Urban Evolution

Urban Evolution

How Species Adapt, or Don't, to City Living
Jonathan B. Losos [3.31.17]
We realize evolution can occur very rapidly. Yet, despite this realization, very few people have taken the next logical step to consider what's happening around us, where we live. Think about the animals that live just around you. Look out your window in your backyard. . . . All the animals living around us are facing new environments, coping with new food, new structures, new places to hide, and in many cases new temperatures. These are radically different environments. If, as we now believe, natural selection causes populations to adapt to new conditions, why shouldn't it be happening to those species living around us in the very new conditions?
JONATHAN B. LOSOS is the Monique and Philip Lehner Professor for the Study of Latin America and Professor of Organismic and Evolutionary Biology at Harvard University, and Curator in Herpetology at the Museum of Comparative Zoology. He is the author of Improbable Destinies: Fate, Chance, and the Future of EvolutionJonathan B. Losos's Edge Bio Page


Lately, I've been thinking a lot about squirrels. Not just squirrels, but raccoons, 'possums, cockroaches, pigeons, all the animals that live around us, in and near our homes. I've also been thinking about the plants, the trees that line the streets, the bushes, grasses, even the weeds that pop up in the cracks in the sidewalk. I'm an evolutionary biologist, and when most people think about evolution, they think about what happened millions of years ago, in the distant past. They think that evolutionary biologists study that ancient history, and we do. We look at fossils to trace the evolution of species through time. More recently, we look at DNA. We compare DNA of species to draw inferences about ancient events from millions of years ago.

Why would an evolutionary biologist care about animals that live around us? Over the last fifty years or so, one of the most exciting findings in biology and maybe even in all of science is that evolution can happen very quickly. Around us today evolution is going on, and it's going on at a pace that we can actually study. That's something that Darwin didn't think was at all possible, but now we know it's true.

People who are studying contemporary evolution are studying it out in nature for the most part, in the woods, on islands, and so on. They hadn't been looking at the plants and animals that live within our urban lifestyles and what's all around us. I'm interested in whether evolution is happening in cities. Is it affecting the species that live in our midst? Are they adapting to our presence? This is a topic that evolutionary biologists have just begun to pick up on. It's very exciting to think that the animals we coexist with are adapting to live with us. That's the question I've been thinking a lot about.

Before getting into that question I just want to give a little bit of background. As I mentioned, Darwin would have been surprised that people would be interested in this. Darwin thought that evolution proceeded incredibly slowly, that it would take thousands of years for a perceptible change to occur. But if you think about it, there were no data back in Darwin's time about evolution. He created the field. His intuition about the pace of evolution was just that: intuition. There was no evidence.

In fact, he borrowed his ideas not from science, but from Victorian sensibilities about the appropriate pace of change in life in general. In the middle of the 19th century, when Darwin voyaged and wrote his great book, life was changing very quickly. The Industrial Revolution and lots of other changes were taking place. People of the upper class, such as Darwin, weren't so thrilled about this. In their worldview, change should occur very slowly. Darwin borrowed this idea from the social context and applied it to his thinking about evolution; that's why he thought that evolution must proceed very slowly.

Darwin was an incredible scientist. He had amazing insights about the natural world. He figured out all kinds of interesting ideas. He figured out coral atolls and how they form; he figured out the role that worms play in aerating the soil; and, of course, his great idea was evolution by natural selection. You're not going to make a lot of money betting against Darwin. When Darwin said that evolution proceeds slowly, the field took him at his word. For about a century, all scientists followed Darwin's lead in presuming that evolution must occur very slowly.

Then, about the middle of the last century, things began to change. The first evidence of that was the development of antibiotics such as penicillin. Penicillin was the wonder drug that was going to make infectious disease a thing of the past. But once we started using penicillin, bacteria very quickly evolved resistance to it.

As every new antibiotic came along, resistance evolved very quickly. There was one antibiotic that only lasted two years before resistance appeared. At the same time we were developing pesticides to apply to insects and to rats, we were developing herbicides to apply to weeds, and the same thing happened: The species quickly evolved resistance to our chemicals. We could see that evolution was occurring quite rapidly.

Then came the famous study in the mid‑1950s of the peppered moth, in England, a moth that initially had been light in color so that it blended into light-colored trees. During the Industrial Revolution, soot was blown into the air by all these factories, so the trees became dark in color. The moths evolved to become dark in order to match the background of these trees. In the 1950s, a scientist documented how that happened over the course of just a few decades.

These examples illustrated that evolution could occur very rapidly. But there still was one caveat—one out, if you will, for Darwin: All of these changes that species were adapting to were man-made changes in the environment. We were rapidly changing the environment through pollution, through the use of chemicals, and so on. Maybe these evolutionary changes weren't typical of what goes on in untrammeled nature, unaffected by humans. You could still give Darwin his out that in the natural world, this didn't apply.

It was in the 1970s and '80s that this began to change. Probably the two most significant events in that regard were, first, the now-famous work of Peter and Rosemary Grant, on a tiny island in the Galapagos, where they studied populations of Darwin's. They followed them over forty years.

What they discovered as they were studying this population was that the environment changed very radically from one year to the next. In one year there would be a very big drought, and the seeds would disappear. The only seeds left would be very large seeds, so only the birds with the biggest beaks could crack the seeds. Natural selection very strongly favored birds with big beaks. Sure enough, in the next year there was a big evolutionary jog, and the population had bigger beaks.

A few years later there would be incredible rains, which produced a huge amount of seeds, most of them small. Then it was the birds with the little beaks that could quickly manipulate the little seeds that had the advantage. A year later the population evolved again. They demonstrated that evolution in a natural context, something unaffected by humans, could occur very rapidly.

At the same time, there were some famous experiments in Trinidad on guppies. Scientists noticed that in the absence of predators, the guppies were very colorful, because females prefer colorful males. In pools where there were predators, however, the guppies were much blander. They did a simple experiment in which they moved a population of guppies from a pool with predators to another pool a little higher up the mountain that had no predators. Those guppies evolved to be very colorful, over the course of just two years. Again, this was an example of nature not affected by human pollution or other things, where evolution occurred very rapidly. We realized that, in this case, Darwin was wrong. When natural selection is strong, when conditions change to a great extent, populations quickly evolve adaptations.

The last thirty years have seen a series of more and more research demonstrating rapid evolution all around us. Twenty years ago I published a paper in which we documented that lizards placed on tiny islands in the Bahamas would adapt very quickly, over the course of about ten years, to the islands they lived on. We took them from places where they lived on big trees and put them on tiny islands that had very narrow bushes. Over ten years they evolved significantly shorter legs.

As I said, we published this twenty years ago, and it was a big deal. The New York Times covered it; it was on the front page of the Boston Globe and USA Today; and ABC News almost sent a film crew to where we were working in the Bahamas to film us when the paper came out. It was a big deal. Evolution occurred rapidly. But that was twenty years ago. The same paper today would get almost no attention. It has become the expectation that if you study populations in a changing environment, you will see rapid change. We now have scores, hundreds of papers, demonstrating this.

We realize evolution can occur very rapidly. Yet, despite this realization, very few people have taken the next logical step to consider what's happening around us, where we live. Think about the animals that live just around you. Look out your window in your backyard. There are squirrels running up and down the trees. You might recall the New York City pizza rat, a video of a rat running down stairs to the subway with a big slice of pizza in its mouth. All the animals living around us are facing new environments, coping with new food, new structures, new places to hide, and in many cases new temperatures. These are radically different environments. If, as we now believe, natural selection causes populations to adapt to new conditions, why shouldn't it be happening to those species living around us in the very new conditions?

For a long time, people didn't think about that. They just assumed that the species living around us were those that happened to have the traits that allowed them to survive in human settings. We have a term in evolutionary biology—it's not used very much anymore—called pre-adaptation. The idea is that a population already has the traits that just happened to make it do well in a new environment. Most people believed that the urban plants and animals we see around us are just the ones that happen to have the traits that allow them to live in our cities and in our environment. But there's no reason to believe this. Why shouldn't they be adapting to our conditions just like any other evolutionary situation? Scientists are now beginning to study urban species to see if they are adapting to modern-day situations.

Let me give you an example from the lizards that I study. I mentioned them once already. My research looks at a type of lizard called anoles, in the Caribbean, in Central and South America, and the southeastern United States. There are 400 species of these lizards. I've spent much of my career studying how this particular type of lizard has diversified, and how particular species are adapted to the parts of the environment they are in.

If you go to, say, the forest in Puerto Rico, where there are ten species of these lizards, and you sit quietly, after a few minutes the lizards forget you're there and they become active. You're able to see them. What you see is that the species have adapted to using different parts of the habitat. There's one high in the trees that has big toe pads that let it cling to smooth surfaces high up. There's one on twigs with short legs to move very carefully on the narrow surfaces it uses. There's another one on the tree trunk near the ground that has very long legs, which allow it to run rapidly down to the ground, to capture prey and to confront other males and so on. Each species has its own adaptation to the part of the environment it uses.

When we go into the cities in Puerto Rico, you don't see all the species. In fact, you see primarily one species: the tree trunk-near-the-ground specialist. That's the one you see everywhere. It's on the buildings, it's on the trees in the streets, it's on the fences; it's all over the place. I, and many of my colleagues, just assumed that this species is pre‑adapted to living in cities, and that its adaptations for living on big trees make it suited for living in cities. We didn't think about whether it was actually adapting.

No one thought about that, until recently. A graduate student at the University of Massachusetts, Kristin Winchell, decided to study city lizards in Puerto Rico. She focused on this one species called the crested anole. She compared the crested anoles in three different cities in Puerto Rico, and for each city, she studied a population out of the nearby forest. Her question was, have they changed from the forest to the city, in three separate comparisons?

What she found was that in each case they had changed. The lizards in the city had evolved longer legs—presumably useful for hanging onto broad surfaces, like walls—and they had evolved bigger toe pads—again, presumably to hang on to the smooth surfaces in cities. She demonstrated that they had evolved to adapt to living in cities. It wasn't just a matter of them being pre-adapted, they had evolved, and that's why they're doing so well in cities.

She's now looking at another aspect of their adaptation, which is their physiology, because cities tend to be warmer than surrounding forests, so they're probably having to deal with hotter temperatures. Have they adapted to tolerate higher temperatures? My understanding is that the preliminary work shows that they have adapted in this way as well.

We can imagine lots of other ways these lizards may be adapting to live in the urban setting. One particularly interesting way is that these are diurnal lizards, which are active during the day and at night they sleep. But there are lots of lights in cities, and people regularly see lizards active at night under lights. They'll be on the wall, right around a light fixture and, as insects are attracted, they catch the insects and eat them. They found a new niche. Is it that they're just taking advantage of what's there, or are they adapting to these new conditions?

We can well imagine that there are changes in their visual system to let them see under dimmer light perhaps, or maybe even to be active at cooler nighttime temperatures. We don't know. There are lots of aspects of living in the city that these lizards may be adapting to, and Kristin and her colleagues are just beginning to look at them. I should mention that the nighttime work is being done by Jason Kolbe, at the University of Rhode Island.

That's just one example, but in recent years scientists have found lots of interesting examples of adaptation to cities. Just to give you a quick few examples, researchers in Europe discovered that there was a species of plant that grows in the cracks of the sidewalk. Normally, that species has very small seeds that the wind blows away, and the seed lands somewhere and then germinates. But that's not a very good strategy if you're in a parking lot, say, where the whole place is concrete, because you're likely just going to land on concrete. The urban populations have evolved bigger seeds that don't get blown by the wind, they just drop down, so they're much more likely to land in a little patch of dirt and be able to germinate. This is a clear evolutionary difference, adapted to where they live.

Another example in plants is that researchers in Canada compared a type of plant that occurs in a gradient, from out in nature towards the center of the city. They looked at a number of cities, including New York, Boston, and several others. What they found is that in almost every case, as the populations got closer to the city center, the plants were able to tolerate colder temperatures. They had physiological changes, actual genetically based changes that let them tolerate colder temperatures. That's probably a result of the fact that cities don't have as much snow cover. Snow insulates the ground in the winter, so the ground is colder in cities, and the plants have adapted to withstand that.

I have one more example from animals that is particularly interesting. Think about roads, for example, in northern areas. In the winter, lots of chemicals get thrown onto roads to deal with ice and so on. Salt, for example, runs off the road and runs into the surrounding area. Bodies of water near roads there tend to be pretty polluted. Well, some researchers looked at frogs and salamanders that breed in these ponds. What they found was that, number one, it's a pretty tough place to live for an amphibian, but they do persist. If you study the larvae that develop in those ponds, sure enough, they've evolved a physiological capacity to adapt to these polluted waterways. They've adapted to living in these circumstances.

These are just a few of an increasing number of examples showing that animals and plants are adapting to living in our urban settings. But we're just scraping the surface. People are just now realizing that there's an interesting phenomenon going on right under our noses that is worth investigation. The examples I've been talking about are examples of evolution by natural selection. We all know that natural selection favors individuals that can survive better and reproduce better, and they pass on their traits to the next generation. This is what Darwin proposed. But there's more to evolution than natural selection. There are other ways populations can evolve. These may be relevant to living in cities as well.

For example, one thing that evolutionary biologists have long been aware of is that small isolated populations tend to diverge genetically from other populations. The reason is that random events will happen through time. A particular mutation might come along that's not necessarily beneficial, but just due to happenstance it becomes established in the population. Conversely, a mutation might disappear just due to bad luck. The individuals that have that allele—that's the technical term—all died out, and that allele is lost from the population. This phenomenon, which is called genetic drift, is particularly common in small populations, random fluctuations. What that means is that if you have isolated populations, they can become different genetically over time.

Researchers in New York City realized that there are many parks in New York City that have become isolated from each other, and the animals that live there used to occur throughout Manhattan. But Manhattan is now mostly a city with these little patches of forest. They went to look at the populations of these different patches and, sure enough, they are genetically different. They looked at a type of rodent, the white‑footed mouse, which doesn't live in the city itself, just the little parks. Sure enough, they're genetically different from one park to the next within the city.

People are also looking at the subways to see if genetic connectedness of rats and other things occur down there as well. Genetic connection and isolation is affecting the genetics, and not just of mice. Research on salamanders is showing the same thing. The effect of changing the geography of a city, of populations, is affecting the characteristics of those populations as well.

However, there's the flip side of this. Another thing that occurs in evolution is mixing together of different populations, populations that for some reason have evolved differences, and if you allow them to contact each other, their genes will intermingle. This too is happening in cities. One particular example comes from my own work with Jason Kolbe at the University of Rhode Island and a number of others. We looked at a lizard that is native to Cuba called the brown anole. It's been introduced into Florida and has spread throughout Florida, and is moving west into the Gulf states, up as far north as South Carolina. It's an invasive species.

In Cuba, Anolis sagrei is genetically different from one region of the country to the other. Throughout the island of Cuba, there are genetically differentiated populations. What Jason's research demonstrated is that there have been many introductions of the brown anole to Miami, and those introductions have come from different parts of Cuba.

Basically, Miami has become a brown anole melting pot. All of the genetic differences from different parts of Cuba have been brought together in Miami, producing a hypervariable population that is different from any population in the native land—in Cuba—and perhaps has greater capability for adaptation because it has so many different genetic variants for natural selection to work on.

How do these introductions occur? These lizards are great stowaways. They get carried in shipments of lumber, or particularly in plants. They get shipped around a lot in nursery plants. You may take the plant home and find there's a female hunkered down or, probably more commonly, she's laid eggs in the pots. They're very good at stowing away and then establishing themselves. The mixing of populations is also occurring, leading to genetically different, more genetically rich populations.

Evolutionary biologists have become very interested in recent years in a higher-level analog of this, and that is the mixing of different species—hybridization—when two different species interbreed and are able to successfully reproduce. In the old days, the traditional wisdom was that hybridization was a negative thing, a constraining force. It might take two species and cause them to coalesce into a single species by amalgamating their genomes.

Evolutionary biologists used to think of hybridization as a restraint on evolution. But that viewpoint is changing now, as there have been a number of well-documented examples illustrating that hybridization can provide a new jolt of energy to a species, and can bring in new genetic variants that allow species to adapt in new ways. They're looking at hybridization now in a positive light.

That too is something that is happening in cities around us. My favorite example of this is the animal that everyone knows from cartoons: the wily coyote. The coyote is an animal that traditionally is from the Great Plains states, in the Midwestern and Western United States. A hundred years ago the coyote was not a very large animal; it was about the size of maybe a border collie. It ate mostly birds and rodents, relatively small prey.

In the last century, even in the last fifty years, the coyote has spread across the entire United States. It covers the entire United States, including being found frequently now in cities. It's frequently in Los Angeles and in Chicago. There's a story about one riding the subways in Portland, Oregon. It's definitely in New York City. It's even shown up in Central Park a few times. Moreover, this is not your father's coyote. This is a very different animal. It is much larger than the coyote used to be, maybe the size of the German shepherd now. Occasionally, it will take down deer. Again, it's a very different animal.

The question is, how has the coyote changed so greatly over such a short period of time? The answer seems to involve two factors. The first is opportunity. We humans were very successful in eliminating the coyote's major nemesis, the grey wolf. Gray wolves, being larger than coyotes, both competed with them for food, but also regularly killed coyotes. When wolves were around, coyotes were small. But, you get rid of the wolf, suddenly, there's this great opportunity for coyotes to evolve to get bigger and eat the bigger food that the wolves aren't taking, such as deer.

But then you might ask, where did the genes come from for them to evolve to get larger? Are they just mutations for larger size that then get taken advantage of by natural selection? That can happen, it might play a role, but it seems more likely in the case of a coyote that it's the result of hybridization. This is what I was getting to. The genes are coming in from coyotes occasionally mating with wolves and getting their larger size genes or, more importantly, in most cases mating with domestic dogs.

Coyotes mate with German shepherds, great danes—large dogs. They're getting the dog genes for being large. That has been responsible for how the coyote has been able to evolve to a large size. Perhaps, and we don't have evidence of this, but perhaps it's also responsible for them living in an urban area. Coyotes are now quite common in cities throughout the United States. They're a fascinating case study in adaptation to living in and around humans.

All of these examples show that species are adapting to a human environment, just as we would expect if you think about it. Does that mean that if you see a population in a city that is different it's evidence for evolutionary adaptation? The answer to that is no. It may suggest adaptation, but there is another explanation. The other explanation relies on the fact that genetically identical organisms can produce different phenotypes, different behaviors, that can look different depending on how they grow up. This is the old nature versus nurture debate that we've heard about in many respects. The environment in which an organism grows up affects how it will develop into an adult.

Think about weightlifters, for example. If you spend your life lifting weights, you'll develop very big muscles and thick bones. But that is not a genetic change. You have not evolved. Those differences will not be passed on to your offspring. It's just a response to the environment you've exposed yourself to. In more clinical terms, think about a plant. Everyone knows that if you don't water the plant or if you put it in the shade, it won't grow as tall as if you give it a lot of fertilizer and water and put it in the sun. Organisms have the capacity to develop in different ways. It's a phenomenon that technically we call phenotypic plasticity.

It turns out that some of the examples that we now know of phenotypic plasticity are quite remarkable. For example, if you raise a tadpole, a young frog, in water in which there are chemicals of a predator—in other words, the predator isn't there, but the smell of the predator is in the water—the tadpole will grow a bigger tail, which allows it to swim more rapidly. The same thing happens with snails exposed to water of predators; they grow a thicker shell. These are just responses that an individual organism can make. It's flexible in how it grows.

What that means is that if we see populations in cities that seem different from their relatives out of the forests, we can't assume that those are genetic differences. It might just be a result of phenotypic plasticity. How the capacity for phenotypic plasticity itself evolves is an interesting topic, and one that I'm not going to get into. The fact is, many organisms have the capacity to produce different body shapes, different physiologies, depending on their conditions.

There are lots of interesting situations that we see in cities, and the question is, are these the result of evolution or of plasticity? Just to name a couple, it turns out that many species of birds and of frogs in cities will change their call to make it higher-pitched or lower-pitched so it can be heard against the background noise of the city. They will alter their calling behavior so they can communicate. Is this a genetic evolutionary change, or is it just that they have the ability to learn to change without evolving? We don't know the answer, but that's something that people are currently researching.

Let me give you one example from my own work about the role of the environment in inducing changes. This applies to the experiment that I mentioned earlier, where we found that when we put lizards on islands with narrow vegetation, they got shorter legs. For years when I presented this work at conferences, someone would raise their hand—usually a pesky botanist—and they would ask if this could be the result of phenotypic plasticity. Could it be that when a lizard grows up using narrow surfaces, it just grows shorter legs?

To me, that sounded ridiculous, but eventually I decided we needed to do an experiment to find out. So we did. We got baby lizards and grew them at the St. Louis Zoo. We grew them either in some aquaria, where they had a very narrow dowel, or in other aquaria, where they had a much broader surface to sit on. We let them grow up from babies to adults. At the end of the experiment, we measured their legs. To my amazement, the ones that had grown up using broad surfaces actually had longer legs. To some extent, there is a latent flexibility in limb length that is affected by the environment in which a lizard grows up. These things do happen.

When you see a population that is different in a city than in the forest, how do you determine whether that's an evolutionary change or not? The old way of doing that, what we did with the lizards, is we basically got individuals and raised them in what's called a common garden. You put all the individuals in the same circumstances and raise them to adults.

If the environment is what's responsible for the differences you see in nature, then when you grow them in the same environment, they should grow up identically. If they don't, then the inference is that the differences must be the results of genes, because they're not affected by the environment. That's what many people have done, and they find that sometimes the differences are genetically based and sometimes they're just phenotypic plasticity.

More recently, with the incredible capacity we have to look at the genes themselves and the genomes, people now look for the genes responsible for the trait—the genes for leg length or color—to see if they are differentiated genetically. It's hard to do, but increasingly we're able to do it, and then you can directly test whether populations have evolved genetic differences or not. More and more this is becoming feasible.

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I never thought that I would be interested in what's going on around me in cities. Like everyone else, I didn't pay much attention. Sure, squirrels are entertaining to look at, and raccoons, and so on, but they're just the animals that live around us, they're not subjects of scientific study. But the way I got to this is I started studying lizards. I was an undergraduate at Harvard University. I took classes from the greats in the fields: Stephen Jay Gould, Richard Lewontin, and from many others who taught me the dynamics of evolutionary change.

There were great debates going on at the time. Is evolution rapid or slow? Gould argued that evolution could occur very quickly. Other people thought it had to be slow and gradual, like Darwin said. They argued about the importance of the genetic basis of change versus phenotypic plasticity, and so on. I myself studied these lizards that I mentioned already. I've studied them my entire career.

The lizards are a great example of adaptation. We see it occurring all around us. As I said, my research took me to the Caribbean—it's a tough job, but somebody's got to do it—where we saw that the species adapted to different parts of the environment.

The really cool thing about these lizards is if you go to the other islands, say you go to Cuba, or Hispaniola, or Jamaica, you see the same set of habitat specialists have evolved on each island. For example, if you look at the twig specialist in Puerto Rico, a very slender lizard that's very well camouflaged because it can't run away fast, it's very slow. It creeps very slowly on narrow twigs. It is highly adapted to living on twigs. Well, if you go to Jamaica, or Cuba, or Hispaniola, there's a species that looks nearly identical. It lives in the same habitat, behaves in the same way, but it's not the same species; they're not even closely related. They have independently evolved these adaptations.

We see that in each one of these habitat specialists’ types. They evolve on every island. That strongly suggests that natural selection really shapes these lizards in very deterministic ways. That in turn led me to the idea that maybe we could do experiments on the lizards to test how they adapt to new circumstances. To be honest, the idea of that experiment didn't come to me out of the blue. It turns out that a colleague of mine, a very prominent ecologist named Thomas Schoener, had set up an experiment where he introduced lizards to islands in the Caribbean, in the Bahamas, the islands I talked about. He didn't do it to study evolution, he did it to study their impact on the ecology.

When I read the paper, I said, "That's an experiment." These islands differ in their vegetation characteristics, and they certainly differ from the source population where the lizards came from. He set up an evolution experiment ten years ago. When I approached him, I was just finishing graduate school, and I said, "Has it occurred to you that this experiment that you set up to study population biology and ecology is actually an evolution experiment?" His response to me was what I could have only hoped he would say, but never thought he would, which was, "Why don't you come work in my lab and study that?" I did, and we studied it. Effectively, we found that they had changed. We did many more experiments of these sorts. I realized from that experience that evolution could occur very rapidly.

When I was an undergraduate at Harvard from 1980 to 1984, four of the biggest lights in all of evolutionary biology in the 20th century were all there at Harvard, at the Museum of Comparative Zoology: Ernst Mayr, the man who more than anyone else had written the book on how evolution proceeds; E. O. Wilson, who had come up with a variety of remarkable ideas, from how ants communicate, to sociobiology, to conservation biology; Stephen Jay Gould, punctuated equilibrium and many other ideas, very much antithetical to what Wilson was arguing; Richard Lewontin, the population geneticist.

To have them and the people who hung around them made it an incredibly exciting atmosphere, particularly because they didn't agree with each other. You heard stories about how their disagreements were sometimes rather heated, but it just made it an incredibly exciting intellectual atmosphere. You wanted to learn about the arguments. You had to ask yourself, what are the points? Where do they come from? How much is data and how much is rhetoric and polemics?

It was a remarkably exciting time. I don't think there will ever be a department like that again, that has four such amazing luminaries, certainly in the field of evolutionary biology.

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Why should we care if squirrels are evolving in New York City, or rats, or cockroaches? Is there any greater significance to this? There is in a number of different ways. Scientifically, these are the experiments that we would love to do as scientists, but we can't do them ethically. We can't move populations into new places and see how they evolve, because it would be unethical. You'd be introducing an invasive species.

The fact is, cities and urban settings are doing those experiments for us. They are subjecting those organisms to novel circumstances, and we're seeing if they can evolve, and if so, how they evolve. From a scientific level, these are just fantastic opportunities to study the process of evolution and understand how it works.

More specifically, beyond just the general academic interest, these are critical times for the world. We are messing up the environment in many ways. Many species are threatened. Whether they can survive or not, we need to know why it is that some species can adapt, and others can't, and how they adapt. Are there things we can do to promote that, to make it more likely that species will be able to survive in new conditions?

Now, you could take a very rose-tinted view of this and ask why we care about the environment. Species will adapt to it. That's what you're showing. Well, the answer is, most species won't adapt. Some will, but most won't, and that's why cities aren't full of giraffes, and elephants, and many other animals. They can't adapt to our surroundings, so they haven't adapted. But by studying the evolutionary process going on in cities, it's a great opportunity to understand what determines whether one species can make it and another won't, and what we might do to promote the persistence of these species. These are two practical reasons why studying evolution in cities is of scientific value.

But there are also more ethereal reasons, in two respects. One is understanding ourselves. We are a great product of evolutionary adaptation. We are one of the great success stories of evolution. Over a relatively short geological period, over a few million years, we went from being an apelike animal on all fours, to standing erect, to evolving incredible capacities and dominating the world. Everyone would like to know just how that worked. Studying the evolutionary process, even if it's squirrels, and cockroaches, and pigeons, and lizards, ultimately, is the same processes that led to us. That's how we're going to understand our own evolution.

Even beyond that, most people live today in the cities, and their experience with wildlife is mostly what they see around them. I was just driving yesterday and I saw some Canadian geese by the roadside. People were there looking at them. They're fascinated with the animals and plants around them. By understanding the extent to which they are evolving, that makes the story that much richer. It is a sublime sense of this world around us. It isn't just set in stone. It is changing. We can see that in the species around us, in the cities.

An appreciation for that will give people a richer comprehension of the natural world, and almost certainly a richer understanding of evolution. Certainly, there are parts of society here in the United States and elsewhere that have their doubts about evolution. Perhaps if they can understand that it's happening all around us, even in the animals they see every day, perhaps that will help influence at least some people to understand the role of evolution in the world as it is today.