Mounting evidence suggests that humans are now a major driving force of evolution on Earth.
From selective breeding to environmental modifications, we are altering our world so much that we not only direct the climate, but also the direction of life itself.
In a massive project involving 287 scientists in 160 cities in 26 countries, researchers examined how urbanization has influenced evolution on a global scale.
Their study, published last year, used white clover (Trifolium repens) as a model – a plant native to Europe and Western Asia, but found in cities around the world.
“There has never been a field study of evolution on this scale, nor a comprehensive study of how urbanization influences evolution,” said evolutionary biologist Marc Johnson of the University of Toronto Mississauga (UTM).
By collecting more than 110,000 samples along gradients that stretched from cities, through suburbs, and out into the countryside, they found that clover in cities is now more similar to clover in another city than a more distant world than it is to that found in nearby farmland or forest. , whatever the climate.
This is an example of parallel adaptive evolution – when distinct populations are shaped by the same selective pressure for specific traits in different places. This shows that how humans have modified the environment has a greater influence on the formation of these traits than natural phenomena such as local population genetics and climate.
While urbanization obviously shares many characteristics across the world, it was not yet established that these acted together to push evolution in the same direction.
“We have just shown that this happens, often in similar ways, on a global scale,” said UTM conservationist James Santangelo.
“For urbanization to drive parallel evolution, urban areas must converge on environmental features that affect an organism’s fitness,” the researchers said. explained in their article.
On closer inspection, the international team identified that one of the changing characteristics along the urban to rural lines was the plant’s production of hydrogen cyanide. White clover uses this chemical as a defense mechanism against its herbivorous predators. It also helps them resist drought.
Factories in the most remote rural populations were 44% more likely to produce hydrogen cyanide than those in the center of cities. It seems that grazing promotes the production of more hydrogen cyanide in rural areas than in cities, where grazing pressure is lower; in the absence of this pressure, drought becomes the determining factor.
This is despite strong gene flow between white clover populations along each gradient, meaning levels of this chemical are heavily selected, time and time again.
We have already broken the life size spectrum animals in the ocean, in part by selectively fishing out large fish, leaving more fish with small fish genes to create subsequent generations. Many fish are now 20% smaller and their life cycles are on average 25% shorter.
The unintended consequences of our actions also change the shape of birds.
“The wing span of cliff swallows has evolved to be shorter near roads, with road-killed swallows having longer wings, consistent with selection for increased maneuverability in traffic,” said zoologist Sarah Otto. explained in 2018.
These latest findings provide another example of a clear evolving urban signal. Studies have already suggested rates of evolutionary change are higher in urbanized landscapes than in natural, non-urban human systems.
“This is the most compelling evidence we have that we are changing the evolution of life in [cities]. Beyond ecologists and evolutionary biologists, this is going to be important for society,” said UTM biologist Rob Ness. Especially since we are expected to triple the area of urbanized land by 2030 compared to 2000.
The researchers amassed a large database that they can now study further for human impacts on clover evolution. By better understanding how we incidentally drive such changes, we have a better chance of being able to intentionally take the wheel and direct evolution in informed and safer ways.
“This knowledge could help retain some of Earth’s most vulnerable speciesmitigate the impacts of pests, improve human well-beingand contribute to the understanding of fundamental eco-evolutionary processes,” the authors concluded.
This research was published in Science.
An earlier version of this article was published in March 2022.