November 2017

For a large segment of young people in the United States, the bright lights of the big city represent a real temptation. Continuing a trend that started more than 200 years ago, Americans (and now millenials in particular) are flocking to urban areas. Today more than 80% of Americans live in cities. The draws of this new life seem clear: entertainment, excitement, and opportunity. Of course, city life can also change you in not-so-desirable ways. Urban living has been associated with asthma, allergies, depression, and anxiety. But what about the non-human city dwellers — the bugs and beasties lurking in our metropolitan interstices? How is the hustle and bustle of urban life affecting them?

Where's the evolution?

Recent research has investigated the myriad ways that non-human urbanites are evolving in response to the unique challenges and opportunities presented by city environments. All of the stereotypical city-dwelling “pests” — rats, mice, cockroaches, bedbugs — have evolved in some way or another, but so have species that are not as prominent in urban life: lizards, moths, salamanders, gulls, bobcats, clover, finches… the list goes on.

Some of this evolutionary change is not adaptive or directional. Cities and the constructed barriers within them often divide large populations into smaller ones, which diminishes their genetic variation and heightens the impact of random evolutionary changes caused by genetic drift. For example, white-footed mice used to roam over Manhattan, but as the urban landscape grew and was paved, the habitat for this species was reduced to the city’s discrete green patches: parks. Today, the groups of mice that inhabit different parks are distinct from one another at a genetic level. Other aspects of the urban environment, like sudden and large reductions in population size when a pest control measure is introduced, also contribute to such random evolutionary changes. For example, bobcats living in Los Angeles experienced a population bottleneck (and corresponding decline in genetic variation) in the early 2000s when many individuals died after eating rodents that had ingested a rodenticide.

Others cases of city-spurred evolution are adaptive and caused by natural selection. For example, over many generations of feeding at local birdfeeders, Tucson house finches evolved longer and wider beaks that are better for cracking open the tough sunflower seeds with which these feeders are usually stocked; city-dwelling anole lizards in Puerto Rico evolved longer, stickier toes, which seem to be better for climbing over hard surfaces; and urban blackbirds and swans have undergone selection favoring individuals that are less wary of humans. The wide variety of these adaptations highlights the many idiosyncratic ways that cities change the environment.

Other adaptive changes involve prototypical features of city life — pollution, for example. In the best known example, air pollution that blackened trees favored peppered moths that blended in better with this dark background, leading to the adaptive evolution of darker wings. And at least two fish species now have adaptations that allow them to better survive and reproduce in polluted waters. Some studies have shown that urban pollution has the capacity to increase mutation rates, pointing to the fascinating possibility that cities may speed the evolution of creatures that inhabit them, not just by providing new selective regimes and by splitting and combining populations in new ways — but by actively increasing genetic variation, the raw material that is the starting point for the evolution of brand new traits.

All of this change occurring in urban areas highlights both the inevitability of evolution and its relevance to modern life. Yes, biological evolution started billions of years ago, and we can understand it in part by studying ancient life forms. And yes, evolution is responsible for the brilliant biodiversity of wild places, like Earth’s rainforests. But it’s not limited to these situations. In fact, evolution is everywhere; it’s going on all around us, everyday, wherever it is that we call home — whether that’s an island, forest, shoreline, desert, or even the heart of the city.