A pigeon overlooks New York City
Rapid development spells unintended consequences for winged denizens. Courtesy of Jason Munshi-South and Marc Johnson

Urbanization has unintended consequences on city-dwelling creatures, from the peppered moths of the Industrial Revolution to today’s pesticide-resistant bed bug.

Historically, the most convincing case for evolution came from the the parts of the natural world left largely untouched by humans. It was, after all, on the isolated Galapagos Islands off the coast of Ecuador that finches—and their many different beaks—helped shape Charles Darwin’s famous theory back in the 1800s.

Even at the time, though, evidence for evolution could be found inside bustling and rapidly growing cities. Look no further than the peppered moths. Their white-and-black-speckled wings once helped them blend in with similarly colored trees. But as the Industrial Revolution covered those trees with soot, the moths with all-black wings (a genetic mutation) survived and passed on their genes. The moths eventually evolved to become all black in certain parts of England.

Fast forward to today, when the human population is booming, and increasing numbers of people are finding their ways to cities. Urban dwellers currently make up half of the global population, and by 2050, another 2.5 billion people are expected to move to cities. Existing metros will expand, and new ones may even crop up. “Urban areas cover about 3 percent of Earth’s land surface, and that number is continuing to rise,” says Marc Johnson, director of the Centre for Urban Environments at University of Toronto Mississauga. And while researchers don’t fully understand how urbanization affects the evolution of life in and around cities, there have been hundreds of studies exploring the question.

The map shows the cities where urban evolution has been studied. Those marked in blue represent commensal species—those that have evolved to live with humans. (Courtesy of Jason Munshi-South and Marc Johnson)

For their latest paper, published Thursday in the journal Science, Johnson and his colleague Jason Munshi-South, an evolutionary biologist at New York City’s Fordham University, reviewed nearly 200 studies, most of which are from the last five years. “What we find is that [urban development] frequently influences evolution, often very rapidly,” Johnson says. And with unintended consequences.

For that reason, Johnson adds, “Urbanization is the best and largest-scale study of evolution ever attempted, and it’s sitting literally in our backyards.”

CityLab spoke with the researchers about the most notable consequences of urbanization, what it means for the organisms living among us, and ultimately, how the effects all come back to humans.

You start off your study by talking about this idea of convergent cities. Can you explain what that means?

Johnson: If you go to Beijing, New York City, or Los Angeles, you’ll see certain features—more impervious surfaces, increased lights and noises, and [warmer temperatures] inside the city than outside. For some of these variables, two cities in different parts of the world can be more similar to one another than they are to a more natural environment [nearby]. They can lead to completely unique ecosystems, which then lead to natural selection adaptation for certain urban forms.

Some of the best examples come from species that depend on us humans. There are two basic scenarios: One [is] where an organism originally evolved to be adapted to human environments and then is spread around. Then there are organisms that seem to have independently evolved the same types of traits and genetic changes in parallel across these cities. Before the Industrial Revolution, European blackbirds were largely a forest species—but now when you go across Europe, you see them all over the place in urban environments. The genetic data shows that these invasions have happened repeatedly and independently, and that the evolution of a number of behavioral traits—[such as] becoming more familiar and less vigilant to human presence—have also applied in parallel but converge across different environments.

How does this apply to pests?

Munshi-South: There is a set of species referred to as commensal species that evolved to live with us as we became more social and settled in denser agricultural villages and towns. They spread around the world with us, so basically every major city is going to have rats, house mice, pigeons, bed bugs, and German cockroaches. They're experiencing waves of evolutionary pressure: If you look at the types of insecticides that are used against roaches in North America, several populations evolved to avoid them.

Johnson: Understanding how the development of cities and human activity can influence evolution of these pests—both adaptive and non-adaptive—is important to human welfare and to the spread of disease. For example, bed bugs have re-emerged in the last two decades because they have rapidly adapted to human persecution. The use of insecticides initially causes a population decline, but the other effect is that it imposes very strong natural selection on those populations so that if there is any mutation that allows them to resist that insecticide, individuals with that mutation are spreading more of their gene copies.

Are we approaching pest management the wrong way?

Munshi-South: Part of the gap in current pest management practices has to do with not understanding how populations are structured across space and time. Are the rats within a few city blocks all sort of an evolutionary unit, or is it larger than that? You have to address the entire unit, where you still have rats that are related and could continue to replenish the local population.

There are some ideas out there about using evolutionary biology and biotechnology to, if not defeat pests, deal them a serious blow. [These include] engineering genetic elements that could be released into populations, which would cause negative effects on reproduction. Those ideas have been tried on very limited mosquitoes, and in the next 15 to 20 years you’ll start to see those for other pests. We’re not advocating that strategy. The ideas could be pursued, but they’d need to be properly validated for environmental safety and unintended consequences.

The diagram shows the rural-to-urban gradient around cities, and the barriers that can isolate groups of the same species. (Courtesy of Jason Munshi-South and Marc Johnson)

What about organisms that aren’t pests? What is one of the more common evolutionary effects of urbanization?

Munshi-South: We’re talking about genetic differentiation and loss of genetic variation in populations within cities. My work has looked quite a bit at animals that occupy parks in New York City, which we kind of conceive of as islands. The native white-footed mouse can thrive in the little forests that remain in these parks, but they can’t leave them very easily. So over time, their populations within a single park become isolated and slowly accumulate differences from generation to generation. You can detect that those populations are becoming different [from ones in other parks], so if you gave me a mouse from any park, I could probably take it back to the lab and, using a small set of genetic markers, tell you where it came from.

Should we be concerned? Not necessarily. It could be that species maintain large populations just fine in these parks. But it’s also possible that they lose variation over time and become inbred. Then you might worry that those populations may not last very long. That doesn’t seem to be the case with white-footed mice, but we’ve studied salamanders that are isolated on a couple rocky hillsides next to a highway in NYC, and they have very little genetic variation left. Given enough time, they may not be able to withstand the diseases that come through the population or a change in water quality.

An interesting thing about the peppered moths is that some of them eventually reverted back to their original colors. Can we reverse certain effects of urbanization on evolution, and should we?

Munshi-South: With the peppered moths, the selection pressure reversed because over time, people demanded cleaner air and a better approach to managing pollution. In examples like this, if we attempted to reverse evolution, we’re probably benefiting ourselves and our cities. The killifish, which lives in eastern North America, have all evolved the same exact mutation in the same gene to avoid damage from PCB, a pollutant [found in several waterways] that has a very strong endocrine-disrupting effect. But it would almost certainly be good for that broader environment, and for humans, if we could reduce or contain PCB pollution in those waterways. That species was able to adapt, but there are other ones that haven’t been able to.

What should be the main takeaway for other researchers, urban planners, and even policymakers?

Johnson: This is a bit of a wake-up call for both scientists and policymakers that we know very little about how our cities are influencing life around us, and especially the evolution of life. There’s a suggestion that this evolution can have positive consequences in terms of conservation of some native species and their ecosystem-level effects. For example, we find that white clovers are adapting to and persisting in city environments. That’s important because white clover is the single most important source of nectar in urban environments, supplying two-thirds of all the nectar that bees use in these types of environments.

At the same time, pest organisms can adapt to urbanization. If we can use our understanding of evolutionary biology to design better and more effective ways of controlling those species, that’s a win-win.

Munshi-South: This old paradigm where scientists used to go out of their way to avoid the influence of humans has really shifted, and we’re going to see more evolutionary biologists thinking of cities as a major evolutionary force.

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