Keeping Pace with Humans: Urban Evolution
By Cody Sullivan
BU News Service
Scrambling inside the walls of a New York City apartment, rummaging through trash, or sometimes caught in a spring-loaded trap is a pest well known to many urban dwellers: mice. In big cities, mice are nothing more than a problem to exterminate, yet these same urban environments make mice a fascinating example of how humans can impact evolution.
Developed areas, such as New York City, have become ecosystems unto themselves filled with animals such as mice, starlings, and opportunistic weeds sprouting from a sidewalk’s fissures. This urban ecology – far different than the forest or plains ecosystems where the animals first evolved – alters how inhabitants behave, from finding food to picking a place to live.
Now, it’s clear that urban ecosystems modify more than just the habits of its residents; they change the very DNA of the animals and plants living within their concrete jungles.
“Urbanization acts as an incredibly strong impetus for adaptation,” said Max Lambert, a PhD student at Yale University who is studying landscape ecology and evolution.
Urbanization pushes evolution at a rapid rate compared to the rates of evolution in nonurban areas. Certain animals and plants evolve faster in cities because of the constant light, noise pollution, and modern landscapes. These stressors require animals to change their behavior and physiology; and if they don’t adapt, they die. Researchers use common garden experiments to determine whether a trait is a result of evolutionary adaptation or what scientists call plasticity.
“Adaptive traits are not due to the environment affecting a trait within one generation, as plastic responses are,” said Dan Warner, an evolutionary biology professor at the University of Alabama at Birmingham. Plastic responses are physical or behavioral changes that are not grounded in an individual’s genetics, but are instead a result of how that individual was raised.
Hawksbeak weed, Crepis sancta, evolved in response to living in cities. Because paved streets and buildings provide only a patchy growing environment, the weed adapted to release fewer seeds. That way, they don’t waste energy by making seeds that won’t grow anyway. In 2008 Pierre Cheptou, of the French National Center for Scientific Research, discovered that Hawksbeak evolution happens very quickly, within five to twelve generations.
The killifish, a common species along the East Coast salt marshes, also evolved to survive nearby development. Marshes near urban areas started becoming polluted in the 1940s, giving the killifish the option to either evolve or die, as many species do when urbanization arrives. The killifish, thanks to a large population size and genetic diversity, survived by evolving in just a few dozen generations an extreme resistance to the chemical stress placed on them by pollution.
“Some luck is involved,” said Andrew Whitefield, a professor of biology at the University of California Davis, who is one of the researchers investigating killifish adaptation. “Mutations that allow evolution usually are already present in large populations [such as the killifish].”
Many species, especially vertebrates, do not have large population sizes that beneficial mutations can hide in. These helpful mutations then can reemerge once needed for big city survival.
A vertebrate that does show quick evolution in cities is the New York City white-footed mouse, as discovered by Professor Jason Munshi-South and his colleagues at Fordham University. Just as the birds Darwin discovered on different islands in the Galapagos evolved different adaptations, so too have isolated populations of mice in city parks evolved uniquely, but not yet enough to become separate species.
While the existence of mice in cities, weeds along sidewalks, or fish offshore may seem trivial to the well-being of a city, these animals and plants all contribute to a healthy ecosystem. Fish, for example, help maintain healthy salt marshes, and salt marshes act as a barrier against storms, sea level rise, and erosion.
Hurricane Katrina in 2005 and Hurricane Sandy in 2012 were so devastating partly because the offshore marshes along Louisiana and the northeastern coast were unhealthy or destroyed. While the killifish may or may not be a linchpin in the marsh ecosystem, the ability for marsh animals to rapidly adapt and survive human development and maintain healthier marshes so that urbanites, human or otherwise, may have naturally built in protection when the next major storm hits.