Naturally Philly

How big cities like Philly are creating entirely new animal species

Urban evolution happens faster than anything Darwin saw.

Swann Fountain is a rich evolutionary research resource

Swann Fountain is a rich evolutionary research resource

Flickr Creative Commons / Malia111
Alan Jaffe

Updated Feb. 20

The original venues for the study of evolution were pristine places like Borneo and the Galapagos Islands, where researchers collected beetles and birds to compare their diverse characteristics.

These days, the field work has shifted. The relatively new science of urban evolution is happening in the gritty streets and green patches of big cities like Philadelphia.

The waters in Swann Memorial Fountain, for example, are ground zero for a survey that’ll show the evolution of insect life along the Ben Franklin Parkway.

Conducted by Isa Betancourt, an entomologist at the Academy of the Natural Sciences of Drexel University, the study will compare recent findings with the museum’s historical collections. Future researchers will have the benefit of being able to review these “snapshots in time,” which Betancourt began in 2013.

One discovery so far: more dung beetles are flying through our cultural boulevard than many Philadelphians may realize. What this means for the rest of Philly’s urban ecosystem will be a matter of additional study.

Last week, the Academy hosted a dialogue on this evolving science, with featured guest Dutch evolutionary biologist Menno Schilthuizen (say that five times fast!).

Schilthuizen’s new book, Darwin Comes to Town: How the Urban Jungle Drives Evolution, explores the impact of city life on plants and animals and the amazing speed with which they adapt. When Charles Darwin published On the Origin of Species in 1859, he believed natural selection worked very slowly over the course of generations.

But modern urban landscapes have greatly sped up the evolutionary process. The pressures of survival in the city cause species to evolve rapidly, Schilthuizen said. “This process is happening all the time.”

In New York City, a Fordham University study followed the evolution of wild mice, which have survived in the city parks for hundreds of years. In Central Park, which is rich in junk food litter, Schilthuizen said, the mice are evolving to thrive on fatty diets. Because mice reproduce so quickly, with a new generation born each year, the biologists can see the effects of adaptation within decades, or even years.

Crows in Japan display a more obvious form of behavioral adaptation. They are known to enjoy harvesting walnuts in urban areas, and for many years have dropped them from high perches onto the street in order to crack them open. Recently, a group of crows has been observed taking up residence near a driving school, where they place walnuts in front of the wheels of cars. Said Schilthuizen: “They are becoming street smart!”

Philly’s omnipresent house sparrows have also adapted their diets. The species is predisposed to survival in the city because of its natural diet of seeds and grains — which it has seamlessly swapped out for pizza crust and bread crumbs.

Another local example of avian evolution is Philly’s rock pigeon, which is developing darker feathers as a way to get rid of heavy metals it absorbs in the urban environment. “They detox themselves” through their feathers, Schilthuizen explained. “You can observe evolution taking place” through the change in coloration.

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Courtesy of Menno Schilthuizen

Schilthuizen chooses not to make a “value call” on the natural adaptation of animals and plants to their human-made surroundings. But he is certain that it will lead to the development of new creatures.

“We are affecting the evolution of species,” he said. “Evolution will continue all the time, and it will speed up. It will result in the formation of new species.”

However, the planet can’t rely on urban wildlife to replace all those species that are losing their habitat as a result of human impact such as climate change, he warned.

“We are going to lose a lot of species — those that need untouched habitats will suffer,” Schilthuizen said. The biodiversity of urban habitat is much lower than that of the wild, he explained. In the pristine rainforests, “there are species being discovered that we hadn’t even seen yet. Some species may be gone without us knowing they existed.”

Urban planners, however, can use the findings of evolutionary biologists to guide how we grow our cities.

“We have tried to make cities greener” with green roofs, vertical gardens, parks and other verdant spaces, Schilthuizen said, but many of the plants used in these projects come from garden centers and nurseries. Instead, he suggested, or in addition, “we could also create vacant spaces and let them be colonized by urban plant species endemic to that ecosystem.”

Individual efforts also contribute to the biodiversity of urban ecosystems. “We are creating mosaics of small habitats” in each backyard garden we plant, he said. “Every garden is its own habitat for specific species. We are doing a good job of creating those.”

Schilthuizen encouraged people to join the “citizen-science” social platforms that support preservation of our urban cohabitants. “Become an urban naturalist,” he said.