The northern dusky salamander survives on an isolated Manhattan hillside. Sarah Goodyear

We have built a city up around them, but amphibians and mammals alike have persisted by genetically diversifying.

Every spring for the last few years, I’ve made an expedition to a rocky hillside in an upper Manhattan park, where water from underground springs seeps through the granite and spills down the steep cliff faces. That water is pure, but the overgrown park is far from pristine. Pathways sparkle with broken glass. The underbrush is scarred with signs of campfires. People dump old tires in the bushes. On one visit, I looked up and saw that I was being watched from afar by a pack of feral dogs.

But I go back each April, because under the logs and stones of this neglected place you can find salamanders -- mostly northern dusky salamanders. Once common in the city, they have almost disappeared from the five boroughs (the only other population in New York City is on Staten Island). Once you know where to look, you realize they are everywhere. I bring along my young son and his friends, and it is magical to watch them realize that all around them, in the drab, unpromising leaf litter of the forest floor, there is life.

Seeing the salamanders there, with the subway rumbling past just a few hundred yards away, makes me feel a simple happiness. We have built a city up around them, but the salamanders persist.

Now I’ve learned that these tiny, sensitive creatures are even more intriguing than I had realized. According to Jason Munshi-South, an assistant professor of environmental science at New York’s Baruch College, my beloved northern dusky salamanders actually illustrate how evolutionary biology is at work in an urban setting.

Two bridges connect this upper Manhattan park to the Bronx. And the salamanders on one side of the bridges are genetically distinct from the salamanders on the other side. In a recent TED Ed talk, Munshi-South says, “We have built this single piece of infrastructure that has changed their evolutionary history.”

Munshi-South and his colleagues have studied not only the salamanders, but also white-footed mice, a native species that can be found in many parks across New York. And they’ve found that DNA analysis can easily distinguish the mice in one park from the mice in another. What 300 years ago was a population with little genetic variation is steadily diversifying.

Where is evolution taking these mice? Munshi-South’s team is tracking differences between the DNA of country mice and city mice, exploring whether mutations are providing greater immune response and resistance to heavy metals among the urban populations.

Munshi-South wants his students to look at the city around them – a place where raccoons, opossums, and even coyotes are showing up in increasing numbers – as something more than a human construction. It’s much more than that, he says – it is “home to native wildlife that are subject to a grand evolutionary experiment.”

Only time will tell what will come of that experiment. But I hope there will always be salamanders in Manhattan.

(Thanks to my friend the urban explorer Erik Baard, who pointed me to the Munshi-South video and who showed me the salamanders to begin with.)

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