From The Atlantic:

New York City Has Genetically Distinct ‘Uptown’ and ‘Downtown’ Rats

A graduate student sequenced rats all over Manhattan, and discovered how the city affects their genetic diversity.

SARAH ZHANG NOV 29, 2017 SCIENCE

… So as Matthew Combs puts it, “Rats in New York, where is there a better place to study them?”

Combs is a graduate student at Fordham University and, like many young people, he came to New York to follow his dreams. His dreams just happened to be studying urban rats. For the past two years, Combs and his colleagues have been trapping and sequencing the DNA of brown rats in Manhattan, producing the most comprehensive genetic portrait yet of the city’s most dominant rodent population.

As a whole, Manhattan’s rats are genetically most similar to those from Western Europe, especially Great Britain and France. They most likely came on ships in the mid-18th century, when New York was still a British colony. Combs was surprised to find Manhattan’s rats so homogenous in origin. New York has been the center of so much trade and immigration, yet the descendants of these Western European rats have held on.

When Combs looked closer, distinct rat subpopulations emerged. Manhattan has two genetically distinguishable groups of rats: the uptown rats and the downtown rats, separated by the geographic barrier that is midtown. It’s not that midtown is rat-free—such a notion is inconceivable—but the commercial district lacks the household trash (aka food) and backyards (aka shelter) that rats like. Since rats tend to move only a few blocks in their lifetimes, the uptown rats and downtown rats don’t mix much.

When the researchers drilled down even deeper, they found that different neighborhoods have their own distinct rats. “If you gave us a rat, we could tell whether it came from the West Village or the East Village,” says Combs. “They’re actually unique little rat neighborhoods.” And the boundaries of rat neighborhoods can fit surprisingly well with human ones.