Unit 4: Mechanisms of Evolution: Peter and Rosemary Grant

grant.jpgThe hypothesis that the beaks of the diverse finch species on the Galápagos Islands had adapted to different food sources through natural selection went untested for almost 150 years. Then came the classic research of Peter and Rosemary Grant, who have been measuring ongoing beak evolution during their 30-year study of the Galápagos finches. The Grants shared some highlights when we interviewed them in their Princeton University offices.

What is the nature of your scientific partnership?

PG: I was trained mainly as an ecologist, but I was also interested in behavior, evolutionary biology, and genetics. Rosemary's training was rather the reverse. She was trained in genetics, but also had strong interest in ecology and behavior. Over our years of working on the Galápagos, our interests have converged—or rather, we've expanded into each other's fields of expertise.

How did your shared interest in variation within populations become focused on the Galápagos finches?

RG: We wanted a place that was pristine—where there had been little human disturbance—and where the populations are quite isolated and small enough that we could actually keep track of individual birds—and where individuals in the population would vary enough that we would have a chance of measuring this variation. We knew from the work of earlier researchers that some of the most isolated islands of the Galápagos, such as Daphne Major and Genovesa, were promising for such studies. And Peter corresponded with Ian and Lynette Abbott, scientists from Australia who had been studying competition between finch populations in the Galápagos. So this convinced us that it was worth trying to get the money to go down to the Galápagos. Peter went first in March 1973 with the Abbots and banded about 60 or 70 medium ground finches on Daphne. They also banded birds on other islands. Then we went down as a family later that year, in November.

How many trips have you made to the Galápagos?

RG: This year was our 29th year. We normally go for two or three months each year. The longest we've stayed is six months.

What are the camping conditions on Daphne?

PG: We've been kidded by our colleagues that we go down to the Galápagos and lie on the beach the whole time under swaying palm trees. But there is no beach on Daphne. There's just steep rocks. To land on the island, you have to find some little platform that the waves have cut out of the rock and then climb on from the boat when there are no waves. Then you climb up until you reach a slope where you can actually stand up and walk. And you have to get supplies up there too—something on the order of 30 5-gallon water jugs, cans of food, packets of rice, sugar and other basics. Plus a stove and cylinder of gas for cooking, as well as other camping supplies. Our tent fits on an area that's almost flat, about the size of a large table. We store our food and cook in a little cave, close to the landing point. Many people think we're tough. But it's not really too tough if you're accustomed to camping in wild places.

How old were your two daughters when your family started these annual expeditions to the Galápagos?

RG: They were six and eight. They loved it! It was just perfect. They each did little research projects themselves. One studied doves, and the other studied mockingbirds. Later, with Peter's help, they actually wrote two little articles on their research. And of course we took them out of school for as long as six months, and so they had to keep up with their school work, too. They're grown up now—both mothers. And they still think their childhood experiences on the Galápagos were great. Nicola is a physician now. When she was in medical school doing her anatomy, she noticed how surprised other students were that the paths of nerves and other things in their cadavers didn't match up exactly to the drawings and photos in their textbooks. It didn't surprise Nicola at all that all the cadavers were different. By studying different families of mockingbirds on the Galápagos when she was a child, she gained an appreciation for variation. And of course, each human has its own idiosyncrasies too. My father was a physician, and I can remember him telling me how every human responds differently to the same medicine.

PG: Okay, so you've told a story about Nicola. The story about Thalia, our other daughter, who is an artist, is that she was so impressed by her experience on the Galápagos that she has decided to live there! She's raising her two children on Santa Cruz, the main island.

When you initiated your research, did you realize that your family would make so many trips and spend so much time on the Galápagos?

PG: Our first study was designed for just one trip to the islands. But one thing led to another. Initially, we wanted to find out if beak sizes of the populations on different islands matched the differences in food supplies for the finches. In particular, we knew from earlier studies that there was a lot of variation in beak size within the population of medium ground finches, Geospiza fortis. We were looking for an ecological association between beak size and something in the environment. And it had to be food, because the main function of the beaks is to pick up food and crush it. For birds at opposite extremes in a distribution of morphology in the population, the small-beaked birds and large-beaked birds are likely to exploit the food in the environment in different ways.

What happened to change a one-time measurement of variation in beak size to a long-term study of evolutionary change in beak size?

PG: After our first trip, a colleague advised us that if we returned later in the year during the dry season, when seeds are in short supply, the variations in beak size would be more pronounced. And that is indeed what we found when the whole family made that second trip in November. We could find almost all the birds we marked when I visited Daphne earlier in the year. So we thought that if we wanted to follow the fates of individual birds with different beak sizes, this is the place. So we decided to make this a three-year study.

You decided on three years so that you could make measurements on the offspring of the birds you had banded earlier, and on their offpring?

PG: Yes. Peter Boag, a Ph.D student, joined the study very early. He observed that the size of a ground finch's beak was correlated with the size and hardness of seeds it ate. And by measuring the beaks of parents and their offspring, Peter Boag showed that the variation in beak size was heritable. We wondered if we would be so lucky as to see natural selection acting on this variation. We were lucky. It happened in 1977. It was an exceptionally dry year—a drought. There were far fewer seeds for the ground finches to eat. After consuming the smaller seeds, the finches had to eat larger, harder seeds. By the end of 1977, the population of Geospiza fortis on Daphne had dropped by over 80%. And the average beak size of the survivors was larger than our earlier measurements of the population. And then the next question was, well, given the fact that beak size variation is heritable, shouldn't we observe an evolutionary response in the following generation? So we had to follow the fate of the survivors through the following breeding season and observe whether their offspring had larger beaks than the preceding average, as expected. And indeed, we found that, too. The average beak size had increased in the population. And that led to the next question: Was the beak measurement going to stay this high, or increase, or go back to the preceding state? For that, we needed long term data. And, as I say, one thing led to another. And the questions started to accumulate faster than the answers.

Including your questions about hybridization?

PG: Somewhere along the line, it became obvious to us that Geospiza fortis on Daphne was occasionally hybridizing with the cactus finch, Geospiza scandens. We found that the hybrids were surviving, and they were fertile. So we wondered if there were circumstances where they would not survive so well.

RG: One big question about the hybrids is why the two species interbreed in the first place. It can happen when a male learns to sing the song of a different species. The birds learn their songs during a short sensitive period when they are in the nest and when they're being fed by their parents. So most sons sing their father's song—at least they do about 80% of the time. But if the father is not around or does not sing very much, their sons simply imprint on their neighbors' song, even if it's a different finch species. Or, perhaps the father dies, and his sons imprint on the song they hear. Or, a rare situation—but it does happen—is when a finch takes over the nest of a different species and kicks out all the eggs, except for one, and then lays its own eggs. So you have a fortis growing up in a scandens brood, learning to sing a scandens song. And when these fortis males are mature, they occasionally breed with a scandens because they sing its song.

So, what keeps the two finch species from amalgamating into one?

RG: The hybrids have bill sizes intermediate between the medium ground finch and the cactus finch. The hybrids can only survive during wet years, when there are a lot of small, soft seeds. During dry years, the hybrids can't crack the larger, harder seeds that are on the ground, and they can't compete with the cactus finches for cactus seeds. Here's how I think of it: In these very new species that are coming together in secondary contact, there is this occasional hybridization through what is a breakdown of a learned cultural trait, the song. And so you get this balance between an input of genes and then selection, during drought years, keeping the populations on divergent trajectories in spite of the episodes of hybridization.

PG: A large part of the answer to the question of why the medium ground finches on Daphne are so variable is hybridization. The variation in characters such as beak size is broadened by the input of genes from different species. And then the other part of the story is that the ecological environment favors particularly large beaks or particularly small beaks in the population at different times. Selection can act upon that increased variation and produce faster evolutionary responses as the environment changes. Also—this is conjectural, but certainly possible— perhaps hybrids occasionally disperse off one island to another island that has neither the hybrids nor the parent species. The hybrids could start a new population with a range of genetic variation different from the parent species on the island they just left.

Do you think this helps explain the adaptive radiation of the Galápagos finches?

PG: I see no reason why hybridization hasn't been important right from the beginning, from the first divergence of the ancestral finch stock that reached the islands. We don't have the early stages, but that's the big challenge of evolutionary biology—trying to infer from modern clues what happened in the past.

©2005 Pearson Education, Inc., publishing as Benjamin Cummings