Urban Evolution

The water flea Daphnia magna — a freshwater crustacean up to a few millimeters in size — is one species busy evolving in cities in response to heat, pollution and even local predators. These zooplankton can prevent algal blooms that overload ponds with toxic cyanobacteria, so this adaptation may have a big effect on freshwater ecosystems, says Kristien Brans, an evolutionary ecologist at KU Leuven in Belgium, who studies the water fleas.

One basic challenge in such urban investigations is to distinguish between two modes of response to altered environments: evolution (genetic alterations that appear across generations) and phenotypic plasticity (the flexibility to alter physical and/or behavioral characteristics in an organism’s lifetime).

For water fleas, it turns out that both are at play. Fleas raised in lab experiments at temperatures matching urban ponds are smaller, and mature and reproduce more quickly, than fleas reared at rural pond temperatures that tend to be several degrees cooler. (That’s phenotypic plasticity — no genetic changes have occurred.) But over time, urban water fleas living generation after generation in warmer, urban pond waters have genetically changed to have those same kinds of alterations. (That’s evolution.)

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GLUE took white clover’s cyanide production as a model to study three questions. Do instances of urbanization in different cities lead to similar local environments? Do those similar environments lead the clover to evolve along the same lines — display parallel evolution — in a trait of interest (in this case, cyanide production)? And if so, what environmental factors are driving the pattern?

In a new Science paper, the collaborators showed that urban environments do indeed end up quite similar to each other, with less vegetation, more impervious surfaces and higher summer temperatures than their outlying rural areas. (In fact, downtowns of cities such as Beijing and Boston are more similar to each other in such factors than they are to their rural areas, Johnson comments.) Analyzing more than 110,000 clover plants from 160 cities in 26 countries, the GLUE investigators also demonstrated a strong link between urbanization and clover cyanide production. And after sequencing more than 2,000 clover genomes and analyzing the urban-rural differences, the researchers showed that natural selection truly is at work.

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Unfortunately, the genetic biodiversity that can fuel adaptation often dwindles in urban areas. A genetic survey by Chloé Schmidt working in Garroway’s lab, for example, found this to be the case, along with lower population sizes, for North American mammals living in more disturbed environments. That’s a concern during a period when so many populations of animals and plants are seeing their natural habitats degraded or simply destroyed.

Scientists don’t take urban environments as precise models for the impacts of climate change. But they say such studies will provide important clues to how creatures may respond to dwindling access to water and food, and exposure to pollution, heat, drought and other dangers.

“We’re in the Anthropocene, and we don’t understand how we’re changing the environment on every level, from greenhouse gas emissions to changing the evolution of life around us,” Johnson says. “People realize this research is part of the solution.”

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