How Cities Reshape the Evolutionary Path of Urban Wildlife

Charles Darwin 's place in the scientific pantheon is deservedly secure, but he made his share of blunders. One of the gravest was maintaining that the effects of natural selection, the linchpin of evolution, could not be observed in a single human lifetime. “We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages,” he wrote in On the Origin of Species in 1859. “And then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.”But soon after Darwin's death in 1882, the first wave of biologists to have grown up on his teachings took note of a curious occurrence in the realm of insects : During the second half of the 19th century, the predominant color of England's peppered moths had steadily shifted from mostly white to almost entirely black. One theory was that the bugs' wings were being tarnished by all the coal soot in the air, a result of the boom in heavy industry from London to Newcastle. But Darwin's disciples came to suspect that natural selection was at play. As England had become more urban, moths who possessed the rare mutation for black pigmentation appeared to enjoy a fitness advantage over their white peers.
It wasn't until the 1950s that Oxford University's Bernard Kettlewell conducted a legendary experiment that demonstrated why the black moths had evolved much faster than Darwin thought possible. Over a three-year period, Kettlewell tracked the fates of hundreds of marked moths that he released in two English forests, one by the pristine southwest coast, the other near the polluted metropolis of Birmingham. In the Birmingham woods—a stand-in for the industry-ravaged landscape of the Victorian era—black moths avoided predation by birds because they blended into the soot-stained trees; the white moths, by contrast, were easy to spot and thus became snacks for sparrows. The opposite occurred in the coastal woods: The black moths stood out when they alighted on the light-colored trees and were gobbled up.
Kettlewell's experiment on “industrial melanism” became a staple of high school biology textbooks because it succinctly illustrates how species can, when subjected to intense environmental pressures, evolve in a matter of years rather than over millennia. But the next few generations of evolutionary biologists were less attracted to hives of human commotion like Birmingham. Researchers raised on episodes of Wild Kingdom and the books of Jane Goodall gravitated toward fieldwork in remote places populated by animals they'd never otherwise encounter. Their mentors encouraged them to go abroad because they knew that faculty hiring committees were wowed by the exotic. The road to a tenure-track job ran through the jungles of the Amazon, not the parking lots of Houston or Columbus, Ohio.

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For the first chunk of his career in evolutionary biology, Jason Munshi-South harbored all the standard romantic notions about which projects he should pursue. He studied the mating habits of tree shrews in Borneo and the demographics of elephants in Gabon, while earning his PhD from the University of Maryland and doing a postdoc at the Smithsonian. But in 2007, Munshi-South became an assistant professor at Baruch College in New York City, shortly after which his first child was born—two events that curtailed his globe-trotting. Restless, he looked for ways to scratch his fieldwork itch within range of the subway. His search for convenient subjects led him to study the white-footed mice that have colonized New York's parks.
Munshi-South and his assistants trapped scores of live mice and clipped off bits of their tails to get genetic material. Financial constraints and the state of technology at the time meant Munshi-South couldn't sequence the animals' entire genomes. Instead he used a shortcut called transcriptome analysis, which centers on the messenger RNA molecules that carry DNA's instructions for protein synthesis into cells. Since only the crucial bits of an organism's DNA get written into messenger RNA, researchers can work backward to infer, with impressive precision, the composition of the genes where it originated.