The Race Between Climate Change and Evolution

 The evidence for recent, accelerating global climate change is very strong, as it is for the role we humans have played in influencing our Earth’s weather. But for the most part, there have been few direct tests of how climate change could affect the organisms that inhabit our planet. Much of the evidence on this point is either anecdotal, correlational, or based upon predictive models, such as reports that 1 billion people will be forced to relocate due to climate changes or predictions of increased extinction risk due to changes in temperature, ocean chemistry, sea level, and other factors. None of these studies get at a crucial question that will determine the true impact of these environmental shifts: in a footrace between climate change and evolution, what will win?
Two studies published today in Science suggest an answer for one particular species, the small flowering plant Arabidopsis thaliana. Just as the fruit fly Drosophila melanogaster has become an important tool for scientists studying animal genetics, Arabidopsis is the model organism of choice for studying plant genes. Researchers around the world - including the laboratory of Joy Bergelson, professor and chair of evolution & ecology at the University of Chicago - have characterized the genome of Arabidopsis and looked at the variation within the species.
“Arabidopsis is pretty much ubiquitous; it’s been collected in most parts of the world. You find it throughout Eurasia, Russia, up through Scandinavia and down through the Iberian peninsula,” Bergelson said. “So, as a community, we now we have thousands and thousands of lines from all over, many of which are housed in this lab.”
Those resources allowed Bergelson’s laboratory and another group from Brown University to study the relationship between climate change and evolution in this plant species via two separate experiments. In the first, a team led by University of Chicago postdoctoral researcher Angela Hancock applied the popular methodology of genome-wide association studies (GWAS) to the plant, with a twist. Instead of looking for gene variants associated with a particular characteristic of the plant itself, the team looked for variants associated with different climate variables, such as temperature, precipitation, and humidity. (If it sounds familiar, ScienceLife wrote about a similar study Hancock performed with human genetics in 2010.)
Their analysis produced a list of climate-associated genes that reflect familiar biological processes, controlling aspects of photosynthesis, growth, energy metabolism, and other activities important to plant health. To test whether these genes were truly related to a plant’s fitness in a given climate, the researchers tested their ability to predict how well a plant would grow when grown in one particular climate, that of Lille France, based on the number of favorable variants that they carried. Happily, the gene variants were truly predictive, suggesting that the variants produced by their GWAS indeed played a role in a plant’s ability to grow and reproduce in a given climate.
“You can actually predict pretty decently changes in the relative fitness of plants just based on the results from our worldwide scan,” Bergelson said. “It didn’t have to work - it was a longshot really - but we were happy to see that it did work.”
But for the future of climate change, the more critical question was how these variants initially appeared in the Arabidopsis genome. For many of the climate-associated variants, the University of Chicago team found evidence of “selective sweeps,” an evolutionary process where a new, favorable mutation appears and spreads rapidly through a species. In the case of Arabidopsis, this finding suggests that the plant adapted to shifts in climate thanks to the random appearance and subsequent spread of these helpful mutations, rather than drawing from pre-existing genetic variation. But in times of rapid climate change, there may not be sufficient time to wait for those genetic changes to occur.
“The key there is that the variant is novel, and so the extent to which a plant or an organism depends on sweeps to adapt to climate means that there is an intrinsic delay,” Bergelson said. “As the climate continues to change, they may not have the standing variation that they would need to adapt. That’s going to slow things down.”
The Brown study came to a slightly different conclusion by asking a slightly different question. In their study, the researchers conducted a “transplant” experiment, planting a variety of Arabidopsis strains from around the world in four locations with distinct climates: Finland, Spain, Germany, and the UK. They then looked for gene variants that were associated with the success or failure of those plants. For this set of genetic variants, which include those related to climate but also those related to other aspects of the biotic and abiotic environment, there was not strong evidence of selective sweeps.
“I don’t think there’s necessarily a contradiction,” Bergelson said. “We’re taking a very different approach and therefore being led to somewhat different parts of the genome.”
In addition, both studies acknowledge a role for both standing variation and sweeps. Working out exactly how climate change drives evolution in Arabidopsis will require longer-term experiments, keeping track of changes in the frequency of gene variants to observe how the plants adapt as the environment changes. But as Outi Savolainen writes in a commentary accompanying the two articles, these results “may eventually be used to better predict and manage climate change responses.” If the footrace between climate change and evolution has already started, it will be good to know the genetic route it will take.