Read more: 鈥�Unnatural selection: How humans are driving evolution鈥�
In the subways of Europe, mutant mosquitoes lurk. They look exactly like the Culex pipiens found above ground, but behave very differently. They breed throughout the year and can mate in confined spaces rather than in big swarms.
What鈥檚 more, the larvae can extract enough nutrients from the pools of water they develop in, meaning that adult females don鈥檛 need to eat a blood meal for extra nourishment before laying their eggs. Given the chance, though, they nevertheless bite voraciously. And while the above-ground mosquitoes bite birds, the below-ground ones bite anything, including humans, rats and mice.
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The mutants were first identified in the tunnels of the London underground by of Queen Mary, University of London. 鈥淟ondon Underground gets very upset if you mention them,鈥� he says. 鈥淭hey will deny that they are there.鈥�
At first Nichols thought the below-ground strain had evolved in London after the underground was built. But later work by of Rutgers University in New Brunswick, New Jersey, showed that the below-ground strain is found in subways throughout Europe (). 鈥淭here are very few genetic differences between this form and the pipiens form, which supports a recent split, but because of that lack of variation it is hard to get strong inference on timing,鈥� says Fonseca.
So while it appears likely that at least some of the underground mosquito鈥檚 traits are an adaptation to city living, definitive evidence is missing. The situation is similar for the hundreds of other organisms that have taken to living with or near humans, from old acquaintances like rats and cockroaches to recent arrivals such as blackbirds and foxes.
Many urban animals behave very differently to their wild cousins, but these behaviours may be learned rather than hereditary. 鈥淚 think these changes are mainly due to plasticity,鈥� says of the Center for Evolutionary and Functional Ecology in Montpellier, France.
of Stanford University in California agrees, but points out that another form of selection may be at work where animals have managed to adapt to cities by mimicking the behaviour of their parents or other group members. 鈥淚t鈥檚 a case of cultural evolution,鈥� he says.
In some cases urban organisms have been shown to be genetically different to their rural cousins, but this is not necessarily a result of (un)natural selection. 鈥淚t鈥檚 plausible but we just don鈥檛 know,鈥� says of the University of Sheffield, UK, who studies urban birds. Some differences could be a result of genetic drift, he points out, due to urban populations being founded by small groups that by chance just happened to be different.
While it has yet to be proven, Cheptou thinks it is likely that many organisms are evolving rapidly to cope with urban living. His team has shown that this is happening in at least one case, with a weed called Crepis sancta. Its seeds are usually dispersed by the wind. In Montpellier, however, the weed has evolved heavier seeds that fall straight to the ground (). The reason is that wind-blown seeds are likely to end up on paved areas, while ones that fall to the ground end up in the same patch of soil as their parent.
鈥淵ou expect more adaptation to urban environments in plants,鈥� says Cheptou, who is now looking to see if plants are evolving in response to yet another human pressure 鈥� the loss of pollinators. His finding is an example of how some adaptations can be counterproductive in the long term. Plants that cannot disperse their seed widely are much more likely to die out when the environment changes.
Fonseca鈥檚 work, meanwhile, shows how evolution can have unpredictable consequences. In northern Europe, the below-ground and above-ground mosquitoes do not intermingle, probably because the cold kills the subway strain. But in southern Europe, where it is warmer, they form hybrids. Fonseca found that one of these hybrids strains had been introduced to the US, where it has flourished above ground. Because it bites both people and birds, she thinks it played a big role in the spread of West Nile disease in the US.
Read next article: 鈥�Unnatural selection: Wily weeds鈥�
Read more: 鈥�Unnatural selection: How humans are driving evolution鈥�
