Science Talk

Carl Zimmer on Evolution in the Big City

The annual Scientific American September single-topic issue is all about cities. And award-winning author Carl Zimmer recently penned a piece on evolution research in the urban environment for The New York Times. In part 1 of this interview, he talks about urban evolution

The annual Scientific American September single-topic issue is all about cities. And award-winning author Carl Zimmer recently penned a piece on evolution research in the urban environment for The New York Times. In part 1 of this interview, he talks about urban evolution.

Podcast Transcription

Steve:          Welcome to the Scientific American podcast, Science Talk, posted on August 24th, 2011. I'm Steve Mirsky. This week on the podcast:

Zimmer:          Four or five people who are traipsing around in a remote part of a park, and they've got bags of stuff, then unfortunately, you know, they explain what they're doing to the police and the police are usually quite curious about what they're up to and, you know, find it interesting.

Steve:          That's award-winning science writer, Carl Zimmer. He had an article in The New York Times in July about researchers studying evolution in the wildlife and other organisms in New York City, and our September single-topic issue of Scientific American is all about cities. So we thought that even though Carl didn't write this particular piece for us, we'd talk to him anyway, which I did by phone.

Steve:          Hi Carl. How are you?

Zimmer:          Good, good.

Steve:          Good. You have an article that was in The New York Times almost a month ago, July 25th, but it actually should have been in our cities issue of Scientific American. Fortunately, it's available for free on the Web for anybody to read, and it's a really fascinating look at evolution and biodiversity and wildlife in New York City, and by extension in urban environments. How did you wind up writing this article?

Zimmer:          There was a paper that came out in Science earlier this year about some fish in the Hudson River along New York City that had evolved resistance to a pollutant. And I was talking with my editor and he was saying, "Oh! That's kind of neat." And I said, "Well, it is neat that they have been able to show that, but it's not as if that fish is the only thing in New York City that's evolving. I mean, lots of things are evolving in New York." And my editor said, "Oh! Really. Well why don’t you write about that?" So, basically I started, you know, looking around and meeting up with biologists who study evolution in one way or another that's happening today in the city.

Steve:          Let's talk about those fish first, because it's a fascinating example of how environmental pressures can select for particular genetic arrays. And why don't you just tell us what happened with that fish, what their pressure was and how the fish responded?

Zimmer:          Sure. So, you know, in the mid-1900s, the Hudson River was still awash, in a lot of different kinds of pollution. And there were PCBs, for example, that General Electric was dumping into the river. And, you know, there's been a big lawsuit people might be familiar with about actually getting General Electric to start cleaning up the polluted sediments. Well, you know, for some of the fish there, it was really a matter of life or death to be able to withstand these pollutants like PCBs. And there's one fish in particular, the tomcod, which spends a lot of time down in the muck where there is a lot of concentrated amounts of pollutants, where it could be really quite fatal. And it turns out that there was a rare mutation in the population there. It is also found in Long Island Sound. And this mutation makes these tomcod resistant to PCBs. And so basically it prevents the PCBs from interfering with how they develop. And so these tomcod are able to survive in places that will be fatal for lot of other fish. And this mutation is found in just about all tomcod in the Hudson, and what that means is that I has spread very quickly in just a matter of a couple of decades. So, you know, not only is evolution happening in New York City, in the New York metropolitan region, but it's happening very quickly.

Steve:          Yeah, because you're not seeing a 1 percent survival advantage for example, you're seeing a 100 percent survival advantage. So in a situation like that, you're going to get this turnover incredibly fast.

Zimmer:          Yeah, I mean, that's a general lesson of evolution is that, you know, if you have a really, really strong selective pressure and you have just enough genetic variation that some are going to be able to barely survive this pressure on them, there's going to be really fast evolution. You know, I know the cases of evolution that happened in a more stately way, over centuries or thousands of years or even millions of years, but you know there's nothing like a good dose of pollution to really speed up evolution quickly.

Steve:          And you also talk about these worms that had an issue with cadmium.

Zimmer:          Right. So upriver a bit from the city, there was a factory that made batteries, and they have also dumped a lot of waste into the river, including cadmium and other heavy metals. And in the 1980s, some scientists who were looking at worms that grow and live in a cove, actually near West Point. You know, they've found that this cove was loaded with cadmium, and it was so much that if you took a worm from another part of the river and stuck it in that kind of sediment, it would just die. And yet they found that there were worms there that were living, that were doing just fine. So they did a study of these worms and were able to identify the traits that let them live there. Basically they take the cadmium and they, sort of, pull it out of their system and, you know, just sequester it off safely in their, sort of, skin as it were. So actually, this was actually a problem for people. This is an impact the evolution can have on people, because the worms were able to live with this incredibly high amount of pollution, and then they get eaten by crabs and other, you know, predators, and then people who tried to eat the crabs or other things, and these crabs have very high levels of cadmium because they were eating these evolved worms. So what's interesting in this case is that, you know, the EPA actually cleaned up the site in the 1990s, and the scientists, sort of, kept track to see what would happen to the worms as this change in the environment happened, and within just a few generations, vulnerable worms from other parts of the Hudson started moving in and interbreeding with the resistant worms and so now, on average, the worms there are just about as vulnerable to cadmium as any other worm, so evolution has gone in reverse.

Steve:          In that case, because we see as you said, the vulnerable ones moved back in, so it's not that we have selection process so much as there's, it's just swamped out. I mean, the pressure has stopped existing, so the situation just gets swamped away by the incomers.

Zimmer:          Yeah, there's no advantage anymore to being a mutant that can resist a lot of cadmium, there's no premium there. So your chance of, you know, reproducing is just as good as any other. And since you get worms, kind of, you know, migrating around, these resistant worms get kind of swamped out, and they're just going to disappear.

Steve:          And you also talk about these guys who are right in the little urban parks within New York City who are going around catching white-footed mice—which are not the same species of mouse that we'll find in our houses or apartments in the city—and their story is really interesting. What are they looking for?

Zimmer:          Yeah, so, the mice that people are familiar with in New York that get into their apartments, I mean, eat their food, those are house mice, and they're actually not native to this city. So people brought them with them; Europeans did when they arrived. Before, they know there were mice in New York, and one species was this white-footed mouse, and it's still in New York, you know, four centuries later. The thing is that the white-footed mouse doesn't like to go into your apartment. In fact, it really much prefers to stay in forested areas, and you can find it—if you know how to catch mice—in New York City's parks. So there are these scientists that I met up with who were curious about what has been happening to this population of mice in response to this wholesale transformation of their old home. New York City had been, you know, mostly forested, you know, 400 or 500 years ago. I mean, the Native Americans who lived here did manage the land; they did do burning and so on, and some parts were, kind of, open meadows and so on. But in any case, there was lots of kind of continuous woods for the mice to live in. And now, you know, the woods are in parks, and they're scattered around the city. So, they have been catching mice all around New York in Queens and the Bronx and so on. And I went along with them when they went into a park in Washington Heights, in a really, kind of, out of the way place called Highbridge Park. And they catch the mice, they measure them and so on, and they take some of their DNA because they want to see how genetically similar the mice are in different parts of the city. And they got a really surprising result, which is basically that there's just about no flow of genes going from the mice in one park to the mice in other parks. So as I said, it's as if these mice are now living on islands. And you know, in the wild, islands are very interesting places in terms of evolution because, you know, things get isolated and they just adapt to their very small, little place and, you know, you get things like a pigeon living on an island and then becoming the Dodo bird. You get really weird transformations after long enough. So they've been looking at the DNA to see if the mice are adapting to their particular environments, and they're finding that if you compare the mice in New York to mice, like, off in the wild, you will see that there are hundreds of genes that are mutating. And what's interesting is that they seem to be genes that are involved with things that a mouse would have to face in the city. So, for example, resisting chemicals, that's something important. But then also there are immune system genes that are evolving in a different way in the city; and the city mice versus country mice, you know, they may be facing a lot of different kinds of diseases in the city as well. So, you know, you've got evolution happening, you know, right in the city with these little mice.

Steve:          That's pretty amazing. So you can think of it almost as an opportunity for speciation.

Zimmer:          You know, if it went on for a number of more centuries, you know, after a while you would have, you could have barriers emerging as these mice became more and more different from each other and from mice in the country. You know, the thing in evolution, you know, if those barriers aren't high enough, if, you know, animals have a chance to interbreed, they will, and they'll mix the genes back together again, and then there won't be much difference. But if you just kept this process going on long enough, you would have the evolution of new species. I mean, that's how you get new species on islands, like Darwin's finches or the dodo. This is the process that happens.

Steve:          And then sort of an obvious example at the microbial level, I mean, any hospital can be an island for evolutionary change, but you have some examples in the story of, well, one in particular, very clear example of something going on within a hospital setting.

Zimmer:          Yeah. So, you know, evolution is not something that is just happening to a fish off in the river that you'll never meet or a mouse in a park that you'll never meet. It's also happening with bacteria that you might meet, but you really don't want to because they're going to make you really sick. So there's a really well-studied example of bacterial evolution in New York City involving a bacteria called Klebsiella which can cause pneumonia and other kinds of life-threatening infections. And it used to be that you could treat his Klebsiella with certain kinds of antibiotics called carbapenems, and they were pretty effective. But unfortunately in the 1990s, scientists started seeing bacteria circulating around in hospitals in Brooklyn that were resistant to these antibiotics, which is a really terrible thing because these bacteria are resistant to a lot of other antibiotics as well. So, once you can't use carbapenems then you're really up a creek because the only antibiotics you have left are ones that are really nasty, that cause kidney damage and things like that. In fact, doctors had abandoned them in the 1960s once they got better antibiotics, and now those antibiotics don't work. And these scientists were able to chart the evolution and the spread of this new kind of Klebsiella, this new kind of bacteria, through the hospitals in New York City, you know, in Brooklyn and in Queens, in Manhattan and so on. And then after a while, they started seeing this very strange strain in other parts of the country and in countries like France and Greece and Israel. So, it's unfortunately been, you know, New York's gift to the world.

Steve:          It's reminiscent of our multidrug–resistant tuberculosis back in the 1990s.

Zimmer:          Yeah, yeah, exactly. You know, there are lot of these different bacteria that evolve particularly in cities and in hospitals in cities. This is, you know, the point I was trying to make in the article is that we created these environments where evolution is going off in new directions. So, we have these mice that are basically, you know, living on islands in the middle of a city and you have fish that are able to withstand pollutants that they never faced in millions of years before, and you have bacteria that are becoming incredibly resistant to, specifically to the drugs that are currently used to kill them, and all this and more is happening in New York.

Steve:          Tell us a little about,  you talk about the guys who are looking for the white-footed mice and also some other researchers who are examining ant species in medians on busy streets in Manhattan. Tell us about the kinds of particular challenges that these researchers face. Because as you point out in the article, they've had some run-ins with the police, and they've also had run-ins with people who are avoiding run-ins with the police.

Zimmer:          Yeah, yeah doing field work as an evolutionary biologist in New York City has some challenges, you know. Sometimes the cops stop you because they don't know what you're doing and other times, you know, people report you, or they steal your material. So, it's certainly got its challenges. But these researchers, based at Columbia University, who want to figure out what's happening to the biodiversity of New York, you know—and that might sound weird, that New York has biodiversity, but you know, the fact is that when Europeans arrived there were a whole bunch of plants and animals and other species that were living in New York. And you know they were found in, you know, other parts of New England or the middle-eastern part of the United States. But New York was just a really interesting place ecologically speaking. And there are still a number of species that were native to New York then and are still there. So, the white-footed mouse being one example, but there are lots of plants and so on that are native New Yorkers. A lot of those natives have become extinct. They just couldn't withstand urbanization, the way, say, the white-footed mouse could. So there were lots of orchids, for example, that were native to New York City, and they disappeared. And you know, actually Manhattan was home to 21 native species of orchids, and they're all gone. So, these scientists are looking both at what natives are still around and which of the immigrants, the invasive species, have showed up and have been able to establish themselves. So, one of the really interesting ways that they look at this is they go looking for ants. And they thought, "Well, you know, we're really interested in how an artificial environment shapes the balance between native and artificial species; what better artificial environment could you hope for than a median running down the middle of Broadway?" So, they go out there with their ant traps and they jump into trees in these medians looking for ants, and they find all sorts of surprising things. They find that, you know, along some of these medians, they found 13 different species of ants—which in itself is pretty amazing, because these medians are like, you know, 10 feet wide and, you know, 100 feet long, something like that—and nine of those 13 species are native. So, even these medians can be cradles of biodiversity and can be sustaining species that might otherwise become extinct.

Steve:          Yeah, that's pretty amazing. But I guess, you know, from the ant's point of view, it's still a pretty big environment.

Zimmer:          It is a pretty big environment and also certainly for some species of ant, like there's one species called the pavement ant which came from Europe and settled in New York very comfortably. It can just walk around on sidewalks, it doesn't care at all. It can go blocks away from its nest and be perfectly fine and somehow find enough food to get by on. So, there are certainly some ants that are really rugged and able to withstand the urban environment. Others that, you know, might just, sort of, have a very small range, you know, they just stay in the, you know, the areas where there are trees and soil and so on. So, you know, different ants will cope with New York in different ways.

Steve:          And the researchers who are capturing these ants use a device that has an unfortunate resemblance to a crack pipe.

Zimmer:          Yeah, it's called an aspirator. So, you know, you don't want to pick up an ant with your finger tips because, you know, either it will get away or you'll crush it. So what they do is they have a glass tube with a couple of—a glass pipe, I should say—with a couple of rubber tubes coming out of it, and you stick one tube next to the ant and you put the other tube in your mouth and you suck in, and rather than getting a mouthful of ants, the ant just ends up in the glass bottle. And yeah, when you take that out on Broadway with police going by, it can be a little awkward. (laughs)

Steve:          Some of the researchers, the mice guys who were, you know, poking around in the woods, and the ant guys, describe getting confronted by police with their guns drawn!

Zimmer:          Well, yeah, you know, you have to think about that, you know, maybe four or five people who are traipsing around in a remote part of a park; they aren't out in the meadow where other people are sunbathing or playing basketball. They're in, sort of, the part of the park that very few people go to, and they've got bags of stuff (laughs). So, you know the suspicion is, "Oh! They're going to, you know, be whipping up some crystal meth." Something like that. You know, unfortunately, the police, you know, they explain what they're doing to the police, and the police are quite curious about what they're up to, and you know, find it interesting. So, it ends well as far as I know.

Steve:          Oh! I guess it should be pointed out that researchers in the more exotic locations like the Amazon can also find themselves confronted by people with drawn guns.

Zimmer:          Yeah, exactly. And in those cases, things may not end so well.

Steve:          So, it's a fascinating article and anybody can find it on the Web. You don’t even need a Times subscription as long as you haven't used your minimum allotment for the month, but it's also available on your own Web site.

Zimmer:          That's right, that's right. I've put it on I have an archive of articles. So you can look for it from this July.

Steve:          Well, you must have some clout with the Times where they're letting you run the articles on your own Web site.

Zimmer:          (laughs)With their permission!

Steve:          More from Carl Zimmer in part 2 of our conversation, coming your way soon. In the meantime, get your science news at our Web site,, where you can also check out various exclusive Web features related to our September issue, all about cities. For example, you don’t want to miss the toilet. I mean the slide show about the history of the toilet. Hey, follow us on Twitter, where you'll get a tweet each time a new article hits the Web site. Our Twitter name is @SciAm. For Science Talk, I am Steve Mirsky. Thanks for clicking on us.

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