Plants and animals have a long history of acclimatizing to city living – think of raccoons and their expert pillaging of compost bins. But now biologists are beginning to see signs that something more fundamental is happening. They say wild things may be changing at a genetic level to survive cities and their polluting, habitat-fragmenting ways.
Fish in New York's chemically-laden Hudson River have evolved a genetic variation that gives them resistance to PCBs, for example. Birds nesting under highway overpasses in Nebraska have developed shorter, more agile wings, allowing them to quickly swerve from oncoming traffic.
And weeds occupying patches of earth surrounding sidewalk trees in France have evolved to produce fewer dispersing seeds, which travel on the breeze and fall uselessly onto concrete. Instead, they produce compact seeds that drop close to the plant where they can germinate.
On one hand, urban evolution is not new. Peppered moths in Britain changed colour from white to black in heavily polluted areas during the Industrial Revolution. White moths were picked off by predators while the black ones, camouflaged in a newly sooty environment, survived to breed more black moths.
What may be different this time is the number of city-dwelling creatures evolving to live in inhospitable habitats.
As cities grow in population and size, so too does their influence on the environment. One hundred years ago, two out of every 10 people were city-dwellers. Today, more than half of us live in cities that are spreading across more and more of the planet.
A small but growing number of scientists say urban evolution may be accelerating in tandem with that growth. And there could be tradeoffs that we are only beginning to glimpse.
It pays to downsize
University of Tulsa ecologist Charles Brown says he was surprised it took just 30 years for the cliff swallows in his study to evolve shorter wings that help them avoid traffic.
Since 1982, he and Mary Bomberger Brown, an ornithologist at the University of Nebraska–Lincoln, have been studying a group of birds that make their gourd-shaped mud nests under highway overpasses in southwestern Nebraska.
Over the years, they recorded a steady drop in the number of road-killed birds. This came as a surprise, because the colonies were growing and traffic had not declined. But as they compared the wing length of road-killed birds with those caught in nearby mist nets, they were in for another surprise – those caught in mist nets had noticeably shorter wings.
The researchers, who published their results last year in the journal Current Biology, believe net-caught birds avoided road deaths thanks to shorter wings that let them dodge traffic. Unlike their road-killed cousins, they survived long enough to pass down genes for shorter wings.
But is such urban evolution a necessary and positive development, or an evil to combat?
"It often results in an organism becoming better adapted to its environment," says Brown. "I suppose it's good if we are hoping that the organism persists."
Isaac Wirgin has a different view. He is a specialist in environmental medicine at New York University Medical Center and a lead author of a 2011 study in the journal Science on pollution-resistant tomcod fish in the Hudson River.
"In my mind, it's not a good thing," he says. "Usually evolution theory says if you adapt to something – like this resistance phenotype in tomcod – you're less good at reproduction or life expectancy, or you're more sensitive to other stressors."
Written By: Sharon Oosthoek
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