Cities are the engine of evolution | University of Toronto Journal

The common white clover is both everywhere and nowhere: it grows widely in grassy areas of cities around the world but barely takes root in our imaginations. Yet this inconspicuous little plant holds a giant clue to how species are adapting to urbanization.

Like many organisms, white clover defends itself against potential predators. Snails, slugs and voles that browse on this perennial herb encounter an unpleasant bitter taste caused by hydrogen cyanide, which can kill them. But new research from the University of Toronto shows that in the world’s concrete jungles, which are home to fewer foraging herbivores, white clovers have naturally evolved to be less guarded: they’re 44% less likely to ’emit this toxic chemical than their brothers and sisters in rural areas. .

“This is the first time we have documented this kind of pattern that is happening around the world. I think that’s pretty strong evidence that humans are influencing the evolution of life around us,” says James Santangelo, who recently completed a PhD in biology at the University of Mississauga. Santangelo helped discover this new phenomenon of natural selection as a researcher on the Global Urban Evolution Project.

In this one-of-a-kind study, Santangelo and two UTM professors were able to recruit 287 scientists from 26 countries on every continent except Antarctica to study the urban adaptation of white clovers. Deploying on lawns and parks in Tampa, Helsinki, Cape Town, Tehran and dozens of other cities, they collected some 110,000 samples and sequenced the genomes of more than 2,000 individual plants. Their findings, which were published in Science, clearly show how the plant undergoes “parallel evolution” – that is, it changes biologically to adapt to the circumstances of urban life.

“Humans have become the primary driver of evolutionary change on Earth,” says Marc Johnson, one of the study’s lead researchers and professor of biology at the University of Mississauga. “If we want to continue to live here sustainably, we need to better understand this trend.”

Johnson, who directs the EvoEco lab at UTM and holds the Canada Research Chair in Urban Environmental Sciences, first explored this trend in white clover populations in Toronto. In an earlier study, he found that the frequency of factories producing hydrogen cyanide (a process known as cyanogenesis) increased with distance from the city center. A new idea germinated one day while he was in the shower, and as he wrote in a blog post about the project: “Holy cow, we could use white clover to understand if organisms adapt similarly to cities around the world. »

The seed was planted, and Johnson caught the interest of Rob Ness, an assistant professor of biology at the University of Mississauga, and Santangelo, who was already doing similar research. They initially aimed to include scientists from 50 cities, but after Johnson posted an open invitation on Twitter and circulated the proposal among his colleagues, more than triple that number signed up.

The objective of the project was to document the genetic diversity of white clover over a wide geographical range. So, starting on foot in the center of their city, the participating scientists moved outwards on linear paths like bicycle spokes, stopping at least every 200 meters to collect 20 plants out of up to 50 Site (s.

The sampling effort turned into a family affair during Johnson’s sabbatical as his two children, then aged 12 and 9, helped pick clovers during trips to Argentina, Chile and in Japan. Santangelo himself picked up 12,000 clovers during a 16-city road trip in eastern North America, where a stop in some neighborhoods drew the wrong kind of attention from residents: People assume you’re not doing anything right and want you out of there. he says.

Scientists around the world were supported in their work by a kit of equipment assembled and delivered by the project team – and an emailed link to videos outlining detailed protocols for testing samples and recording results .

Johnson coordinated the researchers and said engaging so many remote participants posed unique challenges. For example, while most collaborations took place through Google Docs and Sheets, a different approach was needed for participants in China, where the government primarily denies access to Google products. He also spent a lot of time clarifying how to record data. “With so many different people, regions and cultures involved, there were tons of these hiccups,” Johnson says.

Our best chance of conserving most of the biodiversity is to build more connected and evenly distributed green spaces in the city. »

— James Santangelo, U of T Mississauga alumnus and Global Urban Evolution Project researcher

Rob Ness, another of the study’s lead researchers, performed genomic analysis of the plants, which required some creative problem solving. “Plants without hydrogen cyanide are missing one or two genes, and detecting when something is missing is tricky,” Ness says. So he helped design a new assay to detect missing sections in clover genomes.

The results that emerged from the project are still being analyzed. Some urban clovers actually showed a higher presence of hydrogen cyanide than their rural counterparts, which the researchers believe is caused by drier urban conditions. They believe it triggers cyanogenesis as the plant’s way of storing nitrogen, which has drought-protective functions. Several spin-off projects with project partners are now focusing on better understanding clover genetics and adaptations.

In the meantime, says Santangelo, this study provides fertile ground to reinvent how we approach building and maintaining cities in the future. “I think clover will be fine, but we may have to be concerned about other species, such as birds that rely on song to mate and whose songs are now disturbed by noise pollution,” he says. . “Our best way to conserve most of the biodiversity is to build more connected and evenly distributed green spaces in the city. This is how we maintain large populations that are less likely to disappear. »