Evolution in a Big City

So I'm here today to encourage you to think about New York City, and not just as one of humanity's greatest achievements, but as home to native wildlife that are subject to a grand evolutionary experiment.

So take this forested hillside in Northern Manhattan, for example. This is one of the last areas left in the city where there's clean spring water seeping out of the ground. You could drink this out of your hands and you'd be OK. These tiny little areas of seeping water contain huge populations of northern dusky salamanders. These guys were common in the city maybe 60 years ago, but now they're just stuck on this single hillside and a few places in Staten Island.

Not only do they suffer the indignity of being stuck on this hillside, but we divided the hillside in two on two different occasions with bridges crossing from the Bronx into Manhattan. But they're still there, on either side of the bridges, where you see the red arrows -- about 180th Street, 167th Street. My lab has found that if you just take a few segments of DNA from salamanders in those two locations, you can tell which side of the bridge they came from.

We built this single piece of infrastructure that's changed their evolutionary history. We can go study these guys; we just go to the hillside we know where they are, we flip over rocks so we can catch them. There are a lot of other things in New York City, though, that are not that easy to capture, such as this guy, a coyote. We caught him on an automatic camera trap in an undisclosed location; I'm not allowed to talk about it yet.

But they're moving into New York City for the first time. They're very flexible, intelligent animals. This is one of this year's pups checking out one of our cameras. And my colleagues and I are very interested in understanding how they're going to spread through the area, how they're going to survive here and maybe even thrive. And they're probably coming to a neighborhood near you, if they're not already there.

Some things are too fast to be caught by hand. We can't pick them up on the cameras, so we set up traps around New York City and the parks. This is one of our most common activities. Here's some of my students and collaborators getting the traps out and ready. This guy, we catch in almost every forested area in New York City. This is the white-footed mouse -- not the mouse you find running around your apartment.

This is a native species, been here long before humans. You find them in forests and meadows. Because they're so common in forested areas in the city, we're using them as a model to understand how species are adapting to urban environments. So if you think back 400 years ago, the five boroughs would've been covered in forests and other types of vegetation.

This mouse would've been everywhere in huge populations that showed few genetic differences across the landscape. But if you look at the situation today, they're just stuck in these little islands of forest scattered around the city. Just using 18 short segments of DNA, we can pretty much take a mouse somebody could give us a mouse, not tell us where it was from, and we could determine what park it came from. That's how different they've become.

You'll notice in the middle of this figure, there are some mixed-up colors. There are a few parks in the city that are still connected to each other with strips of forest, so the mice can run back and forth and spread their genes, so they don't become different. But throughout the city, they're mostly becoming different in the parks.

So I'm telling you they're different, but what does that mean? What's changing about their biology? To answer this question, we're sequencing thousands of genes from our city mice and comparing those to thousands of genes from the country mice, so their ancestors outside of New York City in these big, more wilderness areas.

Now, genes are short segments of DNA that code for amino acids. And amino acids are the building blocks of proteins. If a single base pair changes in a gene, you can get a different amino acid, which will then change the shape and structure of the protein. If you change the structure of a protein, you often change something about what it does in the organism.

Now if that change leads to a longer life or more babies for a mouse, something evolutionary biologists call fitness, then that single base-pair change will spread quickly in an urban population. So this crazy figure is called a Manhattan plot, because it kind of looks like a skyline. Each dot represents one gene, and the higher the dot is in the plot, the more different it is between city and country mice.

The ones kind of at the tips of the skyscrapers are the most different, especially those above the red line. And these genes encode for things like immune response to disease, because there might be more disease in very dense, urban populations; metabolism, how the mice use energy; and heavy-metal tolerance. You guys can probably predict that New York City soils are pretty contaminated with lead and chromium and that sort of thing.

And now our hard work is really starting. We're going back into the wilds of New York City parks, following the lives of individual mice and seeing exactly what these genes are doing for them. And I would encourage you guys to try to look at your parks in a new way. I'm not going to be the next Charles Darwin, but one of you guys might be, so just keep your eyes open. Thank you. (Applause)