It’s considered a nuisance or a weed when it pops up in luscious suburban lawns, long the bane of gardeners and homeowners sodding, sprinkling and nurturing the greenest of grasses.
But in other places, white clover has become a plant that marvels. It is one of the most rapidly evolving species of flora, learning quickly how to survive in the toughest of urban environments. Some green thumbs would not be surprised at its stubborn spread, while others might welcome a haven for bee recovery let alone any semblance of flowers bursting through crowds and concrete.
According to a study published Tuesday in the Proceedings of the Royal Society B, white clover (Trifolium repens) adapts equally well to cities of all sizes — with 20 studied in Ontario, Canada, from London, with a population near 400,000 to tiny Everett, population 1,670. Researchers had previously explored cities as large as New York, and now they are expanding their explorations to more than 180 cities across the world, in an effort called the Global Urban Evolution Project or GLUE.
Cities work as great natural test cases for evolution, said Marc Johnson, the director of the Centre for Urban Environments at the University of Toronto, Mississauga, who led the research. “In many ways it’s an unplanned experiment happening throughout the world over and over again,” he said.
With climate change advancing and more than half the world’s population living in cities — a figure expected to jump to 70 percent by 2050 — Dr. Johnson said it would be crucial for scientists to figure out how human encroachment and activity affect the plants and animals that surround us.
White clover makes for a good test species because it has already displayed the stamina to survive in climates from Norway to southern India, Dr. Johnson said. The plant also helps nourish soil with nitrogen and serves as an important source of nectar for bees and other pollinators.
The clover adapts to colder climates by losing its ability to make hydrogen cyanide or HCN, a toxin the plant produces to protect itself from predators, like snails, insects and voles, and in the country, cows, sheep and goats. The number of plants that produce hydrogen cyanide increases with every mile away from the city center, the study found, with small cities showing the same effect as big ones.
White clover that grew in an urban environment was less likely to make hydrogen cyanide, Dr. Johnson said. Although cities can be warmer than the countryside, the heat and human activity result in less snow than in rural areas. Without snow to insulate the plants from the cold, the clover would poison itself if it could not give up its ability to make hydrogen cyanide, Dr. Johnson said.
Since Darwin, researchers have identified three main forces behind evolution: Natural selection, genetic drift and gene flow. This study, Johnson said, shows that natural selection, where some species evolve and are better able to reproduce more than others, is the dominant evolutionary force for urban white clover.
“This is the first paper to actually tease apart what’s the relative importance of those three mechanisms to influence the evolution of populations in cities,” Dr. Johnson said.
It’s only in the last few years that scientists have been paying attention to examining evolutionary processes in urban environments, said Anne Charmantier, an evolutionary ecologist with The National Center for Scientific Research, or CNRS, in Montpellier, France, who was not involved in the research.
“We still know very very little about trait variation, response to selection in urban settings,” said Dr. Charmantier, a bird expert who recently published a study on the reproductive selection of great tits in Montpellier. “It’s going to tell us a lot about evolutionary processes in environments that are changing very rapidly.”
Peter Tiffin, an evolutionary biologist at the University of Minnesota who was not involved in the research, said he was surprised that both big and small cities showed the same rate of change for production of hydrogen cyanide in white clover. He said he’d also like to see research that explores whether changes in urban environments radiate outward to populations in more rural settings.
“We are part of the environment. We are shaping things,” Dr. Tiffin said.
Jonathan Losos, an evolutionary biologist at Washington University in St. Louis, said he was impressed at the rate of change that the study picked up.
“We used to think that evolution moved at a glacial pace, but we now know that’s wrong,” said Dr. Losos, also the director of the Living Earth Collaborative, a joint effort of the university, the St. Louis Zoo and the Missouri Botanical Garden. “Species can adapt very quickly when natural selection is strong.”