Urban areas are home to a large proportion of the world’s population and represent places where environments and ecosystems are altered to suit the needs of humans. Urban environmental change can influence evolutionary processes, such as the ways in which plants and animals adapt to local conditions and selection pressures. Most of the known examples of animal or plant adaptation to urban environments involve a single city, or a few cities in the same region, making it unclear whether species can adapt to urban habitats in similar ways across cities throughout the world.
This would be an example of parallel evolution, where populations evolve in similar ways in response to similar environmental selective pressures in cities all over the world. Urbanization can lead to similar environmental changes across cities, but whether this convergence causes parallel evolution has never been examined at a global scale.
In a study published today in the journal Science, 287 scientists from around the world collaborated to test whether a common species of plant, the white clover (Trifolium repens), has undergone similar evolutionary changes in 160 different cities in 26 countries. The Global Urban Evolution Project (GLUE) was led by evolutionary biologists at the University of Toronto, Mississauga, and their report gives the clearest evidence yet that humans can be a dominant force driving evolution.
White clover is a cosmopolitan species that is found in cities and in rural areas all over the world. In addition, it produces hydrogen cyanide (HCN) as a means to deter herbivores that feed on its leaves, and to cope better with water stress. The collaborating scientists tested the levels of HCN produced by white clover plants in cities and in the nearby rural areas, with the premise that herbivory would be less of a selection pressure in cities.
The research found that clover growing in cities was, in fact, less likely to produce HCN than clover growing in nearby rural fields. The probability that a plant produced HCN increased by 44 percent on average from the center of an urban area to the furthest rural population, indicating that there is less pressure from herbivory in cities and more in rural ecosystems.
The results also found that clover growing in a city was more similar to clover in other cities than it was to clover growing in nearby rural fields, despite the fact that genetic exchange between urban and rural populations took place freely. This result indicated repeated adaptation to conditions prevalent in urban environments. Humans change urban environments in many ways, but the reduction in herbivores appears typical of cities everywhere.
“We’ve long known that we’ve changed cities in pretty profound ways and we’ve dramatically altered the environment and ecosystems,” said study co-lead author James Santangelo. “But we just showed this happens, often in similar ways, on a global scale.”
In addition to observing global adaptation by clover plants to living in cities, the researchers also identified the genetic basis of this adaptation. From the more than 110,000 samples of clover collected by the collaborating scientists, 2,500 individual genomes were sequenced and used to assess genetic differentiation between city and rural populations. This has created a massive dataset that will be of use to scientists for years to come.
“This study is a model to understand how humans change the evolution of life around us. Cities are where people live, and this is the most compelling evidence we have that we are altering the evolution of life in them. Beyond ecologists and evolutionary biologists, this is going to be important for society,” said Professor Rob Ness, who co-led the project with Professor Marc Johnson and their PhD student Santangelo.
The study was made possible by the collaboration of several hundred scientists from around the world, something that Johnson refers to as a model for inclusive science. Men and women were equally represented on the team, and it included not just established researchers, but also students at all levels and from all inhabited continents across the world.
“Nearly everyone we asked to collaborate said yes – and that was kind of remarkable, because we were asking people to take on a lot of work,” said Johnson, who coordinated the more than 280 other researchers who participated in the study. “Our collaborators recognized the importance of this project. There has never been a field study of evolution of this scale, or a global study of how urbanization influences evolution. It would have been impossible to do this without our global set of collaborators.”
Now that the potential impact of humans as drivers of evolution in city ecosystems is understood, this can be used to develop strategies to conserve rare species and allow them to adapt to urban environments. It can also help us better understand how to prevent unwanted pests and diseases from adapting to human environments, says Johnson.
“Urbanization is increasingly transforming rural and natural environments into unique ecosystems that Earth’s biodiversity has never experienced, and these changes are altering the evolution of life,” concluded the study authors. “If adaptation to urban environments is common, then this could have cascading effects on populations and ecosystems.”