For a long time, biologists thought evolution was a very, very slow process, too tardy to be observed in a human lifetime. But recently, we have come to understand that evolution can happen very quickly, as long as natural selection — the relative benefit that a particular characteristic bestows on its bearer — is strong.

And where else to find such strong natural selection than in the heart of a big city? The urban environment is about as extreme as it gets. Temperatures in the city center can be more than 10 degrees higher than in the surrounding countryside. Traffic causes continuous background noise, a mist of fine dust particles and barriers to movement for any animal that cannot fly or burrow. Much of the city is clad in impervious surfaces of stone, glass, steel and tarmac. There is pollution of soil, water and air, mainly human-derived food sources, and an especially motley crew of local and invasive flora and fauna.

With urban environments expanding all over the world, wildlife and biologists alike are starting to treat the city as a true ecosystem. Many species’ original habitats are being squeezed into annihilation. For them, it’s adapt or die. And field biologists like me are following suit. As we have to travel ever farther to find untouched wilderness, we are beginning to realize that the expanding urban sprawl is perhaps not something to be depressed about, but rather something very exciting, as entirely novel forms of life are evolving right under our noses.

A Fordham University biologist, Jason Munshi-South, studies the populations of white-footed mice marooned in New York City parks. These native mice once lived all over the place. But as the city expanded, they became confined to the small pockets of forest left behind in parks. Thus isolated, the mice in each park began evolving a park-specific genetic blueprint. In some parks, Dr. Munshi-South found mice carrying genes for heavy metal tolerance, probably because soils there are contaminated with lead or chromium. In other parks, the animals have genes for increased immune response — maybe diseases spread more easily in some high-density populations.

French biologists have been studying a daisylike weed called Crepis sancta, which normally produces two kinds of seeds: heavy ones that fall to the floor, and light seeds that drift in the wind for long distances. But in Montpellier, in southern France, C. sancta makes reduced numbers of the airborne seeds. Small wonder: The plants grow in pockets of soil on sidewalks, and any seeds that are carried on the wind are likely to land on concrete. The heavy seeds that land at the parent plant’s feet, on the other hand, are pretty certain to find a patch of fertile soil. So plants genetically predisposed to produce more heavy seeds have been favored by urban evolution.