Can Animals Evolve To Survive The Anthropocene?

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

When you think of evolution, you might imagine a slow process that takes millions of years, right? I mean, it took 375 million years from the first fish climbing out of the water to get to the humans you see now. And now that we’re around, we’re changing the world at an unprecedented rate. Threats like climate change, deforestation, and pollution are wiping out entire animal species in just one generation.

So scientists are wondering, can evolution act fast enough to keep up, or are some species just doomed for extinction? Here to discuss is Dr. Shane Campbell-Staton, assistant professor of ecology and evolutionary biology at Princeton University, in Princeton, New Jersey. Dr. Campbell-Staton wants to figure out how humans drive evolutionary change. He studied all sorts of critters, from lizards to elephants.

Welcome back to Science Friday.

SHANE CAMPBELL-STATON: Thank you so much for having me. I appreciate being back.

IRA FLATOW: Nice to have you. Let’s start with the basics, shall we? What are some of the big questions you are trying to answer?

SHANE CAMPBELL-STATON: So the major questions that my group is trying to answer have to do with how humans act as engines for evolutionary change in other species across the tree of life.

So we are changing the planet in a lot of different ways, everything from pollution to hunting and fishing, to climate change, global warming– all these different ways that are both intentional and unintentional. And in doing so, we are putting all of these pressures on the other organisms that share space and time with us. And we want to know not just how they’re being affected now in terms of population declines, but how they will continue to be affected through the process of evolutionary response to all these different things that we’re doing into the future.

IRA FLATOW: I note that to get at these questions you’ve looked at a type of lizard, is that right? What did the lizard tell you about evolution?

SHANE CAMPBELL-STATON: Yes. So we’ve been studying a small lizard, called the Crested Anole, which occurs across the island of Puerto Rico. And we’ve been studying how these lizards have adapted to cities across the island. So it’s a very abundant lizard that occurs in multiple cities across the island, where it’s colonized from the surrounding forests.

So myself and my collaborator, Kristin Winchell, have been really interested in how these urban environments are acting as selection pressures for these very small, thermally sensitive animals. So one of the things about cities is that they are very hot because of all the concrete and metal, a lot of reflective surfaces.

IRA FLATOW: Yeah. They call them heat islands, right?

SHANE CAMPBELL-STATON: Exactly. Right. This is what we call the urban heat island effect. And we’ve been studying how thermal tolerance, how heat tolerance in Anoles, that occur in cities has changed from their forest counterparts. And we’ve shown that, again and again, every time these lizards colonize cities, that they become more heat tolerant. And this effect seems to have a very significant genetic component, suggesting that it is evolution by natural selection that’s driving this change.

IRA FLATOW: So the lizards have evolved over generations, you’re saying, to better tolerate the heat?

SHANE CAMPBELL-STATON: Exactly.

IRA FLATOW: And so what about when it gets cold, do they die out quicker?

SHANE CAMPBELL-STATON: So there doesn’t seem to be a tradeoff in terms of cold tolerance. So urban Anoles and forest Anoles are just as cold tolerant. So they’ve been able to increase their heat tolerance without sacrificing their ability to perform at the other end of the temperature spectrum.

IRA FLATOW: So are these lizards, then, an example of rapid evolution?

SHANE CAMPBELL-STATON: Yes, absolutely. So we’re talking about the oldest cities in Puerto Rico, like old San Juan, maybe a couple of 100 years old, but a lot of these cities are even younger. So we’re talking about 100 or less generations. This urban heat island effect is a much more recent urban effect, which means that they’ve had to evolve even more rapidly.

IRA FLATOW: I find this as amazing. Because when I learned about evolution as a student, I remember the teacher drilling into our heads that evolution takes place over many, many, many generations. But you’re saying it can happen fast.

SHANE CAMPBELL-STATON: Yes.

IRA FLATOW: How do you know that it’s evolution that’s happening?

SHANE CAMPBELL-STATON: So we know that it’s evolution because we can actually look at the signatures of selection at the genetic level. We can actually read the story of a population in its DNA. And over and over again, we see that that story is a story of evolutionary response by way of natural selection. So it leaves distinctive footprints at the genomic level that we can identify and pick up on.

IRA FLATOW: So they pass these traits down?

SHANE CAMPBELL-STATON: Exactly.

IRA FLATOW: Yeah. I understand that you’re also showing how elephants are evolving in response to poaching– sort of the same thing. Can you tell us more about that?

SHANE CAMPBELL-STATON: Yes. So when we’re talking about rapid response, things like lizards are better equipped to deal with those sorts of pressures because they have short generation times, they have really large population sizes. They’re able to respond to selection pressures more quickly than larger, more long-lived organisms, just because it takes them longer to reproduce, they have smaller population sizes. All of that is working against them.

But we see that even in species like elephants, we can look at the effects of human-mediated selection and evolution. So in this case, with African elephants, almost all African elephants have tusks. So all males have tusks. The vast majority of females also have tusks. But there’s a small proportion of females that are naturally born without their tusks, and they never grow them.

But in regions where there has been intense hunting for ivory, we’ve seen an increase in the frequency of tuskless females in those populations. So in Gorongosa National Park, during the Mozambican civil war, which happened between the late ’70s and early ’90s, there was large-scale hunting of all of the large mammals, and the elephants specifically were being hunted for their ivory.

Before the Mozambican civil war, about 18% of the females in the Park were tuskless. Which is already a pretty high number versus what we see in other parts of the range. Then, after the war, half of the surviving females were tuskless. This seems to be, again, due to a selection response. If you’re living in an environment where individuals are being disproportionately killed because they have tusks, not having those tusks gives you a selective advantage. You’re more likely to survive, reproduce, and then pass on those genes for tusklessness to the next generation.

The oldest elephants in the Park, those are the elephants that actually lived through the civil war. They’re still around. And then, amongst their offspring, we still see an increase in the frequency of tusklessness amongst their daughters. About 33% of their daughters are still tuskless. Which is still significantly higher than what we saw before the war, even though those offspring themselves, they never experienced the war at all.

IRA FLATOW: And that didn’t take very long. You’re talking about a 15-year Mozambique civil war. Did it happen in one generation?

SHANE CAMPBELL-STATON: Yes, exactly. It happened in a single generation. So when we talk about evolution, most people know evolution by natural selection. But natural selection plays out within a generation. It can happen almost instantaneously. As long as there’s some event that disproportionately favors some individuals over others, then that selection can play out in the course of days or months– in extreme cases, maybe even hours. But it’s not until those survivors have offspring, until they pass those genes on to the next generation, that you actually get evolutionary response to selection. And that is evolution by way of natural selection.

IRA FLATOW: Would that mean that for you to see this change or for the elephants to really exhibit this tusklessness that there would have to be many, many elephants killed who had the tusks for the other ones to survive?

SHANE CAMPBELL-STATON: Yes. So in a lot of these stories, I think we have a tendency to see them as success stories. And they are, in a way, in terms of the resilience of life. But selection always comes at a cost. And that cost is death. So the stronger a selection pressure is, the more individuals have to die in order to get a response to that selection pressure.

IRA FLATOW: Yeah. I get it. And just to be clear– these species aren’t evolving because they’re developing new adaptations, like Lamarck would say, right– natural selection is happening?

SHANE CAMPBELL-STATON: Yes. This is natural selection. So it’s selection on standing genetic variation. When most people think about evolution, you think about a novel mutation popping up in a single individual, and then being slowly spread. Which does happen, but it takes a very long time.

The examples that we’re talking about, you already have variation that exists in populations or in species that then become adaptive under specific circumstances, and then drive those traits and then their genetic underpinnings to higher frequencies in a population. And that occurs much faster.

IRA FLATOW: So evolution is like having a toolkit.

SHANE CAMPBELL-STATON: Exactly.

IRA FLATOW: And if you have a full toolkit, you get better results.

SHANE CAMPBELL-STATON: Yeah. So genetic diversity is essentially the toolkit that any species has to respond to any type of pressure that it would possibly encounter. So the more genetically diverse you are, the more potential tools you have to respond to any given challenge. The thing about natural selection and evolution is it’s not a forward-looking process. It’s not trying to predict anything. It’s not planning on anything. It’s a responsive process. So something happens, and then you get an evolutionary response.

So the more tools you have in your toolkit to deal with whatever may come, the more adaptable a species or a population is. But, again, once you go through a large selection event, because that selection requires death, it also requires a loss of genetic diversity.

So in the case of the lizards in Puerto Rico, they may be very heat tolerant, the survivors in cities, but what happens if an extreme cold snap comes through? Well, the genetic diversity that may be needed for the physiological traits that would be able to survive that type of event may be very different than the traits that have been selected for in the past. But you’ve also lost a lot of the population, and a lot of that genetic diversity may then be gone. So selection always comes at a cost.

IRA FLATOW: This is Science Friday, from WNYC Studios.

In case you’re just joining us, we’re talking to Dr. Shane Campbell-Staton about how climate change is driving animal evolution.

I guess you were talking about some of these elephants being really, really old. And I’m thinking, if I’m one of these older elephants, I must have been around to watch this massive die-off of the other elephants, who couldn’t survive.

SHANE CAMPBELL-STATON: Yes. So that’s one of the things that I learned very quickly when I undertook this project. So when I started the elephant project, I’d never worked on elephants before. I’d never been to continental Africa before, which is a big deal for me as an African-American. So everything was brand new.

And seeing an elephant up close and personal, a sedated elephant, right in front of me, it was a completely different experience than anything I’d ever experienced before. Because I’m looking at this adult female. She’s in her ’40s or ’50s. She actually lived through the Mozambican civil war and survived, while literally nine out of 10 of the individuals in her species died during this 15-year period. And she survived and then went on to have children and grandchildren, became the matriarch of a herd. And now she’s still living her life.

That sort of individualistic story, I mean, it’s such an intense point of connection that I think we can have when it comes to the organisms that are responding to all the different things that we’re doing. But those individual success stories, they’re always marred in some sort of a tragedy. And ultimately, we happen to be the cause of a lot of those tragedies in the Anthropocene, in the age of humans.

IRA FLATOW: I imagine, and from talking with you, I can see that you have been through and watched a lot that has been going on in the Anthropocene. Has your research changed your perspective on what biodiversity might look like in the world we leave behind?

SHANE CAMPBELL-STATON: It has. I think it’s really driven for me the idea, this sort of conundrum that is life. So life is both incredibly resilient and incredibly fragile at the same time, and simultaneously. I spend my life studying all these examples of the strange, incredible, amazing, weird ways that organisms have figured out how to respond to all of the things that we’re doing. And it makes me hopeful, and it makes me proud to some extent, to be a part of this story that is life.

But, ultimately, I feel like a lot of these organisms, if things don’t change, as much as they’re doing, they may be fighting a losing battle. Because we’re just changing the planet so quickly, in so many different ways simultaneously, that even the most diverse toolkit, I have no idea how a species would be able to survive all of the different things that we’re throwing at the planet all at once.

So will life survive the Anthropocene? Absolutely. I have almost no doubt in that. I mean, it survived five major extinction events in the past, and I think it will survive us. But what that biodiversity looks like that comes out on the other side, it may be very different than the diversity that we know now.

IRA FLATOW: Yeah. Well, that is the state of affairs, isn’t it, Dr. Campbell-Staton?

SHANE CAMPBELL-STATON: Exactly.

IRA FLATOW: Thank you very much for joining us today.

SHANE CAMPBELL-STATON: Thank you so much for having me. It’s been a pleasure.

IRA FLATOW: Dr. Shane Campbell-Staton is an assistant professor of ecology and evolutionary biology at Princeton University.

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