beaches.
“In The Bahamas, it would blow your mind how common these things are,” said Michael Logan, a post-doctoral fellow at the Smithsonian Tropical Research Institute in Panama, who studies them. “Pick any bush along the side of the road, look closely, and I’ll bet the bank you see a brown anole — or three — in there.”
You would think that with so many brown anoles covering so much of the planet there would be a lot of genetic variation within the species. Some lizards would be larger or smaller, faster or slower, lighter or darker — meaning that, by chance, a few anoles here or there would be adapted to new challenges, like climate change, and they would pass on these traits to a younger generation of climate-tolerant lizard. But new research suggests that isn’t happening.
The findings, recently published in the Proceedings of the Royal Society B, could have significant implications for the future of all cold-blooded species — the anoles, as well as other reptiles, amphibians, and fish — whose body temperatures vary with that of the outside environment. Studying these species, known as ectotherms, can help scientists better understand the perils of global warming because their lives are so precisely connected to fluctuations in temperature.
“It was surprising to find such low genetic variation in traits that are important under climate change, such as the body temperature at which lizards run the fastest,” a trait essential to outrunning predators, Logan said. “So how have these guys invaded and adapted to such disparate regions of the globe if they lack what’s necessary for genetic adaptation?”
That’s a question still in need of an answer, although one possible explanation is that generations of anoles have faced challenging environments, and as the species evolved to meet these challenges, it wound up in something of a genetic cul de sac. The result is little variation among brown anoles. Logan explained how, over time, evolution yields very limited variability in certain traits.
“A trait like ‘number of limbs in tetrapods’ — amphibians, reptiles, birds, and mammals — is almost entirely determined by genes,” Logan said. “But because there is no variation in those genes among individuals — nearly all tetrapods are born with four limbs.” He added that “selection cannot act on a trait that has no variation.”
For their study, the scientists captured adult lizards from two very different habitats, one cool and forested, the other a hot, sun-soaked peninsula. They then bred the anoles in captivity and raised their offspring in the same laboratory setting.
“We did this because any differences between the populations would be due entirely to genetics,” Logan explained. “In other words, we controlled for ‘nurture’ so we could see if ‘nature’ played a role.” He said there were marked differences between cold-weather lizards and warm-weather lizards, despite growing up under the same conditions.
Using a high-speed camera, they filmed the lizards running across a wooden dowel rod after exposing them to different temperatures. They used gel packs and heating lamps to create temperatures from around 70 degrees F to around 120 degrees F, recording their responses. Predictably, the warm-weather lizards performed better at higher temperatures, but neither group displayed a lot of genetic variation.
“Our results suggest that natural selection may have used up all the available genetic variation to get these populations adapted to their thermal environments in the first place, leaving them with nothing to evolve further as the global climate continues to change,” Logan said.
The study showed that thermal traits have a genetic basis “because they differ between populations when we control for the effects of the environment,” he added. “But we also show that within each population these traits lack genetic variation, which means they can no longer evolve in response to selection.”
The brown anole, because of its large population and ability to colonize novel environments, is unlikely to be especially vulnerable to climate change — although it turned out to be more vulnerable than expected, he said. But the study raises troubling questions about the fate of other species in a warming planet.
Other species tend to cover a more limited area than the brown anole, and they tend to live only in one kind of thermal environment, Logan said. “If the brown anole with its success as a global invader lacks the necessary genetic variation to evolve rapidly, what does that say about the rest of biodiversity?” he said. “Species that are less prolific and more specialized should have even less genetic variation for selection to act on.”
Evolutionary biologist Shane Campbell-Staton, a postdoctoral fellow at the Universities of Montana, Missoula and Illinois, Champaign-Urbana — who was not involved in the study — said the research provides a critical link to understanding how cold-blooded creatures respond to changing temperatures.
“There have been several studies that have shown that extreme weather events and rapid shifts in the environment can cause selective events — organisms with greater resilience to droughts, heat waves or cold snaps are more likely to survive and pass their genes on to the next generation,” Campbell-Staton said. “However, as [the study] points out, evolution — which occurs across generations — can only happen if the traits that allow survival can be passed on to offspring.” With the brown anole, “the traits under selection in high temperature environments don’t seem to be passed from generation to generation very efficiently, meaning that adaptive evolution of those traits would presumably happen on a much slower time scale than the anticipated changes due to global warming,” Campbell-Staton added.
This mismatch “could potentially be a disastrous combination,” Campbell-Staton said. “It means that as the planet warms over time, selection on thermal performance may increase — meaning more individuals may die in a given generation — but the offspring of the survivors may only be slightly better fit to deal with continually rising temperatures, or no better off at all. The end result of this mismatch, if temperatures continue to rise, would inevitably be extinction.”
While the study isn’t directly applicable to warm-blooded humans, “it is clear that many species around the world, including the plants we depend on for food and oxygen, the insects that pollinate those plants, and many other ectothermic species that are important players in ecosystem health could be drastically affected,” Campbell-Staton added.
For species with small home ranges, lacking the ability to migrate, evolution should provide “their main avenue of escape,” from the effects of global warming, Logan said. But this study “hints that many of the species we love and care about may not be able to mount a rapid evolutionary response to climate change.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.