When Charles Darwin developed his theory of evolution in the mid-19th century, he believed that this fundamental process occurred very slowly on a geological scale, taking millions of years. But is this true, or can evolution occur more quickly? How fast can evolution happen?

First, let’s define what evolution is, the process by which a species’ genes or physical appearance gradually change over time. The driving force is natural selection, where individuals with more beneficial traits survive, reproduce, and pass these traits on to the next generation. Over many generations, this is called adaptive evolution.

According to Timothée Bonnet, an evolutionary biologist at the French National Center for Scientific Research and the University of La Rochelle, natural selection and adaptive evolution allow “species to follow changes in the environment.”

In Darwin’s famous example of the finches of the Galapagos Islands, different species evolved different beak shapes and sizes over decades to specialize in feeding on different types of nuts and insects. This conclusion caused a sensation after the publication of “The Finches’ Beak: An Evolutionary History in Our Time” (Knopf, 1994).

There’s a third component: speciation, which is when a species eventually branches into two different species. Bonnet says this happens much more slowly than adaptive evolution.

In the early and mid-20th century, scientists realized that evolution could happen much faster than Darwin thought, using the theory of natural selection to make crops tastier in just seven years and domesticate dogs in just a few generations. “We’ve made evolution possible,” Bonnet told Live Science. “We’ve seen that changes over generations can be quite dramatic.”

This artificial selection has accelerated evolution, but how fast could this process happen naturally?

To find out, Bonnet and an international team of researchers analyzed decades of genetic data from 19 species of birds and mammals. They found that the rate of adaptive evolution was two to four times higher than previously thought. Specifically, each generation increased survival and reproduction by an average of 18.5% under completely stable conditions.

This means that if survival and reproduction were reduced by one-third, adaptive evolution would help the population recover within three to seven generations. As hunters hunted the bigger-horned sheep, the horns of bighorn sheep (Ovis canadensis) became 0.7 inches (2 centimeters) shorter over 20 years, or three generations, than they were before. Snow voles (Chionomys nivalis) became 0.1 ounces (3 grams) shorter over 10 years, or eight generations, likely due to changes in snowfall.

However, conditions in nature are never constant.

“Populations are adapting, but we don’t know what they’re adapting to,” Bonnet said. He explained that changes in the environment, competition, diseases and humans can all trigger rapid evolution. “Evolution exists to compensate for or at least mitigate some of the changes that occur in the environment,” he added.

Climate change is another major driver of adaptive evolution, Bonnet said, but scientists aren’t sure whether populations will have time to cope. As temperatures and weather patterns change and sea levels rise, some species have responded by moving to cooler areas or adapting to saltier conditions. But the environment could deteriorate too quickly for evolution to keep up, he said.

“Evolution happens all the time,” evolutionary biologist James Stroud of the Georgia Institute of Technology told LiveScience.

If natural selection is strong, then “why doesn’t everything evolve quickly all the time?” Stroud wonders what motivated him to publish research on the subject. He and his colleagues found that natural selection in tropical lizards is very dynamic over short timescales (generations or years), but stabilizes over longer timescales (millions of years), leading to almost no change.

Speed ​​of change
How do scientists measure the pace of evolution? University of Michigan paleontologist Philip Gingerich developed a method using a unit of measurement he calls Darwinian. He discovered that evolution occurs slowly on long timescales and rapidly on short timescales.

“The pace of evolution can be incredibly fast because of the constant changes in the environment,” vertebrate paleontologist Michael Benton of the University of Bristol told Live Science. “But the shorter the time frame, the faster the pace, and that’s after you’ve adjusted for time.”

Stroud and his colleagues at the University of Miami now use non-native green iguanas as an example of rapid evolution. The heat-adapted lizards have been known to freeze and fall out of trees during Miami’s rare cold spells.

“We found that some lizards died, some survived, and the survivors were actually able to withstand lower temperatures than we had measured before,” Stroud says. “This suggests that evolution can occur.”

Fossils also hold some clues. During the Triassic period (251.9 to 201.3 million years ago), after the Permian extinction, large marine reptiles called ichthyosaurs grew to gigantic sizes in less than 3 million years (faster than whales) as they became the ocean’s top predators.

According to Benton, factors such as adapting to new conditions, filling new niches, avoiding predators and competing with other animals often increase the rate of evolution of animals.

“Maybe the answer is that everything has the capacity to evolve incredibly quickly when it needs to,” Benton says.