The Heat Is On: Evolution in Action

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The DNA Files:
Unraveling the Mysteries of Genetics

As heard on National Public Radio

The Heat is On: Evolution in Action

Hosted by John Hockenberry


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JOHN HOCKENBERRY: This is The DNA Files. I'm John Hockenberry. By now, everyone's heard about climate change, but we tend to think that the ecological effects of a warming world are mostly down the road.

PAUL EHRLICH: It's already happening. Plants and animals and micro-organisms are changing their distributions and behavior in response to climate change.

JOHN HOCKENBERRY: Perhaps even more surprising, species have already started to evolve.

WILLIAM BRADSHAW: We think evolutionary time takes decades or centuries or a millennia to occur, and we don't think of evolutionary time being on the order of a few years, and when we saw the shift over a five year period, that said to us, "This is evolution occurring at a breakneck speed due to climate warming."

JOHN HOCKENBERRY: On today's edition of The DNA Files, "The Heat is On: Evolution in Action," coming up after the news.
JOHN HOCKENBERRY: Welcome to The DNA Files. I'm John Hockenberry. Today's program is about climate as an evolutionary force. We know that the planetary and the genetic are connected. The big trends on earth have, since the beginning of life, shuffled the genes, and shaped the evolution of life on earth. But making real connections between the two, that--that's tricky. I mean, it's a long distance from the planetary to the genetic.

ADAM BURKE: Don't forget to tell them we're orbiting in our spaceship, John.

JOHN HOCKENBERRY: Oh, I was just getting to the spaceship. Folks, meet Adam Burke. He's our program's producer, reporter. He's a mechanic, right? Think of him as the person who keeps our scientific metaphors in tiptop shape. Now, he's currently repairing a few mechanical things before we get underway in our spaceship. How's it going, Adam?

ADAM BURKE: Fine. Almost done.

JOHN HOCKENBERRY: As I was saying, it's a long way between the planetary and the genetic, but scientists are making some connections.

ADAM BURKE: Okay, all fixed up and ready to go.

JOHN HOCKENBERRY: Yeah, right. We've got a program to do here. Okay. So imagine you're above planet earth as we are right now in our spaceship. Man, that is one nice place to look at, Earth.

ADAM BURKE: Not a bad view at all.

JOHN HOCKENBERRY: Not a bad view at all? It's earth, for heaven's sakes, and if you stayed up here a long time, you'd start to see things. That's right. You'd see changes in response to a warming world. The glaciers at the poles are changing, as many have heard, but when we talked to Camille Parmesan--she's a biologist at the University of Texas--she told us that some of the biological changes underway right now would also be visible over time.

CAMILLE PARMESAN: From space, what you'd be seeing is sort of an expansion of the Tropics, tropical species moving into temperate zones, temperate zone species moving into boreal zones, the boreal forest moving into the tundra, and then the polar regions getting much smaller.

JOHN HOCKENBERRY: We tend to think that it's just a few exotic and polar species that have already started reacting to climate change. After all, the earth has only warmed .7 degrees Centigrade on average over the past century.

ADAM BURKE: That would be 1.2 degrees Fahrenheit, John.

JOHN HOCKENBERRY: Thank you, Adam. A few years ago, Camille Parmesan and a colleague looked into just how many species had responded in some way to climate change. They compiled data on more than 1,700 species worldwide--everything from plants to birds to butterflies to inter-tidal invertebrates to plankton. Across all of these groups, half of the species are showing some kind of a response.

CAMILLE PARMESAN: Either they're breeding earlier or they're shifting the ranges northward, or they're accelerating their generation time. So you know, it's a little different, depending on who you're looking at, but that's absolutely astounding, that 50% of species are doing something different from what they were doing 30 or 40 or 100 years ago.

JOHN HOCKENBERRY: Parmesan says that about 40% of the species she's looked at have shifted their ranges away from the equator and toward the poles.

CAMILLE PARMESAN: And this is a combination of shifting their breeding grounds, if they're a migratory species, or actually shifting where they live. So if they're a very sedentary species that doesn't move very much, they're actually shifting the whole region in which they live.

JOHN HOCKENBERRY: So this is one strategy for dealing with climate change. Species are moving to follow their climate. Not necessarily a new strategy, though.

CAMILLE PARMESAN: During the glacial and interglacial cycles, the Pleistocene glaciations, species would move around by 1,000, 2,000 miles, trying to track their climate.

JOHN HOCKENBERRY: Of course, back then, creatures didn't have an obstacle course of freeways, farm fields, and cities to deal with when they moved. So this time around, some are managing and some aren't, but it definitely seems to be a popular coping strategy. Are there any others, Adam?

ADAM BURKE: Well, there's timing. Parmesan told us that species are also shifting when they do things, too. So take spring, for example.

JOHN HOCKENBERRY: I love spring.

CAMILLE PARMESAN: So we're seeing birds breeding earlier, frogs breeding earlier, trees leafing out earlier, flowers blooming earlier.

ADAM BURKE: And you know, there's a growing body of science that says at least some of these shifts are genetic.


ADAM BURKE: Yup. Ooh, speaking of timing, pardon me.

JOHN HOCKENBERRY: What are you doing?

ADAM BURKE: Seatbelt.


ADAM BURKE: Okay, hold on.

JOHN HOCKENBERRY: Hold on? Hey. Does that mean we're going in?

ADAM BURKE: Oh, yeah, we're going in and out into the living room of two scientists in Eugene, Oregon.

JOHN HOCKENBERRY: Whoa. [laughs] Warn me about that next time, will you?

ADAM BURKE: Yeah, like I'm going to warn you. It's way more fun not to warn you. Now, listen, these scientists study an insect called the pitcher plant mosquito. The scientific name: Wyeomyia smithii, although William Bradshaw refers to it affectionately as Wyeomyia.

WILLIAM BRADSHAW: Wyeomyia is a mosquito that breeds only in the stomach of a carnivorous plant.

ADAM BURKE: That plant grows in bogs all the way from the Gulf of Mexico to Northern Canada.


ADAM BURKE: Yeah, and its leaves are shaped like a small water pitcher.



CHRISTINA HOLZAPFEL: This is a pitcher plant that collects rainwater throughout the year.

ADAM BURKE: And that's Christina Holzapfel. She and her husband, William Bradshaw, are evolutionary biologists, and they have been studying the pitcher plant mosquito for over 35 years. So they're fairly well acquainted with the host plant and the mosquito larva, which is a teeny --

CHRISTINA HOLZAPFEL: Teeny, tiny, little --

ADAM BURKE: Little wrigglers they're called. Now, these wrigglers live in the watery bowl of the plant, feeding on a buffet of mucky rainwater and disintegrating insects that are trapped there.


CHRISTINA HOLZAPFEL: They have little orange mouth brushes, and those little brushes are filtering the food that they eat.

ADAM BURKE: So that's what these little wrigglers do. All summer long, eggs hatch in the plants, and juveniles wriggle around, eating and growing, and eventually becoming adults. It takes between three to four weeks on average, but around late summer, early fall, these larva come to a fork in the road.

JOHN HOCKENBERRY: That's right. They have to go back to mosquito school, right? I mean, they have to buy their lunch boxes. They have to get their fashions--new shoes, right?

ADAM BURKE: No, John. Adult pitcher plant mosquitoes can't make it through the winter. So after a certain point in the year it’s no longer safe to turn into an adult. Around late summer or early fall, the larva shift into a sort of holding pattern called dormancy. So they won't develop into adults until spring.

JOHN HOCKENBERRY: Wow, so--so, they turn into like Peter Pan mosquitoes all winter long, just stuck in childhood, eating and wriggling around in the belly of the plant?

ADAM BURKE: Yeah. Even under the snow.

JOHN HOCKENBERRY: Whoa. But wait a minute, didn't you say that the larva start to go into dormancy around August or September?

ADAM BURKE: That's right.

JOHN HOCKENBERRY: But how do they know winter is coming? I mean, fall hasn't even arrived yet.

ADAM BURKE: Ah, that's where the plot thickens, John. They use the length of day.

WILLIAM BRADSHAW: When days are getting short, winter is coming. When days are getting long, spring is coming, and Wyeomyia has a highly accurate ability to measure the length of day. They can measure the length of day to within five minutes, and they use day length then to predict into the future when winter is coming, and to time the onset of their dormancy at the optimal time.

JOHN HOCKENBERRY: These Wyeomyias can measure the length of day to within five minutes?

ADAM BURKE: Yup. Birds use similar cues to determine when to migrate.


ADAM BURKE: Yeah, they're not conscious decisions. They're genetic cues, hardwired into the animal.

JOHN HOCKENBERRY: So little clocks hardwired into mosquitoes and birds, what does any of this have to do with climate change?

ADAM BURKE: Well, when Bradshaw and Holzapfel started their research in the early 1970s, they didn't know it had anything to do with climate change. They were just studying the timing of when pitcher plant mosquitoes were entering dormancy. But in Spring of 2002, they suddenly realized their mosquito work might have something to say about climate change and evolution. They analyzed data comparing the timing of dormancy over a 24-year period, then, a bit shocked, they checked a five-year comparison.

CHRISTINA HOLZAPFEL: What we expected to see in the analysis of the five-year data set was nothing. What we actually saw was that the mosquitoes were evolving in a tiny, short time frame--five years. That was just stunning to us.

WILLIAM BRADSHAW: We talk about evolutionary time, and we think evolutionary time takes decades or centuries or a millennia to occur, and we don't think of evolutionary time being on the order of a few years, and when we saw the shift over a five year period, that said to us, "This is evolution occurring at breakneck speed due to climate warming."

JOHN HOCKENBERRY: So wait a minute, wait a minute. Evolution at breakneck speed. What does that mean?

ADAM BURKE: That means that populations of pitcher plant mosquitoes are evolving to shift the timing of their dormancy to later in the year. The timing of dormancy and the pitcher plant mosquito is genetic, remember?

JOHN HOCKENBERRY: Okay, right, right.

ADAM BURKE: Okay, good. So here's how it works. Each population of mosquitoes has evolved to go into dormancy at a particular time of the year.


ADAM BURKE: But within any one population, there will be a range of responses.

JOHN HOCKENBERRY: Well, of course.

ADAM BURKE: Some individual larva will go into dormancy earlier, some go later.

JOHN HOCKENBERRY: Okay. I think I'm still with you.

ADAM BURKE: The thing is, there are evolutionary consequences to the timing of all of this.

WILLIAM BRADSHAW: Timing is essential.

CHRISTINA HOLZAPFEL: Dormancy is all about timing. If you go into dormancy very, very early into the season, you haven't had an adequate opportunity to make lots of progeny or young.

WILLIAM BRADSHAW: If you go dormant too late, you die.

CHRISTINA HOLZAPFEL: And that's a bad thing as well.

ADAM BURKE: So this is something Bradshaw and Holzapfel call "strong selection pressure."

JOHN HOCKENBERRY: [laughs] Meaning there are rewards for getting the timing right. You get to live.

ADAM BURKE: And negative consequences to getting the timing wrong. Now, here is what's interesting about all of this. The factors that determine what's too late and what's too early for the mosquito are shifting with climate change.


ADAM BURKE: What happens when you go into dormancy too late?



ADAM BURKE: With climate change, winters are milder than they used to be. So the cut-off point separating the mosquitoes that die from the ones that live is actually arriving later in the year, or another way to look at it, mosquito populations in the far north go into dormancy around a week later than they did 30 years ago.

JOHN HOCKENBERRY: It sounds like that there's some sort of optimal window in time, and that that window is shifting later in the year.

ADAM BURKE: Well, it's thanks in part to that variation, right? By having a range of timing responses within a population, the pitcher plant mosquito always has a chunk of individuals that get the timing right.


ADAM BURKE: And make it through the window to pass on their genes.

JOHN HOCKENBERRY: And because they have short generation times, the mosquito can evolve in a time period as short as five years, right?

ADAM BURKE: Exactly, though it's kind of a bittersweet thing for Bradshaw and Holzapfel.

CHRISTINA HOLZAPFEL: We feel the fact that Wyeomyia is able to adapt and evolutionarily change in response to rapid climate change is a very positive thing. It makes us feel happy. At the same time, we are not so naive to think that we can take its success, and say that that reflects what is going to happen to large mammals and to humans, for example.

ADAM BURKE: If you think about it, a mosquito can go through many generations in the time it takes a polar bear to learn its first baby steps.

JOHN HOCKENBERRY: Yikes. So that means many species aren't going to be able to bob and weave here. I mean, they're not going to be able to adapt, especially in the hardest hit areas of the globe. We've heard about the poles, but there's another red zone. Coming up after the break, the heat is on in Australia. You're listening to The DNA Files. We'll be back in a minute.
JOHN HOCKENBERRY: Welcome back to The DNA Files. I'm John Hockenberry, here with reporter Adam Burke. Adam?

ADAM BURKE: Hi, again.

JOHN HOCKENBERRY: We're exploring climate change as an evolutionary phenomenon, a force that's re-jiggering the planet on both a physical level and a genetic level, and we've sent our producer, Adam, out into the world, out into the world--did you hear that Adam?--to find out how global warming is pushing species around. Before the break, we heard how species are shifting around geographically, changing their seasonal timing, and adapting genetically in response to a warming world.

ADAM BURKE: But if, on the other hand, you don't have that kind of wiggle room --

JOHN HOCKENBERRY: For example, if you happen to be a little colony of branching coral on Australia's Great Barrier Reef --

ADAM BURKE: And we're not saying that you are.

JOHN HOCKENBERRY: But let's just say for the sake of argument, that you are a small reddish brown colony of coral, about the size of a head of cauliflower attached to a lumpy stretch of reef. It's not like you can pack up the store and head south or north or wherever things might be better. You're stuck. You just sit there.

It sounds like we're on a boat. What happened? Are we on a boat?

ADAM BURKE: We are on a boat, near the Lizard Island Research Station, which is at the northern end of the Great Barrier Reef.


ADAM BURKE: And a team of divers from James Cook University are getting ready to collect the kind of coral you were just talking about.

JOHN HOCKENBERRY: You mean, the small, reddish brown, about the size of a head of a cauliflower coral?

ADAM BURKE: Yeah, that one. Pocillopora Damicornis, the kind marine biologist Morgan Pratchett studies.

MORGAN PRATCHETT: It's one of the more important corals in terms of habitat for fish and other invertebrates, which tend to live in the branches of corals. So we're going to go collect a heap of these corals, and see what's inside them.

JOHN HOCKENBERRY: Inside of coral? What's inside of coral?

ADAM BURKE: Well, he's looking for little crabs and shrimp and starfish and other critters that live in the nooks and crannies of the branches.


ADAM BURKE: So soon they're swimming around 15 feet below the surface, prying loose colonies of Pocillopora Damicornis. That's just one of over 300 species of coral in the Great Barrier Reef. They come in all sizes, shapes, and colors.

MORGAN PRATCHETT: Typically, that ranges from brown to green to blue and really quite dark colors.

ADAM BURKE: Some look like big thickets of branching antlers. Others are round and solid, textured like brains. But the structure you see is not the coral animal. It's a calcified skeleton of sorts.

JOHN HOCKENBERRY: It's like the house of the animal, I guess, right?

ADAM BURKE: More like a high-rise building, and the tiny coral animal is attached to the calcified structure. And living inside the cell tissue of this coral animal are microscopic single-celled plant species called--brace yourself for this, John--zooxanthellae.

JOHN HOCKENBERRY: Zooxanthellae. [laughs] Hey, how do you like me now? That is a mouthful, though.

ADAM BURKE: Yeah, and it's this symbiosis that allows corals to grow all these fancy structures. The photosynthetic plant harnesses energy from the sun and passes on nutrients to the coral animal, and the coral animal gives nutrients to the plant.

JOHN HOCKENBERRY: So this animal and plant working together, build all these fabulous [laughs] apartment buildings. Is it a happy relationship?

ADAM BURKE: It's a happy relationship most of the time. But it falls apart when times get tough--pollution, changes in ocean salinity, temperature extremes.

MORGAN PRATCHETT: Any number of these factors could cause the coral to become physiologically stressed, and as a consequence, it will undergo a bleaching. And bleaching is a process where the symbiotic algae, the zooxanthellae, essentially are evicted from the coral host.

JOHN HOCKENBERRY: This bleaching business sounds serious I mean, can the coral roommates get back together?

ADAM BURKE: Well, that depends on how extreme the stress factors are. Historically, bleaching events were small and local, but over the last few decades, warmer ocean temperatures have brought regional scale bleaching. In 1998, it's estimated that 12 to 15% of the world's corals died. So with global temperatures going up, Morgan Pratchett is trying to understand the relationship between reef biodiversity and the presence of living coral.

MORGAN PRATCHETT: We don't know all the myriad of organisms which live on a reef, but if we just think about coral reef fish, at least 70% of those species decline in abundance after a bleaching event.

ADAM BURKE: And don't forget, it's the fish that bring in all of those snorkeling tourists --


ADAM BURKE: Worth about five billion Australian dollars a year.

JOHN HOCKENBERRY: Whoa. That's how the food chain works, I guess. It's not just a few scientists who are wringing their hands over climate change in coral reefs, eh?


JOHN HOCKENBERRY: So it seems that the key question here is: Can coral actually adapt to warmer oceans, even though it's stuck right where it is?

ADAM BURKE: Well, some scientists say no.


ADAM BURKE: And the reason is that symbiosis we were talking about earlier.

JOHN HOCKENBERRY: You mean the zooxanthellae business with the little coral animal you were talking about?

ADAM BURKE: Yeah, it's a tricky evolutionary relationship. I mean, let's say, one of these symbiotic zooxanthellae is floating around in the ocean.


ADAM BURKE: Just a little cell, and then that little cell bumps into the particular species of coral host that it likes to live with.


ADAM BURKE: That symbiont has to get inside a cell of the coral animal, which is not the easiest thing to do.

OVE HOEGH-GULDBERG: Cells spend a lot of energy trying to prevent things from invading them, because they're a little lump of energy, and they're trying to keep it for themselves, right?

JOHN HOCKENBERRY: Meet Ove Hoegh-Guldberg, a marine biologist at the University of Queensland.

OVE HOEGH-GULDBERG: These are these immune systems, and there's ways of destroying invading pathogens and bacteria and so on, and then suddenly it's, "Come inside. Live inside my cell. You know, I'm not going to fear you. I'm going to live with you." And, of course, that's a very bizarre situation, when you think about it.

JOHN HOCKENBERRY: I'll say. But they are coral, after all. And how do these coral know if you're a friendly cell or an enemy cell?

OVE HOEGH-GULDBERG: What we do know is that there are some molecules in the surface of the right type of zooxanthellae, which are being recognized by systems on the coral side. This is acting essentially like a set of keys. You get the key to the first room, if you get into the cell. A broad range of cells might actually get taken in. What happens next is if they're the wrong type, then they get attacked by little parts of the host cell called lysosomes, and they're eliminated that way. So if you have the first set of keys, you get into that first room. If you have the second set of keys, you avoid digestion.

Then the next trick is, of course, you've now got to integrate into the physiology of the host. So this is like the third set of keys. This is where you have the ability to take up nutrients in the host and to pump out sugars, and to make sure that you start to grow in the tissues.

ADAM BURKE: So it's pretty complex and delicate on the molecular level. We're talking about two species that have both evolved together, and Hoegh-Guldberg believes that co-evolution took a long time.

OVE HOEGH-GULDBERG: I think the problem with this is it's not a single character. It's a whole set of genes, which are involved in symbiosis, which have to be concerted in their evolution. You know, maybe hundreds of genes have to slowly co-evolve with the host to get to a point where they've got this sophisticated and intimate relationship between the two cells.

ADAM BURKE: Now, that's not saying corals can't evolve to cope with increased temperatures. In fact, there's evidence that they can and have. The Great Barrier Reef is 1,400 miles long. Temperatures range from one end to the other, and you can find the same coral symbiont combination living in different locations with different temperature tolerances. So Terry Hughes, who runs the ARC Center for Marine Excellence in Townsville, Queensland, says it's premature to say whether corals will adapt or not.

TERRY HUGHES: Clearly, corals are locally adapted to the temperature regime that they're currently found at. We don't know how long that local adaptation took. It might have taken millennia. So the million dollar question is: Can corals adapt to rapidly rising temperatures? And we don't know the answer to that.

ADAM BURKE: They do know these mass bleaching events can be somewhat selective. Some species are surviving better than others.

TERRY HUGHES: We're already seeing a shift in the composition of species in favor of species that are more resistant to bleaching.

ADAM BURKE: So in the short run anyway, Hughes believes we'll continue to have what he calls "vibrant coral reefs" if we can slow the rate of climate change.

TERRY HUGHES: I don't want to overstate the optimism. I think coral reefs are going to be degraded, but I don't think we're going to see them destroyed in 30 years. We're going to see a shift in species composition, but if we can limit the temperature rise to two or three degrees --

ADAM BURKE: That's four to five degrees Fahrenheit.

TERRY HUGHES: And if we take strong action now, then I think we've got considerable optimism in terms of the future of coral reefs.

JOHN HOCKENBERRY: So Terry Hughes sounds hopeful here. I'm going to have to say it's kind of a relief to hear that reefs won't be destroyed in the short run.

OVE HOEGH-GULDBERG: On the raw face of it, yes, that's correct.

JOHN HOCKENBERRY: There's Ove Hoegh-Guldberg again.

ADAM BURKE: Yeah, and he would tell you that changes that Terry Hughes is talking about aren't just cosmetic.

OVE HOEGH-GULDBERG: I think it's a little misleading to say that they’re going to be anything like a coral reef as we know it today, because corals, I think, will be quite rare organisms. They've got huge ranges. They can reproduce asexually. So there's no problem about corals in the short term as far as extinction, but in terms of their dominance of ecosystems like the Great Barrier Reef, I think they will be rare members of the community.

JOHN HOCKENBERRY: When it comes to extinction, it's difficult to know exactly which species are next, but that doesn't stop scientists from trying to understand how rare species will fare with this climate change business. I mean, I'd want to know if I was next.

ADAM BURKE: Me, too.

JOHN HOCKENBERRY: So just west of the Great Barrier Reef, along the northeast edge of Australia, there are scientists who are studying species at risk. And you went out with them into the woods, Adam, right?


JOHN HOCKENBERRY: Tell me if I have this right. These are a chain of mountains in Australia known as the Wet Tropics.

ADAM BURKE: That's right. That's right.

JOHN HOCKENBERRY: High altitude, right? Cool, misty rainforest?

ADAM BURKE: That's exactly right. The Wet Tropics are some of the last shreds of an ecosystem that once blanketed Australia, and Steve Williams studies the plants and animals that live only in the Wet Tropics--in other words, found nowhere else in the world. He's an ecologist based at James Cook University in Townsville, Queensland, but more often than not, you can find him out in the rainforest with a research team, documenting biodiversity.

STEVE WILLIAMS: There's another possum way up in the canopy.

ADAM BURKE: Oh, yeah.

STEVE WILLIAMS: There's another lemuroid.

ADAM BURKE: So it's the middle of the night, and we're out with flashlights, counting the marsupial possums that we can spot in the tree canopy. These animals are about the size of a cat. They're hundreds of feet up. You can actually identify them by the color and shape of their eye shine--or, at least, Steve Williams can. We're also on the lookout for other nocturnal animals--owls, bats, a rare species of kangaroo that lives in trees --

JOHN HOCKENBERRY: Oh, man. Dude, you saw kangaroos that live in trees?

ADAM BURKE: Yeah, we didn't see any.


ADAM BURKE: But one of the students caught a lizard called a leaf-tailed gecko, and we all gathered around--a foot long, mottled green and tan coloring, bulging, yellow eyes.


STEVE WILLIAMS: You know, these things rely on camouflage. You can see by the pattern. They look like the lichen on tree bark, and their tail is leaf-shaped, which is what gives them their name of leaf-tailed geckos. And even their eyes have a camouflage pattern on them. [laughs] You can hear a little squeak. Hi, cutie.

ADAM BURKE: Wow, they are really beautiful.

STEVE WILLIAMS: Oh, yeah, they're fantastic, one of my favorite animals.

JOHN HOCKENBERRY: With a tail shaped like a leaf and yellow camouflaged eyes --

ADAM BURKE: It looks like a creature from another planet.


STEVE WILLIAMS: The belly is just like soft as silk, isn't it?


STEVE WILLIAMS: Here, feel the belly.


ADAM BURKE: Can I feel the belly, too? [Gecko squeaks.] Oh. [laughter]

ADAM BURKE: In the last few million years, climate has fluctuated wildly in the Wet Tropics. For example, the last Ice Age, 18,000 years ago, caused the region to dry out considerably, leaving just tiny islands of cool misty rainforest on a few mountaintops.

STEVE WILLIAMS: As the rainforest contracts, each one of those mountaintops becomes more and more isolated and smaller and smaller area. Sometimes if it gets too small, the rainforest completely disappears, the populations in that area would have gone extinct. And as the rainforest has expanded again, over the last 5,000 years, species have been able to re-colonize those mountains.

JOHN HOCKENBERRY: So climate has crunched the forest down to these tiny isolated pockets, and then they've expanded out again and crunched down again, and expanded out again.

ADAM BURKE: And the tiny isolated pockets that remained became kind of these life rafts for many species of vertebrates. That's where they hunkered down and rode out the dry times. Creatures like the lemuroid ringtailed possum.

JOHN HOCKENBERRY: And I'm almost afraid to ask, the tree kangaroo?

ADAM BURKE: Yup, and the tree kangaroo, and birds, and frogs, and our friend, the leaf-tailed gecko.


ADAM BURKE: My friend. All of these different critters survived in tiny patches of forest that were left on mountaintops.

JOHN HOCKENBERRY: So whatever happened to your little leaf-tailed gecko pal? I mean, what did the scientists do with it?

ADAM BURKE: Well, they weigh it and measure it, record the coordinates where it was found, and they take a DNA sample.

JOHN HOCKENBERRY: That's interesting--I don't know how interesting for the gecko, but it's interesting.

ADAM BURKE: It is really interesting. Part of the story of past climate change can be found in the genes of these animals.


ADAM BURKE: Yeah, which is why Steve Williams is working with a population geneticist named Craig Moritz at the University of California, Berkeley to decode this story of past isolation.

CRAIG MORITZ: Take a population down to a very small size, it loses a lot of those genetic variation, and that loss of genetic variation takes a long time to recover. You have to get it back by mutation, and that takes tens of thousands of years to do. So there's a record of history sitting in the DNA that we can use to try and estimate what was the population size like through time from the pattern of variation.

ADAM BURKE: So for example, if a population of leaf-tailed geckos were confined to a mountaintop thousands of years ago, that population is going to have a recognizable genetic signature.

JOHN HOCKENBERRY: Just because of a mountain? Why?

ADAM BURKE: Well, remember, Craig Moritz said, "Take a population down to a very small size, and it's going to lose a lot of its genetic variation." Okay, here, let's think about it this way. See this bag of candy?

JOHN HOCKENBERRY: This bag of candy right here?

ADAM BURKE: That bag of candy.


ADAM BURKE: There are 13 different colors of jellybeans inside.

JOHN HOCKENBERRY: You've already inspected this?

ADAM BURKE: I have already inspected it. Now, reach in, grab a small handful, and put them in that bowl.


ADAM BURKE: Just a small handful, John.

JOHN HOCKENBERRY: All right. There's some for you, don't worry.

ADAM BURKE: Okay. So what colors do you have?

JOHN HOCKENBERRY: All right. I've got some dark blues, two yellows, a brown one, some red ones.

ADAM BURKE: So just four of the 13 colors.

JOHN HOCKENBERRY: Yup, that's all I got.

ADAM BURKE: So it's not really a surprise, right, that there's going to be less variety in the bowl --


ADAM BURKE: Than there is in the whole bag? I mean, odds are really low that you're going to have all of the colors, right?

JOHN HOCKENBERRY: Right. So 13 colors in the bag, only four in my little bowl here.

ADAM BURKE: The same thing happens with leaf-tailed geckos when their habitat crunches down to a little island on a mountaintop.


ADAM BURKE: That bag represents the diversity before the crunch. The candy in your hand represents the genetic diversity of geckos remaining on a particular mountaintop after the crunch.


ADAM BURKE: Okay. So now reach in the bag and get another handful and tell me the colors.

JOHN HOCKENBERRY: All right. All right. We've got one brown, some yellows, some orange ones, and some green ones.

ADAM BURKE: So that represents the genetic diversity of geckos on another mountaintop.

JOHN HOCKENBERRY: Different combinations of colors.

ADAM BURKE: And each combination represents a different genetic signature.

JOHN HOCKENBERRY: Which allows our geneticist Moritz to tell descendants of one population from another.

ADAM BURKE: Exactamundo. So they start with some genetic data collected from leaf-tail geckos throughout the Wet Tropics.

CRAIG MORITZ: So we look at the leaf-tail geckos, we see two genetic groups. Within each population, the level of variation is exceptionally low, which suggest they contracted back to a very small population.

ADAM BURKE: And because they mapped where each genetic gecko sample came from, they know something about where these creatures survived during the rough times.

CRAIG MORITZ: There are different numbers of genetic clusters, if you like, within each species. So the leaf-tail gecko had just two--one north, one south--a very, very clear picture. In others, there could be five or six or seven different genetic groups, which implies that they survived the glacial periods in multiple places.

ADAM BURKE: There are actually a whole range of genetic signatures that tell Moritz where species hunkered down, where they went locally extinct, and how long it took them to spread back out again.

JOHN HOCKENBERRY: It's like DNA's a map. I mean, Steve Williams is out there, running around in the woods, catching lizards and frogs, and taking tiny tissue samples, which he sends to Craig Moritz.

ADAM BURKE: And Craig Moritz uses the genetic signatures to reconstruct a story of how that map has changed over time for the gecko and many other species in the Wet Tropics.

CRAIG MORITZ: It's like trying to read molecular tea leaves. You know, whoa, what's in the bottom of the cup, and what did that mean about the past? You need to draw multiple lines of evidence, and it's when you put all the different lines of evidence together, we can build up a fairly consistent picture of what happened.

ADAM BURKE: Williams and Moritz are also using their information to estimate how these species are going to cope with rising temperatures now, based on what happened in the past.

JOHN HOCKENBERRY: How much warming is predicted here?

ADAM BURKE: Between three and ten degrees Fahrenheit.

JOHN HOCKENBERRY: That Fahrenheit thing again. And what do they say will happen to the Wet Tropics species?

ADAM BURKE: Similar to what happened in the past. As temperatures increase, these species will retreat up to the highest reaches of the mountains until there's nothing above but blue sky.

CRAIG MORITZ: I think the consensus at the moment is that this is a type of climate that these fauna simply haven't experienced in the last few million years, and these species, which formerly evolved in much cooler, wetter climates are going to get hammered. So I desperately hope these models are wrong, and we're doing everything we can to disprove them.

JOHN HOCKENBERRY: Coming up, a species that is changing the climate. This is "The Heat is On: Evolution in Action." We'll be back in a minute.
JOHN HOCKENBERRY: This is The DNA Files. I'm John Hockenberry, and Adam Burke is with me. Adam?


JOHN HOCKENBERRY: So we've been talking today about how a little bit of climate warming over the past century--just 1.2 degrees on average has already shaken up the biological world, and is pushing species towards extinction. But what we tend to forget in all this is how ordinary extinctions are--Earth, you see, is not a stable place. It never has been. You've got continents moving around. You've got changes in temperature, changes in ocean chemistry. You've got volcanoes, even asteroids hitting the earth.

STEVE JONES: At the end of the Permian Era, 241 million years ago, there was a massive extinction.

JOHN HOCKENBERRY: That's evolutionary biologist Steve Jones of the University College, London.

STEVE JONES: Over 90% of all creatures disappeared. Whole groups of animals like the trilobites--one of the most abundant of all, just went. Fortunately, a few things were left, and the evolution went back almost to the beginning of the chess game and started again.

ADAM BURKE: Lucky for us. [laughs]

JOHN HOCKENBERRY: [laughs] That's right, and you might see this as a story of resilience, but it also reminds us that even the most robust life on earth could be a hair's breadth away from the whirling blades of extinction, at least if you consider evolutionary time.

STEVE JONES: I often think of natural selection as a factory--a factory for making almost impossible things--you're an almost impossible thing. I am. Every bird is, every plant is, every bacterium is. You and I and all those other creatures stand at the summit of an enormous mountain of extinct life--creatures which did not survive to pass on their genes.

JOHN HOCKENBERRY: It's amazing. Jones is describing an entire parallel universe of extinct life. Our current warming trend here on earth could add another heap of species to that collection of extinct organisms. It kind of makes me wonder. What kind of a world are we headed for here?

STEVE JONES: Evolution in some ways is the triumph of the weeds, and the weeds are already taking over. Climate change is going to make it worse.

ADAM BURKE: I don't suppose Jones is talking about dandelions or crabgrass.

JOHN HOCKENBERRY: Uh not really, but sort of. I mean, he is talking about all the species that can live in a wide range of habitats. You know, the cockroaches, the raccoons, the coyotes--

STEVE JONES: And the weediest species of all, of course, is Homo Sapiens, in the sense that we're the species that's come and ruined everybody else's habitat and trampled all over it for our own delectation rather than staying rare, specialized, and rather beautiful in a valley in East Africa.

JOHN HOCKENBERRY: Get used to it, folks. Homo Sapiens, the ultimate weed. We live on almost every continent, and in almost every climate on earth. Why didn't we remain rare, specialized, and beautiful in a valley in East Africa?

ADAM BURKE: I got a theory to share.

JOHN HOCKENBERRY: Well, aren't you just the little theory meister here?

ADAM BURKE: It's a good one.

JOHN HOCKENBERRY: Okay, let's--let's hear it.

ADAM BURKE: Okay, good. So anthropologists have long theorized that climate change played a role in making humans the wily generalists that we are.

JOHN HOCKENBERRY: [laughs] Oh, you wily generalist, you. But I've heard that theory. Five millions years ago, human ancestors were living in dense forests, and then when East Africa transitioned into an open grassland environment all of a sudden, our ancestors had to come up with a very serious Plan B.

ADAM BURKE: Yeah, and anthropologist Rick Potts of the Museum of Natural History in Washington, D.C. says this was a prevailing theory for a long time.

RICK POTTS: We kind of all thought that we knew what the setting was, that basically Africa dried out, and that was the cauldron in [laughs]--in which human evolution took place. It bubbled along, and out came new adaptations to the challenges of the open grasslands.

JOHN HOCKENBERRY: That sounds like the standard evolutionary story.

ADAM BURKE: Well, it is. The environment changes, and human ancestors evolved to cope with the new environment, giving rise to the many things we prize in ourselves.

JOHN HOCKENBERRY: Like tool use, like walking upright, social cooperation, hunting, meat eating.

RICK POTTS: And you could basically unfurl all the different qualities that we are so proud of in terms of human uniqueness just by getting early humans into one environment.

JOHN HOCKENBERRY: Now, as I'm listening, I assume there's a flaw in this theory, right?

RICK POTTS: Well, the problem with that is that there's no longer any evidence for it.

ADAM BURKE: Now, East Africa is drier than it was five million years ago, but when you consider multiple sources of climate data, things like ancient pollen samples, lake beds, sediment cores from the Indian Ocean --

RICK POTTS: None of that indicates that the savanna was contemporaneous with the earliest evidence of fossil humans. And so, the question is: Well, what is the environment?

ADAM BURKE: It turns out it wasn't any one environment at all. Rick Potts turned to paleo-climate modelers like Peter DeMenocal of Columbia University. And DeMenocal says East Africa saw dramatic climate swings between wet and dry over the last five million years, or as he puts it, there were greener times and browner times.

PETER DEMENOCAL: Greener times with more vegetation, browner times with less vegetation.

JOHN HOCKENBERRY: So in some periods, rainfall increased, and forests expanded. In others, rain was more scarce, and things dried out.

ADAM BURKE: Right, but remember this swinging back and forth is happening over periods of tens of thousands of years. DeMenocal's data lets him watch these fluctuations like a movie.

PETER DEMENOCAL: If you were to play this movie, you would see this pulsing of greener times and browner times, going back and forth, but then as we move toward the present, you notice that the range of variations increases as well.

ADAM BURKE: In other words, there were times--stretches of a few hundred thousand years where the swings between wet and dry became more dramatic.

JOHN HOCKENBERRY: So greater contrast between the wet, wet and the dry, dry during these periods.

ADAM BURKE: Yeah, and Rick Potts began to believe that the increase in climate variability was an important piece of the human evolution story.

RICK POTTS: It dawned on me that we really didn't have any idea about what that meant. We always treated it as just noise in the signal. [laughs] The signal represented the constant qualities of environments, the stable qualities, but to my mind, the variability might itself represent a very important signal of uncertainty and unpredictability that might help us understand the emergence of new adaptations, new traits.

ADAM BURKE: This is something Potts calls variability selection, and the idea is that environmental instability over long periods of time will favor organisms that can cope with a range of environments.

RICK POTTS: What ultimately will survive that are those genes and those strategies of behavior that allow versatility, allow versatile or adaptable response to when things do change.

ADAM BURKE: What's interesting is that the fossil record of human ancestry seems to bear this idea out. For example, take the well-known fossil, Lucy.

JOHN HOCKENBERRY: Oh, yeah, I know her--female, about 3.2 million years old, discovered in Ethiopia in the mid-1970's, likes to date anthropologists.

ADAM BURKE: That's the one. Lucy is the most famous fossil member of a species called Australopithecus afarensis. Lasted in Africa for one million years in a region that fluctuated dramatically between wet and dry.

RICK POTTS: What was amazing is that Lucy's lineage was found in all of the layers through all of these different environmental changes, and Lucy was an amazingly adaptable early human, even without a large brain and stone tools. Lucy's lineage, Australopithecus afarensis, became extinct some time around 2.9, 2.8 million years ago, and that's a time when the amount of environmental fluctuation in Africa increased even more, more than it was during Lucy's time.

ADAM BURKE: Around the same time, two new hominids emerge--including the earliest appearance of our own genus, Homo, the first hominid with a slightly larger brain.

RICK POTTS: And what's interesting is that what comes out of Lucy's extinction is the emergence of two new branches in the human family tree that are more adaptable than Lucy was.

ADAM BURKE: So Potts argues environmental instability rewarded those that could branch out and diversify, and he lumps cultural innovations in there, too. This same period, 2.8 to 2.5 million years ago, marks the earliest evidence of eating meat and the earliest evidence of stone tools, which Potts says was like getting access to a whole new set of teeth.

RICK POTTS: All sorts of different foods open up to the possibility of being processed with these stone tools, with these teeth outside of your body.

ADAM BURKE: Stone tools were an early indication that human ancestors were becoming more adaptable, not only by evolving new physical characteristics, but by tinkering with the environment and sharing their know how with each other.

JOHN HOCKENBERRY: So better brains meant more elaborate cooperation, snazzier tools. We get language, agriculture--all these are adaptive responses to a volatile environment

ADAM BURKE: That's what Potts argues.

RICK POTTS: We are tinkerers. We change, and we like to change everything. And what we face right now is an experiment in earth's climate that has never really been tried. We are now a new factor on the volatile planet. It's not that the planet is inherently stable. We know that that's not the case. The problem is is that we're now pulling on some of the same strings that have in the past led to climate change and led to extinctions.

JOHN HOCKENBERRY: Okay, that's a problem, but to deal with it, we've got our plasticity and our adaptability as a species, an evolutionary product of a volatile world, right? I can feel good. But I have a feeling that there are people who disagree with Potts, right?

ADAM BURKE: Sure, there are other theories on what drove human evolution and then some anthropologists say it's an interesting idea with no conclusive data.

JOHN HOCKENBERRY: But whatever disagreement there may be about the mechanisms that drove human evolution, I mean, [laughs] few can argue about the net result. We're doing pretty well here.

ADAM BURKE: It's hard to argue with the reality of 6.7 billion Homo Sapiens.

JOHN HOCKENBERRY: That's right. [laughs] We talked with a guy who spent his life thinking about human population numbers, Paul Ehrlich of Stanford University. He says the way we have literally infested the planet is a kind of evolutionary success.

PAUL EHRLICH: One can argue about whether certain bacteria or so on are dominant over us, but there's no question that as animals go, we're the top dog on the planet.

JOHN HOCKENBERRY: And it's those very same human top dog qualities forged over the last five million years that Ehrlich says explains why we have become the life of the party here on earth.

PAUL EHRLICH: So we have a relatively small, but very effective set of genetic information, which produces among other things this marvelous brain and the capacity to manipulate gigantic amounts of cultural information.

JOHN HOCKENBERRY: We pass along our genes, and in a similar way, we pass along our culture. Ehrlich argues that culture is what gives Homo Sapiens the power to respond to environments in very rapid and sophisticated ways. In early human societies, you didn't need to invent stone tools from scratch every generation. You could learn how to make them from your pals. In modern societies, we screw in the light bulb and flip the switch without ever needing to know how to make one. We evolve to do this, and we do it very, very well.

PAUL EHRLICH: Human beings have become dominant by manipulating their environments. In other words, rather than just reacting to whatever the environment does, we change environments, and that's what we're still doing. We are now changing the environment of essentially every organism on the face of the planet.

JOHN HOCKENBERRY: And now we've tilted the climate systems, too. It's an unintended byproduct of the human way of life, but the reality of climate change challenges Homo Sapiens to change the way we live, and the stakes are high. Rising sea levels, disrupted food supplies, rising temperatures that could throw Earth's systems into ecological chaos.

PAUL EHRLICH: So it's basically a problem in cultural evolution. How do you transform what we know scientifically and what we know we should be doing into actual social action? And there's lots and lots of things we could be doing this very day. So the issue is: Why don't we use the tools we've evolved?

ADAM BURKE: Yeah, John, why don't we?

JOHN HOCKENBERRY: You're asking me?

ADAM BURKE: I'm asking you.

JOHN HOCKENBERRY: You're asking me?

ADAM BURKE: [laughs] I'm still asking you.

JOHN HOCKENBERRY: Well, I was thinking of the story Rob Boyd tells. He's an anthropologist from UCLA.

ADAM BURKE: And that would be the story of?

JOHN HOCKENBERRY: You know the story. You know the story. Eric the Red.

ROB BOYD: Eric the Red settled in Greenland from Iceland right around 1000 A.D., and there was a fairly thriving community of Norse settlers on the southwest coast of Greenland. Think of these people living on, you know, beautiful, grassy valleys on the edge of fjords and stone houses, and living and marrying and drinking, and doing all of the things that people did.

JOHN HOCKENBERRY: So there they were --

ADAM BURKE: Minding their own Nordic business.

JOHN HOCKENBERRY: Growing hay in summer and storing enough to last the winter months, and it lasted that way in Greenland for a few hundred years, and then the little Ice Age happened.

ROB BOYD: When it got a little colder, it shifted the balance so that the system they had--the Norse economic system stopped working.

ADAM BURKE: Uh what does he mean it stopped working?

ROB BOYD: There were occasional visits from trading ships from Iceland, and the last one came, and everyone was dead.

JOHN HOCKENBERRY: Meanwhile, living up the coast, there were Inuit hunter gatherers who also had to brave the little Ice Age, but they survived in different ways.

ADAM BURKE: What, kayaks? Harpoons? A delicious seal diet?

JOHN HOCKENBERRY: Hey, Adam, what are you doing? Reading ahead? Yeah, that's what they did.

ROB BOYD: And the interesting thing is, there was lots of contact between the Norse and the Inuit, and so even though their neighbors were a success, the Norse never copied this way of life from their neighbors, and stuck to their guns, and in the end, went out of business.

JOHN HOCKENBERRY: Score one for the Inuit. So the question, of course, is: Why? Why did these Norse settlers starve to death when the keys to survival were right in front of their highly evolved, adaptive faces?

ADAM BURKE: You know what I would have done?


ADAM BURKE: I would have put down my horned Viking hat and started taking harpoon lessons from my Inuit buddies.

JOHN HOCKENBERRY: Some Viking you are. I mean, you're a sensitive, international guy, Adam. Of course, you would have done that. And maybe there were some Nordic folks who traded their myths for mukluks, but not enough did. Most didn't. No one knows why for sure, but Rob Boyd thinks that culture had something to do with it.

ROB BOYD: So even though, the Inuit system seemed to be better, you could see a--a Viking guy thinking, "Well, my neighbors will think it's ridiculous if I stop farming and start paddling around in a kayak. That's not what we do. That's what those other people do." And so I think that it's completely understandable how this happened, but it does illustrate that despite the fact that we're great adapters, sometimes we--we don't adapt well enough.

JOHN HOCKENBERRY: So culture is kind of our mixed blessing. It's brought humankind a long, long way. But sometimes culture also prevents us from recognizing the path to survival, even if it's right there in front of us.

ROB BOYD: We have the capability of adapting that's unprecedented, I think, and that comes from the fact that we have these cultural tools, which let us adapt non-genetically on really rapid time scales, but whether we'll channel those in our collective interests, that's the hard question.

JOHN HOCKENBERRY: Hard question? Hard question? [laughs] That is the only question. There is no other question than this.

ADAM BURKE: Okay. So where does that leave us?

JOHN HOCKENBERRY: Well, it seems like it leaves us facing the same mind boggling global problem we started with, Adam. I mean, climate change is going to hit us, right?

ADAM BURKE: Right. I mean, it's already hitting us.

JOHN HOCKENBERRY: The question is: What human changes is that going to bring?

PETER DEMENOCAL: Right now, we're dealing with an earth that's warming, because of human activities. That's just the fact, Jack.

CAMILLE PARMESAN: The climate space in which species now exist is gone if we have business as usual.

CRAIG MORITZ: Humans are just one species of perhaps 10 million on the planet.

CHRISTINA HOLZAPFEL: Who among us cannot give up one or two luxuries?

WILLIAM BRADSHAW: If all we could do is slow the process of climate change, we would provide greater opportunities for plant and animal communities to adapt.

PAUL EHRLICH: We're either as a culture--and we're a global culture in this sense with this problem--we're either going to solve it as a global culture or we're not going to solve it as Homo Sapiens.

JOHN HOCKENBERRY: Ehrlich said it. We are an evolutionary experiment living on the edge.

ADAM BURKE: An almost impossible thing.

JOHN HOCKENBERRY: I love that. A hominid with a big adaptable brain.

ADAM BURKE: Capable of unlocking Nature's secrets and solving the puzzles inside.

JOHN HOCKENBERRY: And whether we like it or not, we are the authors of the latest chapter in the human evolutionary story. What do you out there think should be written? Here with Adam Burke, I'm John Hockenberry. Thanks for listening to The DNA Files.

To find out more about climate change, evolution, and genetics, visit our website at where you can download a podcast of this program. This series, The DNA Files, was produced by SoundVision Productions with funding by the National Science Foundation, U.S. Department of Energy, National Institutes of Health, and the Alfred P. Sloan Foundation. This program, "The Heat is On: Climate Change and Genetics" was produced by Adam Burke. The DNA Files is managing editor, Loretta Williams, editor, Deborah George, science content editor, Sally Lehrman. Research director is Adi Gevins. Production support by Noah Miller, Julie Caine, and Raynelle Rino. Office support provided by Steve Nuñez and Beverly Fitzgerald. Our web director is Ginna Allison. Technical engineer and music director is Robin Wise. Our host is John Hockenberry. Our theme music was composed and performed by Steve White. Additional music by Adam Burke, Conrad Praetzel and Robert Powell. Marketing of The DNA Files is by Schardt Media. Legal services by Cooper, White and Cooper and Spencer Weisbroth. Special thanks to Murray Street Productions. Send your responses and letters to For CDs and transcripts, call 888-303-0022. That's 888-303-0022. The executive producer is Bari Scott. This has been a SoundVision production, distributed by NPR, National Public Radio.