During World War II, Londoners fled to hide in subway tunnels, bloodthirsty mosquitoes waiting for them. They stung like angry. These were mosquitoes of the species that commonly live in London parks and attack birds preferentially. Although mosquitoes found safe shelter from natural enemies in the metro area, the birds were missing for understandable reasons. But there were plenty of rats.
And so the mosquitoes retrained as mammalian blood drinkers. People looking for shelter from German bombs in the subway were a great feast for them. This earned underground mosquitoes a zoological designation Culex pipiens molestus that is, a mosquito difficult. The London mosquitoes, adapted to the subway tunnels, were one of the first revealed examples of evolution that took a whole new direction in the city.
We often imagine evolution as a process whose results are not visible until millennia ago. But this is a mistake. Evolution can pick up a frantic pace. For example, a recent study showed that octomiles change hereditary information from generation to generation so quickly that it allows them to adapt to different seasons. Sure, it records a rapid multiplication, but even so, we are witnessing significant evolutionary changes within generations, which we would count on the fingers of both hands.
Not all creatures can cope with the demands of city life. One of the conditions is the relatively large variability of behavior, which today scientists often do not hesitate to call a personality. In an urban environment, creatures that are more fearless and resistant to stress usually prevail. For example, the little owl in the 300,000-strong Argentinian city of Bahía Blanca managed it.
The tiny owl, about twenty centimeters tall, is conspicuous by its long legs. They are useful for her when hunting insects and other small animals, which she does not chase from the air, but chases after them on the ground. The little owl also differs from most of its relatives in that it prefers prey at dusk or dawn to night hunting.
The owl got its name from the fact that it nests in the abandoned burrows of smaller mammals. This is an absolute exception among owls. Urban owl populations are thriving in Argentine cities. If in the Argentine countryside their nests are usually about fifteen kilometers apart, it is no exception in the city when two nests are only ten meters apart.
Cooper’s Hawks are similarly successful in the American urban environment. They are attracted by a large population of pigeons. They are doing great. Maybe too much, because the new Mexican Albuquerque, for example, is already crowded with hawks. Many young predators are forced to leave their hometown and find a place to live on its outskirts.
There is thirty birds per hawk that moves into the city, which gives the city its ramparts. It might seem that the birds raised in the city will be outsiders in a rural environment. But the exact opposite is true. Urban outcasts often push rural natives out of their nests.
Urban blackbirds use street lighting to extend the time they can forage, and they also use courtship on a day stretched by hours of artificial lighting. They have a hereditary internal biological clock adapted for this. In the hereditary information of urban blackbirds, researchers have identified several gene variants that are beneficial to urban life, and evolution clearly favors them in urban birds.
The degree of adaptation to the urban environment sometimes goes surprisingly far. The city is a large source of pollution and many times the animals are exposed to highly toxic substances. But even some creatures can adapt. An example is the stocks of some fish living in the American Hudson River.
Naturally, there were individuals among them with talents for increased resistance to dangerous polychlorinated biphenyls. In pure water, the talent offered no advantage and therefore remained very rare. However, as factories began discharging polychlorinated biphenyls into the river, the rare talent increased the chances of survival and began to spread in the fish population.
Between 1947 and 1976, 600 tons of highly stable and toxic polychlorinated biphenyls ended up in the Hudson River. The poisons remained in the water and in the sediments at the bottom long after their release into the river was stopped. Many species of fish species have been carried away. They had severe deformities, such as a jaw failure.
Tomkod cod (Photo: Joanie Côté, Public domain)
Fish originally protected by a rare variant of the gene, but thrive in a poisoned environment. Tomkod cod is resistant to polychlorinated biphenyls. The American fish Fundul Mummy has adapted to waters contaminated with polycyclic aromatic hydrocarbons.
Pollution also carries with it the presence of substances that damage hereditary information. Mutations induced in this way can increase resistance to contamination. the industrial environment wants mutations of hereditary information not only of humans but also of animals, as evidenced by DNA analysis. Extreme examples are offered by the surroundings of the crashed nuclear power plants in Chernobyl and Fukushima, where the incidence of DNA damage in bacteria, plants and animals is significantly higher than in areas where the organisms did not face radiation.
However, relatively common pollution does not remain without an effect on hereditary information. Gulls living in the American countryside carry fewer mutations in hereditary information than gulls inhabiting large cities. If these birds lived near cities with a developed steel industry, where the level of pollution is significantly higher, it affected their DNA even more. It is not clear whether these mutations were such as to make it easier for seagulls to live in an infected environment.
Swallow (Photo: Don DeBold, CC BY 2.0)
The American Swallow, like our swallows, builds nests out of clay that sticks to the walls of buildings. In Nebraska, these birds began to build nests on the walls of bridge piers after 1982. They liked the bridges and overpasses of the busy highways the most. Swallows paid a bloody tax on this choice. Many of them died after crashing into a high-speed car on the artery during the flight.
In thirty years, however, the number of birds killed by cars has dropped to a fifth. A closer look at the swallows showed that the birds’ wings had shortened during that time, improving their maneuverability. Thanks to this, birds can avoid cars today and life on a busy highway is no longer so dangerous for them.
In cities, the average temperature is several degrees higher than in the surrounding countryside. In many places, walls, roads or sidewalks are embedded in the sun, and then their temperature reaches extreme values. Animals can adapt to this as well. For example, urban ants of the species Temnothorax curvispinosus they resist the temperatures at which their rural relatives run into serious difficulties.
The plants can also adapt to the city and its specific conditions. Weed species Crepis sancta has adapted to life in the city in twelve, but perhaps only five, generations. It has developed much heavier seeds that do not fly far in the air and will end up not far from the mother plant.
While in nature spreading seeds over long distances is advantageous, in the concrete jungle of the city, places with open land are rare. If the seed lands close to the mother plant, it has a good chance of falling on the soil where the mother plant grew and has a chance to germinate. When flying at a greater distance, it will most likely fall on concrete, asphalt or other paved surfaces and will not take root there.
The latest grandiose Global Urban Evolution Project (GLUE), led by evolutionary biologists from the University of Toronto, has followed the urban adaptation of the creeping clover. Researchers have looked at whether evolution is taking place in parallel in cities around the world.
287 scientists took samples of the white clover plant in 160 cities and nearby rural areas in 26 countries. The team led by James Santangelo thus collected a total of samples from 110,019 white clover plants from 6,169 populations of this weed plant.
Crepis sancta (Photo: Bernard DUPONT from FRANCE, CC BY-SA 2.0)
The results of their research, published in the scientific journal Science, are so far the clearest evidence that people and the cities they build are the dominant force driving the development of life around the world. From Toronto to Tokyo, from Melbourne to Munich, white clover often develops in direct response to changes in living conditions imposed by the urban environment.
In cities, it produces less cyanide, which serves as a protection against herbivores. It is a logical evolutionary act, because the synthesis of cyanide depletes the clover and reduces its drought resistance. In the city, saved herbivores are not in danger of clover, but the lack of moisture will be plentiful and plenty. Without cyanide production, it has a better chance of survival.
“Urbanization is increasingly transforming the rural and natural environment into ecosystems that terrestrial life forms have not yet experienced. And this transformation shapes the development of life, “write the authors of the study. “If adaptation to the urban environment is common, then it could have a knock-on effect on populations and ecosystems.”
Urban development can drastically change the local environment and create new ecosystems that can drive rapid evolution in a wide variety of organisms. It could then decide the fate of some of the most vulnerable species of plants and animals on our planet.