Humans are changing the way plants evolve worldwide, study says

TORONTO, Ontario — Human behavior is changing the way life evolves on Earth, a new study finds. Researchers from the University of Toronto say plants adapt in unusual ways because of human actions, like urbanization.

The phenomenon – dubbed “unnatural selection” – is one of the greatest biological forces on Earth. It has created “parallel urban worlds,” where wildlife has had to respond or die.

“We’ve long known that we’ve changed cities in pretty profound ways and we’ve dramatically altered the environment and ecosystems,” says co-lead author and PhD student James Santangelo in a university release. “But we just showed [the reverse] happens, often in similar ways, on a global scale.”

The findings come from an analysis of white clover collected from 160 cities and nearby rural areas in 26 countries. A common weed in European gardens, it is famous for its three “trefoil” leaves. The study provides the clearest evidence to date that people in towns and cities are driving nature to adapt.

The shamrock-shaped plant is seeing genetic alterations from London to Lisbon, Toronto to Tokyo, and Melbourne to Munich. Researchers say humans are profoundly transforming their surroundings, which is dramatically impacting organisms they share the land with.

Life is evolving differently in cities

The Global Urban Evolution Project (GLUE) study showed environmental conditions tend to be more or less the same in nearby rural habitats. So, downtown Toronto is more comparable to downtown Tokyo than to farmland and forests just outside the city.

For instance, white clover produces hydrogen cyanide to deter herbivores and increase tolerance to drought. The international team found city varieties produce less due to changes in the presence of plant eaters and water stress. These results apply across different climates and the implications reach far beyond the humble clover.

“This study is a model to understand how humans change the evolution of life around us,” says co-lead author Dr. Rob Ness. “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.”

White clover is present in almost every city, providing the perfect tool to investigate human influence on evolution. It originates from the same time as the domestication of dogs 30,000 years ago. Now, industrialized farming, introduced species, urbanization, pollution, and climate change are creating unprecedented selective pressures.

How is life evolving to account for humans?

Evolution – at least for larger, more complex organisms – can be slow. It leaves many animals unable to adapt fast enough. However, rapid change is also possible, through an inbuilt genomic plasticity. The most famous example is the peppered moth.

It changed from speckled white to black in response to soot and air pollution rising from the chimneys of the Industrial Revolution in Britain. Today, worker bees in industrial beehives – transported from farm to farm across the United States in convoys of trucks – are one-third larger than their wild cousins.

Over the past 100 years, North American songbirds have modified the shape of their wings to cope with habitats fragmented by deforestation. Under pressure from poaching, Zambian elephants are born without tusks.

Since the introduction of cane toads to Australia in 1935, to deal with beetle infestations in sugar plantations, the mouths of black snakes have shrunk. Succeeding generations learned to avoid toad-sized prey, while the toads themselves have become cannibals, victims of their own success as predators.

Sea-snakes in Papua New Guinea have developed darker bodies and shed their skins more often in response to toxins in the zinc-polluted waters they inhabit. One species of mosquito has evolved to live only in the tunnels of the London Underground and lost the capacity to breed with its surface-dwelling cousins. Similar declines in genetic diversity have been observed in mosquitoes in the New York and Chicago subway systems.

Blackcaps have shifted their migration routes from the Iberian Peninsula to the U.K. as climate change extends their range. Swans that avoid cities have a genetic difference from the ones that are human-tolerant.

Finding a way to preserve life on Earth

The study in the journal Science opens the door to developing strategies to conserve rare species in towns and cities. It can also help us better understand how to prevent unwanted pests and diseases.

The 287 scientists sequenced more than 2,500 clover genomes from over 110,000 samples. They then created a massive dataset that will be studied for years to come. Interestingly, the unprecedented initiative began with a single tweet on social media.

“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,” says study author Professor Marc Johnson. “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.”

South West News Service writer Mark Waghorn contributed to this report.

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