Humans Are Forcing the Rapid Evolution of Wildlife. Can Animals Change Fast Enough?

Human activities are causing wildlife species to evolve faster than scientists once thought possible in a phenomenon known as rapid evolution

By Katarina Zimmer
Conservation
Dec 18, 2024

Illustration by Armando Veve.

KRISTIN WINCHELL IS USED TO GETTING STRANGE LOOKS FROM LOCALS when she walks through San Juan, Puerto Rico, with a lasso-bearing fishing pole. Winchell, a New York University evolutionary ecologist, is on the prowl for crested anoles, catching them on fences and buildings. These little lizards have taught her some important lessons about wildlife’s ability to adapt to our human-dominated world. Anoles in Puerto Rican cities, she has found, tend to sport longer limbs than their forest-dwelling relatives, helping them sprint across treeless expanses to avoid getting snapped up by predators. Their larger and more intricate toe pads may allow them to cling tightly to the smooth surfaces of buildings.

The city lizards also excel at handling the blistering urban heat. When Winchell and her collaborator, Princeton University evolutionary biologist Shane Campbell-Staton, placed some of them under a heat lamp and slowly dialed up the temperature, they stayed active until around 105 degrees F, about 1.8 degrees higher than their forest counterparts. Indeed, in a series of studies published over the past decade, she and her colleagues have learned that the lizards have evolved an entire suite of adaptations to urban living, fundamentally rewiring their bodies to survive the crushing pressures of the city. And they’ve done so in as little as a few decades, Winchell marvels. “I think it’s pretty incredible that these animals are changing so rapidly, and that we can actually measure it and see it unfolding.”

Scientists long considered evolution to be a slow, incremental process that unfolds over millions of years—or “many thousands of generations,” according to Charles Darwin, who came up with the theory of evolution in the 19th century. That paradigm provided little hope that wildlife would be able to evolve ways to withstand the mounting human threats they face, from pollution and poaching to habitat loss and climate change.

But studies like Winchell’s increasingly are finding that wildlife species can evolve much, much faster than scientists once thought possible—sometimes in a matter of decades or just several to dozens of animal generations. These potential evolutionary shifts have been documented across a wide range of species, including insects, birds, reptiles and mammals—and the term “rapid evolution” is now a solid part of the scientific vernacular.

“It gives us some level of hope that … nature has some kind of resilience,” says freshwater ecologist Rick Relyea of Rensselaer Polytechnic Institute in New York state. “We don’t want to push these ecosystems too far, or these animals won’t be able to handle those pressures. But [this] can buy us some time until we get human impacts on ecosystems mitigated and reversed.”

A collage of a Puerto Rican crested anole and its toepad morphology.

In just a few decades, some Puerto Rican crested anoles (above, left) evolved a suite of adaptations to city living, including longer limbs and larger, more intricate toe pads (above, right) to better race across pavement and cling to buildings. Well suited to desert life, a Cape ground squirrel (below) in South Africa shades itself with a tail used as a parasol while foraging for bulbs, grasses, insects and other foods.

Analyzing genomes for evidence

Faced with an existential threat like unbearable temperatures, wildlife populations can respond in several ways, such as moving to cooler climes. Individual animals also can acclimatize during their lifetimes, learning to seek out shade or adjusting their physiology. Evolution is different because it involves a change in a species’ DNA over several generations.

The process happens via natural selection, in which animals with poorly adapted versions of genes die or fail to reproduce. Only individuals with better-adapted gene variants reproduce and pass their genes on to the next generation, so that the population as a whole becomes increasingly well adapted. In this way, “adaptation can build novel solutions to challenges that we can’t imagine happening within a single lifetime of an organism,” says evolutionary ecologist Rachael Bay of University of California, Davis.

Because evolutionary adaptation is at heart a genetic process, it can be tricky to prove. Scientists often analyze genomes in search of evidence. Winchell, for example, found that city lizards were indeed genetically different from their forest relatives in genes related to limb and skin development and heat tolerance. Researchers also may test how animals from different populations—such as lizards from cities versus forests—fare under heat, to help rule out that individuals aren’t just quickly acclimatizing. Some scientists also track animal families over time to determine that beneficial traits are indeed being inherited and aren’t just showing up due to random changes in a population. These kinds of studies haven’t been done for all claims of rapid evolution, but it’s clear the process is occurring in at least some species.

Some of the most notable examples are warming-related, where some bird or mammal species seem to be getting smaller and gaining longer limbs or beaks—traits thought to help off-load heat. In Cape ground squirrels of South African deserts, the rodents’ spines have become around 5 millimeters stouter and their feet slightly larger over 18 years of monitoring rising temperatures, according to ecologist Jane Waterman of Canada’s University of Manitoba. As with many other shape-shifting examples, it’s unclear if the animals are evolving to adapt or if, for example, temperatures are causing animal embryos to develop differently. If the squirrels indeed are evolving, it would be remarkable, she says, given they’re already so well suited to desert life, using their bushy tails as shade parasols and cooling off in burrows.

Following two decades of heavy poaching for their tusks, many female elephants in Mozambique’s Gorongosa National Park were born without tusks.

How hunting spawns evolution

Among the most striking examples of rapid evolution are responses to hunting. At Ram Mountain in the Canadian Rockies, evolutionary biologist David Coltman of Canada’s Western University has been studying male bighorn sheep, which in most parts of Alberta can be shot legally only when their horns, viewed in profile, are large enough to nearly complete a circle. Over several decades of trophy hunting, Coltman and his colleagues saw average horn size in the population decrease. Coltman’s studies suggest that rams with long horns were killed more often, leaving mostly short-horned individuals that passed this trait on. One ram with especially short horns sired 25 offspring, he recalls. “He was the most successful ram in the study period because he could never legally be harvested.”

Other scientists have been amazed to find evidence of a similar evolutionary phenomenon in even longer-lived, slow-to-reproduce African savanna elephants. In Mozambique’s Gorongosa National Park, elephants endured unprecedented levels of poaching for their tusks during a 20-year civil war. In the postwar 1990s, many female elephants were born without tusks, Campbell-Staton says. “In the turnover of literally just a single generation,” he says, “you see this … selection and an evolutionary response.”

In the Bahamas, a more benign human activity—building roads across tidal creeks—also may have influenced evolution. When some roads ended up isolating creeks from the rest of the ocean, predatory fish such as barracuda and needlefish could no longer reach or survive in those creeks. That, in turn, caused populations of their mosquitofish prey to balloon. Those smaller fish then developed larger, more streamlined body shapes to better compete with one another, explains evolutionary ecologist Brian Langerhans of North Carolina State University.

Even the most unintentional effects of human activities can force evolutionary shifts. In Hawai‘i, researchers noticed in 2006 that male Pacific field crickets had lost their mating call, probably to avoid being detected by a parasitic fly humans inadvertently introduced to the islands. Then in 2018, they found the crickets had evolved a new call—which sounded like purring—likely undetectable to the fly. “I don’t think I can overstate how remarkable that finding is: In response to an invasive parasite, a species rapidly evolves from having a song to not having a song and then to having a different song,” Campbell-Staton says.

In North America, bighorn sheep rams in one population from the Canadian Rockies evolved to grow shorter horns as an adaptation to trophy hunting for large horns.

Trade-offs and traps

Evolutionary adaptation often comes with trade-offs, however. Large horns in bighorn rams may signal that they’re high-quality, strong males worth mating with, so by hunting the highest-quality animals, “you might actually be depressing the population as a whole,” Coltman says. In many U.S. cities, evolutionary biologist Sarah Diamond of Ohio’s Case Western Reserve University has found a potential trade-off in acorn-inhabiting ants that have evolved a higher tolerance to heat. Ants with greater heat tolerance tend to have a higher metabolic rate, she says, meaning they might need more food than they feasibly could find.

Such trade-offs can happen because genes that underlie complex traits like heat tolerance may be linked to other traits, so changing one disrupts the other. When evolution occurs over long time periods, species have a chance to evolve ways of compensating, Campbell-Staton notes. “But in cases of rapid evolution, there’s just not enough time.”

Rapid evolution also can lead species into traps. In the western United States, that happened to Edith’s checkerspot butterfly, which was adapted to lay its eggs on native plants such as the maiden blue-eyed Mary. But on one Nevada cattle farm colonized by a nonnative plantain introduced by European settlers, the butterfly had evolved to use the invasive plant instead, scientists reported in 1993. The plantain’s longer life span allowed checkerspot larvae to feed longer, boosting their survival. When, in 2005, farmers stopped ranching the meadow, native grasses overgrew the plantain. With nowhere to lay its eggs, the butterfly population went extinct. As rapid as rapid evolution can be, it’s often not rapid enough to keep pace with the fast-changing habits of humans.

Even for slower-burning threats like climate change, species are struggling to keep pace. One of Bay’s studies found that the fate of some corals in the Pacific’s Cook Islands hinges on the speed of change. Using computer simulations, she and her colleagues predicted that the corals would go extinct by mid-century if warming continues at current levels, but they could adapt and persist if nations take more action to fight climate change. And when Diamond recently examined several previous studies on the adaptability of a range of urban animals, including lizards and acorn ants, she found that although many species have adapted to small temperature increases, they’d falter at several degrees of warming. “They’re definitely giving it their best shot,” Diamond says. But as of now, “they’re not quite keeping pace with the changes in the environment.”

The fate of evolving wildlife depends not only on the speed of human activities but also on a species’ biology, including the genes underpinning traits. Complex adaptations like heat tolerance, which involves many genes, take longer to evolve than traits with a simple genetic basis. In Atlantic killifish, for instance, pollution tolerance appears linked to only a few genes, which helped fish living by toxic sites along the U.S. East Coast evolve tolerance in just a few dozen generations.

An image of Edith's checkerspot butterfly feeding on Parry's Arnica.

After it evolved to lay eggs on an invasive plant, one population of Edith’s checkerspot butterfly went extinct when native plants overgrew the nonnatives.

The imperative of genetic diversity

Adaptation also depends on the overall health of a wildlife population. The larger a population, the more genetically diverse it will be, and the higher the likelihood that some individuals will harbor useful genes to cope with threats. Connectivity to other populations also helps, says evolutionary biologist Kristen Ruegg of Colorado State University. She and her colleagues recently discovered that the southwestern willow flycatcher, a denizen of western riparian habitats, has more genetic variants linked to heat tolerance than it did decades ago—and the bird possibly got those variants by interbreeding with a different flycatcher subspecies from San Diego that may have flown in.

Yet the flycatcher is declining in spite of its beneficial DNA, Ruegg says. She and other scientists stress that the onus remains on humans not only to dial down pressures but also to help wildlife adapt by preserving natural habitats. “What we can do,” Ruegg says, “is do our best to ensure the genetic health of populations, and that gives them the best shot at responding to the unknown.”

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