When Rosemary and Peter Grant first set foot on Daphne Major, a tiny island in the Galápagos archipelago, in 1973, they had no idea it would become a second home. The husband and wife team, now emeritus biology professors at Princeton University, were looking for a pristine environment in which to study evolution. They hoped that the various species of finches on the island would provide the perfect means for uncovering the factors that drive the formation of new species.

The diminutive island wasn’t a particularly hospitable place for the Grants to spend their winters. At less than one-hundredth the size of Manhattan, Daphne resembles the tip of a volcano rising from the sea. Visitors must leap off the boat onto the edge of a steep ring of land that surrounds a central crater. The island’s vegetation is sparse. Herbs, cactus bushes and low trees provide food for finches — small, medium and large ground finches, as well as cactus finches — and other birds. The Grants brought with them all the food and water they would need and cooked meals in a shallow cave sheltered by a tarp from the baking sun. They camped on Daphne’s one tiny flat spot, barely larger than a picnic table.

Though lacking in creature comforts, Daphne proved to be a fruitful choice. The Galápagos’ extreme climate — swinging between periods of severe drought and bountiful rain — furnished ample natural selection. Rainfall varied from a meter of rain in 1983 to none in 1985. A severe drought in 1977 killed off many of Daphne’s finches, setting the stage for the Grants’ first major discovery. During the dry spell, large seeds became more plentiful than small ones. Birds with bigger beaks were more successful at cracking the large seeds. As a result, large finches and their offspring triumphed during the drought, triggering a lasting increase in the birds’ average size. The Grants had observed evolution in action.

That striking finding launched a prolific career for the pair. They visited Daphne for several months each year from 1973 to 2012, sometimes bringing their daughters. Over the course of their four-decade tenure, the couple tagged roughly 20,000 birds spanning at least eight generations. (The longest-lived bird on the Grants’ watch survived a whopping 17 years.) They tracked almost every mating and its offspring, creating large, multigenerational pedigrees for different finch species. They took blood samples and recorded the finches’ songs, which allowed them to track genetics and other factors long after the birds themselves died. They have confirmed some of Darwin’s most basic predictions and have earned a variety of prestigious science awards, including the Kyoto Prize in 2009.

Now nearly 80, the couple have slowed their visits to the Galápagos. These days, they are most excited about applying genomic tools to the data they collected. They are collaborating with other scientists to find the genetic variants that drove the changes in beak size and shape that they tracked over the past 40 years. Quanta Magazine spoke with the Grants about their time on Daphne; an edited and condensed version of the conversation follows.

QUANTA MAGAZINE: Why did you decide to go to the Galápagos? What drew you to study finches specifically?

ROSEMARY GRANT: I had more of a genetics background and Peter more of an ecological background. But we were both interested in the same process — how and why species form. We both wanted to choose a population that was variable in a natural environment.

The Galápagos had several things that were very important. The islands are young, and there are lots of populations of finches that occur together and separately on the different islands. The islands were in close to pristine condition, having never been inhabited by humans. We knew that any changes would be natural changes and not the result of human interference.

The climate is extremely dynamic. The archipelago lies astride the equator and is subject to the El Niño–Southern Oscillation phenomenon. There are years with a terrific amount of rainfall, which is very good for finches. But it can also get years of drought, when many birds die. We now know that up to 80 to 90 percent of birds on the small islands die in times of drought. Those extremes would give us the opportunity to measure the climate variations that occurred and the evolutionary responses to those changes.

PETER GRANT: We had three main questions in mind. First, how are new species formed? That’s the Darwinian question of the origin of species. Second, do species compete for food? If they do, what effect does that have on the structure of animal communities? That was a hot topic in the early 1980s. There was very little experimental evidence at the time, so there was plenty of scope for taking a position one way or another. Third, why do some populations exhibit large variation in morphological traits like body size and beak size?

What was it like stepping on the island for the first time?

PG: It’s difficult to convey the thrill of arriving in an exotic location you have thought so much about for a long time, scrambling up the cliff, excited that you have finally arrived, and seeing the boat leave and knowing that you are on an uninhabited island. That first landing is unforgettable.

Your first major discovery came after a severe drought in 1977. What happened?

PG: A student of mine was on the island working, regretting the fact that birds were dying. We got a letter from him about the dismal field season. But we thought this could be of crucial importance for understanding why birds are the shape and size they are. That was the first glimmer.

We went back to the island at the end of 1977 with our two daughters. As a family we scoured the island for dead and live birds. We discovered it was largely the small-beaked birds that had died. The medium ground finches with large beaks had a survival advantage over those with small beaks because they were able to take advantage of large seeds. When we looked at the offspring of survivors, we found that they were large like their parents. There had been an evolutionary change in beak size. This was a clear demonstration of evolution by natural selection.

Was this the first time anyone had observed evolution in real time?

PG: In a natural environment, yes. Scientists had previously demonstrated evolution of insecticide resistance and resistance to bacterial infections. But for continuously varying ecologically important traits, this was the first demonstration of evolution in a natural environment.

RG: That’s why it was so important for us to use a pristine environment. We knew it hadn’t been influenced by humans at all.

In 1981, you spotted an unusual-looking finch, which you dubbed Big Bird. What was so special about him?

RG: When Big Bird arrived on Daphne, we caught him and took a blood sample. It showed that he was with high probability an introgressed bird — a hybrid medium ground finch and cactus finch that had backcrossed [bred with] one of the parent species.

Big Bird bred with two medium ground finches, and those offspring started a lineage. Daphne had another serious drought from 2003 to 2005, and all the birds from Big Bird’s lineage died except for a brother and sister. When the rains came again, the brother and sister mated with each other and produced 26 offspring. All but nine survived to breed — a son bred with his mother, a daughter with her father, and the rest of the offspring with each other — producing a terrifically inbred lineage.

Why is that so significant? Was Big Bird the beginning of a new finch species?

RG: In all respects, this lineage was behaving like a different species. The lineage was much bigger than its nearest relative, the medium ground finch. These birds all sang a different song that had never been heard on Daphne, the song of the original colonist. They bred in one part of the island and held territories that were continuous with each other’s but overlapped those of other species. The other species completely ignored the Big Birds, and the Big Birds ignored them.

The original colonist had a genetic marker that we were able to trace all the way down through the generations. The brother and sister that survived the drought had two copies of that marker. From then on, all the birds in the lineage carried that marker.

Were you surprised by the Big Bird lineage?

RG: We had often argued that if birds that had genes from other species flew to another island with different ecological conditions, then natural selection would shape them into a new species. We never thought we’d see it happen, but we did.

What does the Big Bird story tell us about interbreeding? That it can possibly stimulate the development of new species?

PG: Several years ago, people thought that when populations interbred, exchanging genes would not lead to anything other than a fusing of two populations. It’s almost a destructive force, undoing the generation of a new species. But in the Big Bird story, interbreeding can actually generate something new. We see the same thing in the butterfly literature. Some populations of butterflies are the product of interbreeding of two others.

RG: By putting two genomes together, you can get a new genetic combination. Then the process of natural selection can act on the new population and take it on a new trajectory. Some will fail. Some will produce offspring that are extremely variable. Some of those individuals will be in a new or a changed environment. This is where they could have some advantage.

We know now that certain genes came from Neanderthals to modern humans, which gave us some immune advantages. We saw the same sort of thing in finches.

During your tenure on Daphne, you witnessed a new group of finches colonizing the island. Why was that so interesting?

PG: With the heavy rains of the 1982 El Niño, five large ground finches from another island decided to stay and breed on Daphne. They built up numbers very slowly and had little influence on the other finch species. But when the drought started in 2003, their numbers were high enough to have a material influence on the food supply.

The large ground finch competed with the resident medium ground finch for the diminishing supply of large and hard seeds. As a result, average beak size in medium ground finches decreased, and the difference between the two species increased. Darwin called this the principle of character divergence — traits like beak size diverge as a result of natural selection. It occurs when two species, previously separated, come together and compete for food. It allows species to coexist, as opposed to one species becoming extinct as a result of competition. Ours was the first conclusive and comprehensive demonstration of the process, the cause and the role of natural selection.

What are the biggest changes you’ve seen over the past 40 years in our understanding of evolution?

PG: From our studies and others, I think the general concept of the rate of evolution has changed. It’s a much more rapid process than it was thought to be. When we started, most people would have been skeptical that you could get evolutionary change in one generation — producing a bird with a more pointed beak, for example. The idea that the effects of natural selection are so minute that you can’t measure them has been thrown out.

How has our understanding of speciation — the development of new species — changed?

RG: The [traditional] model of speciation was almost a three-step process. First, there was colonization of a new area. The new area has different ecological conditions, so the species changes as a result of natural selection. Then it goes to another area. Colonization, change and dispersal occur until the two species come in contact again. Then you can get things like character displacement.

Our work has shown that this model of speciation does hold. But in addition, we have shown there are other routes to speciation, such as gene flow from one species to another. We see this in the Big Bird lineage but also in cichlid fishes and butterflies. There are multiple routes to speciation.

What impact has genomics had on the field?

PG: Our understanding of evolution in general and speciation in particular is undergoing a large transformation as a result of genomics. That’s a major difference from when we started. Now we have a genetic underpinning of the processes of evolution that we previously had to infer from morphology [the physical form of organisms].

RG: The really big breakthrough was whole-genome sequencing. We are collaborating with Swedish geneticists, who are sequencing finch genomes. That’s become very exciting.

For the big selection event of 2003 to 2005, we have blood taken from birds before the drought and from the survivors. We’ve shown that one gene, HMGA2, was extremely important. The gene comes in two forms. One is associated with large birds and one with small birds. We could show that the large-bird version of HMGA2 was at a selective disadvantage, and the small-bird version was at an advantage.

PG: There was a major shift in the frequency of these two variants — the variant associated with small size increased. Until this discovery we had plenty of reasons for thinking that evolution had taken place but no genetic evidence of a change in gene frequencies. This was the clincher. That’s why it was so exciting to us.

RG: Sequencing genomes can reveal so much more if you have the actual knowledge of the population in the wild. Putting that together has become enormously rewarding. We’re lucky that we can do this. We always kept our blood samples and song recordings and were able to go back. I hope that in the future, there will be greater appreciation for putting together genomic work with fieldwork.

What new questions are you most excited to explore?

PG: The Big Bird story. We want a genetic underpinning for Big Bird like we have for the selection in 2005. We’re waiting for the data.

You didn’t originally plan to keep going back to Daphne for as long as you did.

PG: No one who does long-term studies expects at the beginning to go back for a long time. We were lucky to have rewards at the beginning.

Do you plan to go back to Daphne?

RG: We stopped intensive work after 40 years, but we do plan to go back.

PG: The oldest person died at 122 years old. That means we have 40 more years.