Why we are living in an era of unnatural selection

Human-induced trait change has been observed in animals on every continent other than Antarctica.

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, and more docile. In 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, originally to deal with beetle infestations in sugar plantations, the mouths of black snakes have shrunk as 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 UK as climate change extends their range.

“There has never been another species that has so quickly changed the course of evolution,” says Sarah Otto, an evolutionary biologist at the University of British Columbia. “Darwin would be shocked!”

We can’t always know what causes a particular change, says Otto, whether it’s plasticity in action or the beginning of cladogenesis, where distinct sub-populations form. But there are enough examples where genetic change is involved to know that something deeper is going on.

“Swans that avoid cities have a genetic difference from the ones that are human-tolerant,” she says. And she points to the difference between UK-migrating blackcaps and birds that still migrate to Iberia as being “very clearly genetic”. “The young carry this difference,” she says. Changes like this are the first steps to the emergence of a new species. “The London Underground mosquitoes are an example where we might be forming a new niche and creating new opportunities for speciation,” Otto adds.

I if we are narrowing the opportunities for species to evolve by interacting with their environments – 36% of the planet’s land surface is given over to agriculture, while urban environments around the world resemble one another. One study found that the mass of plastic is now greater than all living biomass. Biodiversity is haemorrhaging due to human activity, according to many analyses. “We are homogenising the planet in some ways,” she agrees. “On the other hand, we’re making these really extreme environmental shifts. Urban environments are entirely different from our agricultural environments.”


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