When humans settled into larger villages and towns some 12,000 years ago along with the advance of agriculture, wildlife took advantage. Those species “preadapted” to survive in early cities, including mice and rats, grabbed onto the coattails of modern civilization and haven’t looked back since. Now they pretty much depend on the underbelly of human society for survival. A new study published in the journal Science—the first to take a broad look at the way urbanization is affecting evolution—posits that countless other species are now faced with the evolutionary conundrum how to thrive in the shadow of humanity’s ever-expanding footprint.
In reviewing nearly 200 studies, most from within the last half decade, the researchers look at cities throughout the world, viewing them as “an unintended but highly replicated global study of experimental evolution.” This global urban experiment is really just getting underway as the planet increasingly takes on an urbanized landscape. The United Nations predicts that the world’s cities will house 60 percent of humanity by 2030, and already urban areas comprise some three percent of Earth’s land surface. According to the researchers, the nascent field of urban evolution has “rapidly become an important frontier in biology, with implications for healthy and sustainable human populations in urban ecosystems.”
Jason Munshi-South, an evolutionary biologist at New York City’s Fordham University and co-author on the analysis, told Earther he and his coauthor—Marc Johnson, director of the Centre for Urban Environments at University of Toronto Mississauga—were surprised by the sheer scale of urbanization and how many species are likely to be affected.
“There are many, many large cities on every continent and in most major forms of terrestrial habitat,” he said. “The evolution of most species will be affected either directly because they live in cities or indirectly as the effects of cities ripple outwards. Urbanization may be one of the most dominant forces of evolution acting on Earth today.”
Anyone looking at an urban landscape can tell the natural environment has been dramatically altered. Cities are covered in impervious surfaces and altered waterways; they suffer higher temperatures and elevated levels of pollution; and they are rife with invasive species while also lacking the typical abundance of native species. Together, these environmental changes brought about by global urbanization are “creating novel ecosystems with unknown consequences for the evolution of life,” state the study’s authors.
This is a major break from way evolution has been viewed historically as something that plays out deep in the natural world beyond the scope of human activity. Humans are now altering environments much faster than would happen naturally, which is causing certain “rapid adaptations” of native species in urban areas. For example, a video accompanying the study notes how urban birds such as house finches have evolved longer, wider beaks to better consume sunflower seeds packed into backyard birdfeeders. Even humankind itself may be adapting to an urbanized world, as one study in the review shows how people in older cities are more resistant on a genetic level to diseases like leprosy and tuberculosis, possibly because their ancestors were better equipped to deal with these diseases.
Evolution is, at its most basic, a change in gene frequencies in a population from one generation to the next. This change can be caused by gene mutation, chance genetic drift, gene flow between populations of the same species, and natural selection. In the report, the researchers document urban evolution caused by each of these mechanisms, however genetic drift and gene flow are by far the most extensively investigated.
According to the study, cities elevate the strength of genetic drift—basically evolution by chance or luck—while restricting gene flow. Urbanization creates fragmented landscapes, scarred by roads, buildings, canals, and other manmade barriers that can impede gene flow and limit genetic diversity by cutting off populations from each other. Then, within these populations, certain species can prosper not for their inherent strengths, but due to a lack of competition.
“Undoubtedly more species will experience evolutionary change due to urbanization in the future,” said Munshi-South. “Some will adapt, some will disappear from urbanized areas, and others will persist or change in seemingly random ways.
One of the main phenomenons the authors encountered in their research was species losing genetic variation over time as they became isolated in small urban habitats, such as parks, cutting them off from migrant populations.
Munshi-South said one of the big questions about these evolutionary changes will be their impact on humans. For instance, larger species such as raccoons could become even more reliant on humans for food, with those adapted for urban living winning the competition for survival and making life harder for certain other species (including some humans). Already, as wildlife areas confront development, encounters between humans and bears are becoming a bigger problem in many parts of the U.S.
Then there’s the issue of pests. Johnson warned that the ability of some potentially harmful organisms to adapt to city life might increase the transmission of diseases.
“Bedbugs, for example, were scarce two decades ago, but they’ve adapted to the insecticides used to keep them at bay and have exploded in abundance worldwide,” he said in a statement. He also noted that there are now mosquitoes that can live in London Underground stations that no longer need to feed on blood to produce eggs, nor do they become dormant during the winter. What they do do is carry a number of diseases.
In the long run, as this field of study grows, preserving genetic diversity could become a much more important element of wildlife conservation and ecosystem management in urban settings.
“In short, the preservation of genetic diversity can have positive effects on biodiversity and maintaining healthy ecosystems,” said Munshi-South. “These ecological and ecosystem-level effects of genetic diversity have only been studied in non-urban habitats, and an important frontier is to understand how genetic diversity and evolution will affect the conservation of species, as well as the ecology of communities and ecosystems.”