Rapid evolution helped a wildflower survive California’s megadrought

During California’s worst dry spell in the past 1,200 years, some populations of wildflowers defied the odds to survive the ordeal. Researchers say they now believe these flowers relied on a type of rapid genetic evolution — the first time such a phenomenon has been documented in the wild.

The dry spell happened between 2012 and 2015, and killed more than 100 million trees. It was a particularly brutal period during an ongoing megadrought that began in 2000 and which has been made worse by climate change. Even though the drought killed plants that are normally drought-resistant, the scarlet monkeyflower, a bright red wildflower that thrives in wet areas, and along creek beds and springs, showed remarkable resilience.

A team of researchers spent eight years studying 55 populations of the wildflower, whose scientific name is Mimulus cardinalis, by keeping track of its numbers in the wild and sequencing the flowers’ genomes to reveal genetic shifts.

“We were able to show that these populations across the range in California were declining due to this extreme drought, and we found evidence of a rapid evolution across the genome,” said Daniel Anstett, an assistant professor at the School of Integrative Plant Science of Cornell University and first author of a study on the findings, which published Thursday in the journal Science. “And then we were able to relate a metric of this evolution to the ability of these populations to recover and to not go extinct.”

While the entire species was not at risk of extinction, individual flower populations likely were, suffering declines of up to 90% compared to peak population sizes. It took about two to three years for these populations to rebound, according to Anstett.

This rapid comeback is a process biologists call evolutionary rescue, which happens when a species is able to recover from the threat of extinction by an external factor such as a drought, Anstett explained. “Evolutionary rescue occurs when the few individuals that are left have the right genetic makeup to do better than the ones that died, so they do well or thrive within these new conditions, so the population inches back from extinction,” he said.

Evolutionary rescue has been demonstrated in lab settings but researchers only had partial observations of it in the wild before: in cancer resistance in Tasmanian devils and adaptation to pollution in killifish. However, these studies did not provide full evidence that the process was taking place.

This study, Anstett said, offers rigorous evidence that evolution happened and it led to a demographic recovery. “We had a lot of information about the wildflowers’ demography and the genes, so that provided more ironclad information that ultimately evolutionary rescue was happening.”

A great indicator of drought

The scarlet monkeyflower is a perennial herb, meaning it regrows each season from the same roots. It’s pollinated by hummingbirds and can grow up to 3 feet tall and 3 feet wide.

“This plant is found in streams in California, Baja California and Southern Oregon that are often seeps of water that seasonally flow through habitat,” said Anstett. “It really requires that flowing water to complete its life cycle and then be able to grow all the way up to seed, so it’s a great indicator of drought.”

When a drought comes, Anstett explained, water stops flowing, so the plant has two choices: grow rapidly and produce flowers and seeds before the drought really takes hold — or grow much more slowly and perhaps live for another year. This latter strategy is what the wildflowers used. “What we saw was that the plants were, in fact, developing more slowly and going more towards bunkering down, living longer, growing less fast, which is called drought avoidance,” Anstett said.

To understand the process, the researchers kept track of wildflower populations across 19 sites, which were visited every year to assess whether the plants lived or died. They then collected seeds from the plants, grew them in a lab, and then ground up the leaves from the plants they’d grown to extract DNA.

The DNA was then used to sequence the plant’s genome — its genetic instructions. “We were able to see this rapid evolution happen within seven years,” Anstett said, adding that the genetic mutations that the plant leveraged to become drought resistant weren’t new, but probably developed a long time ago. The plants that carried these drought-resistant traits were able to survive and pass those mutations forward via the seeds.

The next step in the study, which Anstett says could eventually span 30 or 40 years, is to see whether these mutations that allowed the plants to survive the megadrought will continue to be beneficial or hinder the plants if the drought stops.

More evidence of evolutionary rescue does not make drought or climate change less concerning, but it suggests that there could be more plants or animals that might be able to show resilience under extreme duress. Key for that to happen is genetic diversity, which means that conservation efforts to keep plant populations as widespread as possible are still crucial.

“The amount of genetic variation can be critical to this adaptation,” said Anstett. “This is a hint to conservation biologists to try to improve the amount of genetic variation and the connectivity between habitats so that organisms are able to ultimately be more resilient to a range of stressors.”

‘A real breakthrough’

Demonstrating that evolutionary rescue happens in the wild and identifying the specific genes responsible has been a major outstanding goal in evolutionary and conservation biology, David Field, an associate professor of applied biosciences at Macquarie University in Australia, said in an email. Field was not involved with the new study.

“The researchers used an elegant experiment to demonstrate that evolutionary rescue actually happens in the wild in our lifetime,” he wrote in an email, adding that the study provides an important demonstration of the possibilities of using similar experiments to help conservation efforts of endangered species globally.

Other researchers who also did not participate in the research expressed similarly positive views: “This is a very important study,” Diane Campbell, distinguished professor emerita of ecology & evolutionary biology at the University of California, Irvine, said in an email. “It provides exceptionally strong evidence that a process known mostly from theory can help wild plant populations recover from the increasing droughts predicted under climate change in Mediterranean-type areas.”

A major question on the mind of botanists is whether plants can evolve in time to survive the extreme droughts that are predicted to get worse with climate change, said Isaac Lichter Marck, a botanical researcher at the California Academy of Sciences. “When faced with extreme drought, animals can move, adapt, or die — but wildflowers are rooted in place, leaving them with a much shorter list: adapt or die,” he wrote in an email.

The study is a critical model for using genomics to predict which wildflower species are vulnerable to extinction as a result of climate change, he continued. But it also raises other concerns: “Pressures like habitat loss and invasive species are eroding genetic variation in the wild. As this research shows, loss of genetic variation could make the already alarming threats from climate change even worse.”

Jeff Diez, an associate professor in the biology department of the University of Oregon, said in an email that the research represents a real breakthrough and stands out for its sheer thoroughness. “The sobering part is that it took an extraordinary effort to show this for one species,” he added. “If we want to understand how whole communities of interacting species will respond to climate change, we need to assess evolutionary potential across far more species, and there is bound to be enormous variation in which species can evolve fast enough.”

Overall, he concluded, the study contains the genuinely hopeful message that some wild plants may evolve fast enough to survive climate disruption. “But it’s also wrapped in a humbling reminder of how much we still don’t know.”

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