Evolution Right Under Our Noses
An ant trap in a bed of ivy in the median of Broadway at 101st Street in Manhattan. Credit Damon Winter/The New York Times
To study evolution, Jason Munshi-South has tracked elephants in central Africa and proboscis monkeys in the wilds of Borneo. But for his most recent expedition, he took the A train.
Dr. Munshi-South and two graduate students, Paolo Cocco and Stephen Harris, climbed out of the 168th Street station lugging backpacks and a plastic crate full of scales, Ziploc bags, clipboards, rulers and tarps. They walked east to the entrance of Highbridge Park. where they met Ellen Pehek, a senior ecologist in the New York City Parks and Recreation Department. The four researchers entered the park, made their way past a basketball game and turned off the paved path into a ravine.
They worked their way down the steep slope, past schist boulders, bent pieces of rebar, oaks and maples, hunks of concrete and freakish poison ivy plants with leaves the size of a man’s hands. The ravine flattened out at the edge of Harlem River Drive. The scientists walked north along a guardrail contorted by years of car crashes before plunging back into the forest to reach their field site.
“We get police called on us a lot,” said Dr. Munshi-South, an assistant professor at Baruch College. “Sometimes with guns drawn.”
Dr. Munshi-South has joined the ranks of a small but growing number of field biologists who study urban evolution — not the rise and fall of skyscrapers and neighborhoods, but the biological changes that cities bring to the wildlife that inhabits them. For these scientists, the New York metropolitan region is one great laboratory.
White-footed mice, stranded on isolated urban islands, are evolving to adapt to urban stress. Fish in the Hudson have evolved to cope with poisons in the water. Native ants find refuge in the median strips on Broadway. And more familiar urban organisms, like bedbugs. rats and bacteria, also mutate and change in response to the pressures of the metropolis. In short, the process of evolution is responding to New York and other cities the way it has responded to countless environmental changes over the past few billion years. Life adapts.
The mice are the object of Dr. Munshi-South’s attention. Since 2008, he and his colleagues have fanned out across the city to study how the rise of New York influenced the evolution of the deer mice.
On this day in Highbridge Park his students, Mr. Cocco and Mr. Harris, spread a blue tarp on the forest floor, while Dr. Munshi-South walked to an orange flag planted in the ground. He picked up an aluminum box sitting next to the flag and pushed in a door at one end. At the other end of the box crouched a white-footed mouse. It gazed back at Dr. Munshi-South with bulging black eyes.
The researchers inspected 50 traps laid the day before and found seven mice inside. They plopped each mouse out of its trap and into a Ziploc bag. They clipped a scale to each bag to weigh the mice. Dr. Munshi-South gently took hold of the animals so his students could measure them with a ruler along their backs.
Dr. Munshi-South and his colleagues have been analyzing the DNA of the mice. He’s been surprised to find that the populations of mice in each park are genetically distinct from the mice in others. “The amount of differences you see among populations of mice in the same borough is similar to what you’d see across the whole southeastern United States,” he said.
White-footed mice live today in forests from Canada to Mexico. They arrived in the New York City region after ice age glaciers retreated 12,000 years ago. In the past few centuries, as their forest home became a city, they survived in New York’s patches of woods. (House mice, which New Yorkers battle in their apartments, arrived with European settlers.) Research by Dr. Munshi-South and his colleagues suggests that New York’s white-footed mice, which occupy isolated patches, are adapting to life in the city.
When Dr. Munshi-South opened the final trap, the seventh mouse had run out of patience. It shot out of its box and raced off into the brush.
Mr. Cocco shrugged. “They are New Yorkers, after all,” he said.
Pollution Forces Change
Evolution is one of life’s constants. New species emerge; old ones become extinct. Environmental changes have often steered evolution in new directions. And modern cities like New York have brought particularly swift changes to the environment. European settlers cut down most of New York’s original forest; towns grew and then merged into a sprawling metropolitan region. The chemical environment changed as well, as factories dumped chemical pollution into the water and soil.
Credit The New York Times
Pollution has driven some of the starkest examples of evolution around New York. Hudson River fish faced a dangerous threat from PCBs, which General Electric released from 1947 to 1977. PCBs cause deformities in fish larvae. “These are important changes,” said Isaac I. Wirgin of New York University Medical Center. “If you’re missing your jaw, you’re not going to be able to eat.”
Dr. Wirgin and his colleagues were intrigued to discover that the Hudson’s population of tomcod, a bottom-dwelling fish, turned out to be resistant to PCBs. “There was no effect on them at all,” Dr. Wirgin said, “and we wanted to know why.”
In March, he and his colleagues reported that almost all the tomcod in the Hudson share the same mutation in a gene called AHR2. PCBs must first bind to the protein encoded by AHR2 to cause damage. The Hudson River mutation makes it difficult for PCBs to grab onto the receptor, shielding the fish from the chemical’s harm.
The AHR2 mutation is entirely missing from tomcod that live in northern New England and Canada. A small percentage of tomcod in Long Island and Connecticut carry the mutation. Dr. Wirgin and his colleagues concluded that once PCBs entered the Hudson, the mutant gene spread quickly.
“When these chemicals first starting getting released, if you had the normal form of the gene, you probably weren’t going to make it,” Dr. Wirgin said.
Evolution has also run in the opposite direction as government agencies cleaned up some of the pollution around New York. In 1989, Jeffrey Levinton of Stony Brook University and his colleagues discovered that a population of mud-dwelling worms in the Hudson had evolved resistance to cadmium. They lived in a place called Foundry Cove near a battery factory near West Point. Dr. Levinton and his colleagues found that the worms produced huge amounts of a protein that binds cadmium and prevents it from doing harm.
In the early 1990s, the federal Environmental Protection Agency hauled away most of the cadmium-laced sediment from Foundry Cove. Over nine generations, the Foundry Cove worm populations became vulnerable again. This shift occurred, Dr. Levinton and his colleagues reported last year, as worms from less contaminated parts of the river moved in. They are interbreeding with the resident worms, and the resistant mutations are becoming rarer.
Today, scientists can scan the entire genomes of New York’s animals and plants to look for evolutionary changes. Last month, Mr. Harris presented new data on white-footed mice at the annual meeting of the Society for the Study of Evolution. Mr. Harris and his colleagues have identified mutations in more than 1,000 genes that are present in all New York City mice, but missing from mice in Harriman State Park. 45 miles north of the city.