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Two CU Boulder researchers are helping clarify how species’ populations with longer lives can still adapt to a changing climate

Our warming climate is leaving many plant and animal species with a choice: either adapt, find a new home or risk extinction. Fortunately, throughout the history of life on Earth, a concept called evolutionary rescue has stepped in to help species adapt to new environments and climates.��

Evolutionary rescue is a biological process where natural selection favors the individuals of a species that carry genetics best suited to the new climate. These individuals are more likely to survive and reproduce and are therefore able to better propagate future generations to ensure survival of the species.

Scott Nordstrom (left) earned his PhD from CU Boulder in 2023 under the advisorship of Brett Melbourne. (right), professor of ecology and evolutionary biology (Left photo from Scott Nordstrom; right photo from Brett Melbourne)��

For example, a smaller bat may be better able to weather a hot summer with multiple heat waves. Or a monkeyflower that's better able to retain water in its leaves may have . These genetic anomalies help move the population toward survival, instead of extinction.��

In the face of anthropogenic climate change, however, conservationists are worried that species with the longest life spans—like giant pandas, elephants, or sequoia trees, for which new generations take years to decades—will be too slow to adapt and avoid extinction.

A mathematical model developed by (PhDEBio’23) proved that that’s not always the case, however. As part of his doctoral dissertation, Nordstrom, in partnership with Brett Melbourne, a University of Colorado Boulder professor of ecology and evolutionary biology, set out to determine just how true it was that long-lived species were resigned to their fate. Their findings were published in in May 2026.

Their model contributes to conversations about conservation, especially when it comes to extinction concerns. “A lot of the more endangered species or the populations that are at higher risk of extinction tend to be longer lived,” says Nordstrom. “So, it's especially relevant for thinking about conservation.”��

Shifting focus: From flour beetles to tortoises

Before taking on this project, Nordstrom and Melbourne had been working with colleagues at Colorado State University to understand , which live for about a month before a new generation is birthed.��

“We found that genetic diversity of the population is really critical for allowing rapid adaptation to occur,” says Melbourne. “And that got us thinking about how things could be really different for longer-lived species.”��

Large tree species, like the Giant Sequoia, can live for thousands of years, but are now more endangered than ever due to increased wildfire activity in the American West. (Photo: Pexels)

The researchers set out t try to understand how relevant their findings were to species with longer lives.

Experimental work tracking the genetic variations in generations of long-lived species was not possible, however, so the pair created the next best thing: A flexible mathematical model and computer simulations that would allow them to map out potential evolutionary patterns of these species.��

For each simulation, they input a sample population into the model, using “good” environmental factors (i.e., the climate that they were already adapted to). Then they switched those factors to “bad” (i.e., a climate with warmer temperatures or less water).��

“Each individual’s survival depended on how well it was adapted to its environment, so when the environment shifted from good to bad, survival was low and the populations started shrinking,” says Nordstrom.

“But because there was genetic variation within the populations, some individuals were slightly better adapted to the bad environment, and those individuals were more likely to survive and pass on their genes, allowing the population to adapt,” he adds.

When nurture beats nature��

Through their simulations, Nordstrom and Melbourne were also surprised to find that long-lived species can experience a complicated evolutionary dynamic in which a population’s traits seem to decouple from their genetics. In these cases, some random environmental event has affected an organism's trait in a way that turns out to be an advantage in the changed environment.

For example, an American alligator might be genetically predisposed to weigh 600 pounds but actually weighs 400 pounds because environmental factors impeded its growth in early development. Perhaps the alligator was born in a drought year, when typical prey like fish and turtles were scarce.��

Ultimately, that smaller alligator may be able to survive heat extremes better in a hotter climate, thus slowing the rate of population decline. And because they are long-lived (up to 50 years), there is a good chance that there will be multiple small alligators in a population at once, thus changing the composition of that population in a way that slows the rate of population decline, allowing adaptation time to catch up and prevent extinction, the researchers speculate.

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Researchers have long thought that species like the American alligator, which can live up to 50 years, are less likely to benefit from evolutionary rescue to help them adapt to changes in the climate of their habitats. (Photo: Unsplash)����

Interestingly, those chances are much less likely to occur to short-lived species like flour beetles. Nordstrom says that’s because their short life spans don’t allow for their non-genetic phenotypic variation (like that seen in the undersized alligators) to remain in the population as time progresses; instead, only their genes are passed on to their offspring, and their offspring will thus not inherit their size advantage.��

“The flour beetles just mate once and pass their genes forward,” says Nordstrom. “Next generation, repeat.”��

That means that natural selection occurring within a generation can be important for evolutionary rescue in long-lived species. Previously, it was speculated that only evolution between generations determined whether populations could adapt to new conditions in time.

“This process of rescue is one part evolution and one part demography,” says Nordstrom. “In the race of evolution versus demography, this definitely helps the demography because it slows down population decline.”��

He adds that this will be surprising to researchers who have up to this point only considered the evolutionary component here. “But we showed that the demography is actually super important, too.”

While Nordstrom and Melbourne can’t say that all long-lived species will benefit from their demography, Nordstrom says it’s important for future researchers and conservation managers to know that evolutionary rescue is not out of the question for endangered species like pandas and bison.��

“Maybe it's a little bit more complicated than we thought,” says Nordstrom. “But this is the first major study finding that it’s not necessarily true that slower generational turnover guarantees that adaptation and evolution will be slower.”

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