Profile of B. Rosemary Grant

Charles Darwin viewed evolution as a process so slow that it often evokes images of changes accumulated over near-geologic timescales. Rosemary Grant's research has revealed otherwise. Through painstaking documentation of the evolutionary process first described by Darwin, Grant has shown that evolution can be observed within a lifetime. Grant, an evolutionary biologist at Princeton University and a member of the National Academy of Sciences, has traced the evolution of Darwin's finches—14 species of songbirds of the genus Geospiza that inhabit the storied Galapagos islands of South America. For nearly four decades, Grant and her husband, Peter Grant, have followed the finches’ fate on the famous archipelago, furnishing proof for a biological process no less fundamental than natural selection. Their work lent the narrative spine to The Beak of the Finch, a Pulitzer Prize-winning account of their adventures in the archipelago written by the journalist Jonathan Weiner.

B. Rosemary Grant and spouse Peter Grant. Image courtesy of K. Thalia Grant.

The Grants in the Galapagos. Image courtesy of Martin Wikelski.

Arnside, an estuarine village nestled at the foot of a hill along the river Kent in northwest England, was Grant's hometown. There, amid a riot of rare butterflies thronging the hillsides, Grant learned to appreciate nature's diversity. Accompanied by her mother, who fed her natural curiosity with field trips to nearby limestone cliffs, Grant discovered the joy of scientific exploration at a young age, collecting plant fossils and comparing them with living lookalikes. “I became curious about differences among the plants we were collecting. I wondered if they were different because of their parents or because of where they grew,” says Grant, whose curiosity may have foreshadowed what came to be her life's quest—the study of natural variation. Thanks to her physician father's attempt to channel her scientific energy, the 12-year-old Grant gamely ventured to read Darwin's On the Origin of Species. “I was quite uncritical when I first read it, and then, I read it more thoroughly as a teenager,” Grant says. Those teenage years at boarding school were largely filled with intellectual restlessness, which Grant sought to replace with scientific pursuits. “I struck up an acquaintance with Charlotte Auerbach, a geneticist at Edinburgh University, thanks to my aunt who worked there,” Grant says. Auerbach, in turn, introduced Grant to Edinburgh geneticist Douglas Falconer, and the two proved pivotal in shaping Grant's decision to attend university at a time when academic institutions were an undeniably male bastion. Despite her school headmistress, who tried to convince her that university education was for men, and an untimely bout of mumps, which kept her from taking the university entrance examinations, Grant enrolled in correspondence courses that prepared her for formal education at Edinburgh University in Scotland, where she graduated with a degree in zoology in 1960.

Origins

At Edinburgh, Grant spent a year studying genetics with Conrad Waddington, whose work helped lay the foundation for the field of developmental genetics. During that year, Grant devised an immunological method to detect differences in cell surface proteins between pathogenic and nonpathogenic strains of soil amoebas. “It was a neat little thing with a nice result,” says Grant. But her real interest was in individual differences that were more meaningful, that led to the splitting of one species into two and that paved the way for evolution. So Grant devised a dissertation project to study differences among populations of fish forever separated from one another in landlocked fjords in Iceland's glacial valleys. But she deferred the project until later, choosing instead to work as a biology lecturer at the University of British Columbia in Vancouver. “I was excited by the prospect of moving to Canada to teach, so when the opportunity came along, I decided that the PhD could wait for a year,” Grant says. The decision marked a plot-propelling twist in both Grant's scientific career and personal life: At the University of British Columbia, she met evolutionary biologist Peter Grant, whom she married a year later.

It was a marriage of minds. Both Grants shared a passion for probing the basis of natural diversity. A doctoral student at British Columbia, Peter focused on the ties between evolution and ecology, a pursuit that soon led the two researchers to the Tres Marias Islands in the Pacific Ocean off the coast of Mexico, where they studied how competition for food among terrestrial birds serves as an engine of evolutionary change in beak size. The work proved a precursor for the Grants’ now-famous findings on Darwin's finches in the Galapagos Islands. Grant accompanied her husband first to Yale University and then to McGill University as he took up postdoctoral and tenure-track positions. While at McGill, the Grants entertained the idea of an expedition to Galapagos, a birthplace of what is arguably the most revolutionary scientific theory of all time, thanks to insights from Darwin's Finches, a book by British evolutionary biologist David Lack.

Adventures on Daphne

In 1973, the Grants made their first trip to Daphne Major, a pristine volcanic island in the Galapagos archipelago that masked boobies, magnificent frigate birds, short-eared owls, lava lizards, and Darwin's finches call home. The island is virtually untrammeled by people, providing an ideal natural laboratory for the study of evolution. Every year since then, the Grants have returned to the Galapagos to study the finches in their habitat for months at a time. The Grants’ goal was to determine how each of the 14 species of finches evolved from the ancestral one, which likely flew in from the South American mainland. They also hoped to find how competition for food and space triggered by environmental changes might affect morphology, behavior, and ultimately, the evolution of species. Nearly four decades after the Grants’ first trip to the archipelago, their findings have helped paint a dramatically vivid tableau of evolutionary forces in action. But the picture was assembled piecemeal.

In 1980, after the pair moved to the University of Michigan at Ann Arbor, Grant decided that the time had come to pursue her long-deferred PhD. Grant pursued her doctoral work under the guidance of Uppsala University evolutionary biologist Staffan Ulfstrand. Her project focused on the patterns and causes of morphological variation in the large cactus finch, G. conirostris, which resides on a low-lying island called Genovesa in the northeastern tip of the archipelago. “I chose Genovesa, because the winds and ocean currents across the archipelago largely bypass this small island, unlike Daphne, which lies more or less in the center of the archipelago. I wanted to see how the finches evolved in the more isolated periphery,” Grant says. Grant's studies on the large cactus finch revealed three insights into the finches’ evolution. First, natural selection operated through competition for food among species as the environment changed over time. Next, Grant's research revealed that related species occupying overlapping niches sometimes interbred to beget hybrids, which then bred with one of the two parental species to jumble the genes among the species of finches. Such backcrossing, known as introgressive hybridization, led to novel admixtures of parental genes that provided grist for natural selection and potentially resulted in speciation. Finally, her findings showed that the morphologically distinct beaks of birds that sang different songs could be genetically inherited, although the songs themselves could not; fathers taught distinct songs to their nestlings. The revelation helped explain not only the high degree of variation seen among the large cactus finches but also how such variation was maintained. “Many of our later findings on Daphne had their beginnings on Genovesa,” Grant says.

Those findings on Daphne Major began with laborious fieldwork carried out with sacramental correctness, as the Grants caught, banded, tracked, and bled the birds year after year. Notable among their many discoveries is the Grants’ uncovering of a phenomenon dubbed character displacement, a process of evolutionary divergence in the face of competition. The discovery was sparked by the arrival on Daphne of the large ground finch G. magnirostris in 1982. Until then, Daphne was dominated by the medium ground finch G. fortis, which had beak sizes adapted for eating either large or small seeds of the plant Tribulus. Larger fortis birds with proportionately large beaks ate large seeds, whereas the smaller birds went after the small seeds. The arrival of the large ground finch made short work of the large seeds and consequently upset the seed cart; even the larger of the medium ground finches were no match for magnirostris when it came to feeding on large seeds. In 2004, as drought parched the island and the large seeds dwindled, most of the finches died. In particular, medium ground finches with large beaks died, whereas those finches with small beaks survived to reproduce. The Grants published their findings in Science, providing evidence for competition for food as an agent of natural selection (1).

Power of Song

Beak size, however, was not the only trait that could be altered by competition. Birdsong, the Grants found, could follow a similar fate. In the 1950s, evolutionary biologist Robert Bowman found that male Darwin's finches are taught to sing by their fathers between 10 and 40 days after hatching, whereas females do not sing. Finches use a combination of visual and auditory cues to lure potential mates, suggesting that beak size and song together serve as a reproductive barrier between species. “So song, which is learned, and morphology, which is under genetic control, come together to keep species apart,” Grant explains. The Grants were puzzled by the song differences they observed among closely related species of finches inhabiting the same island. Even more baffling was the fact that the same species of finch sang remarkably different songs across different islands in the archipelago. To determine how the differences in song arose, the Grants carefully chronicled the finches’ songs for many years. After the arrival of magnirostris on Daphne, they found that nestlings of smaller species, such as fortis and scandens (the cactus ground finch), learned to sing their fathers’ songs—not verbatim but as slightly improvised versions. The improvisations, it turned out, helped distinguish the songs of fortis and scandens from those songs of magnirostris, resulting in less interference from the larger species. The song differences seemingly telegraphed the smaller’ species intent to coexist without conflict. Thus, avoidance of conflict led to changes in birdsong, illustrating the power of character displacement in influencing natural variation. The Grants published their meticulous analysis of songs recorded over 30 years in their 2010 PNAS inaugural report (2).

The Grants are now discovering that song differences can lead to the evolution of new lineages (3). When a finch of one species takes over the nest of another species, it usually evicts all of the eggs, but occasionally, an egg might be left behind. Then, as the eggs hatch, the nestling grows with its foster siblings, learning its foster father's song. This kind of song misimprinting can also happen when the nests of distinct species are found adjacent to each other. In such rare cases, nestlings of both species learn the loudest and most frequently sung song. As adults, birds with misimprinted songs naturally attempt to mate with birds of their song type. The Grants observed such behavior triggered by song misimprinting among fortis and magnirostris finches. However, when male fortis finches sang songs learned from magnirostris, the magnirostris males in neighboring territories often clobbered the serenading suitors, sometimes to death, largely because of obvious differences in appearance between the two species. That said, when the phenomenon involved species that resembled each other, such as fortis and scandens, an occasional hybrid resulted. In 1981, a hybrid individual that likely originated from nearby Santa Cruz Island arrived on Daphne and bred with a resident fortis. A couple of generations later, the 2004 drought reduced the descendants to just two individuals: a brother and a sister. Since 2005, the siblings have bred so prolifically that there are now more than two dozen progeny alive on the island. More interestingly, they have not bred with resident fortis. “Individuals of this hybrid lineage are larger than both fortis and scandens, have a unique song as well as a genetic marker, and hold territories on Daphne that overlap with those of fortis and scandens. They are behaving like a new species. We appear to have witnessed a new species in the making, an extraordinary and completely unexpected piece of luck,” Grant says.

In 1985, the Grants moved to Princeton University, which they have since called their home away from the Galapagos. At Princeton, they pore through mountains of meticulously assembled data to wrest meaning out of the numbers that they measure on Daphne. They also mine the blood samples taken from the finches for genetic information, which might yield clues to the evolution of species. By following the trace of repeating stretches of DNA called microsatellites, which serve as genetic markers of kinship in the finches’ blood cells, the Grants were able to follow the finches’ patterns of inheritance across several generations. “The microsatellites were crucial in determining paternity and in establishing pair-bonds,” Grant says. But the real goal in drawing blood from the finches was to uncover clues to the genetic basis of changes in traits such as beak size, width, and length. Working with the Grants, Harvard University molecular geneticists Cliff Tabin and Arkhat Abzhanov found that a signaling protein dubbed bone morphogenetic protein-4, which plays a role in bone development in people, controlled beak depth and width in Darwin's finches (4). “We had already found, using heritability studies, that beak depth and width were highly correlated traits. So, the molecular discovery of BMP-4 fitted perfectly into the picture,” Grant says. “It was as if we had come to the same conclusion—by looking inside and outside the finch,” she adds. Soon thereafter, the Harvard team found that another human signaling protein, called calmodulin, which plays crucial roles in metabolism, neurotransmission, and immunity, controls the length of the finches’ beaks (5).

Precious Variation

Genetics aside, the Grants’ work in the Galapagos has profound implications for conservation biology. Because subtle environmental changes, not to mention blistering droughts and El Nino-triggered floods that batter the archipelago, nudge the finches in different directions along their evolutionary trajectory, human-induced habitat and climate changes in the Galapagos are likely to influence the nature and pace of natural selection. The Grants and their partners have documented the first signs of a human imprint on evolution in the Galapagos (6). Likewise, environmental shifts elsewhere likely influence the evolution of species by changing the relative abundance of resources needed for survival and reproduction. “Conservation biologists often think in terms of maintaining the status quo ante. Our work has shown the importance of variation to evolution,” Grant says. Variation, which allows organisms to respond to environmental fluctuations, is the path to species’ patent destiny, providing raw material for natural selection. To deplete natural variation, then, is to alter that destiny—sometimes irreversibly. Grant first raised awareness to the importance of maintaining natural variation in her 1989 book Evolutionary Dynamics of a Natural Population (7). To this day, she ends her lectures with the words “neither species nor environments are static entities, but dynamic and constantly changing. To conserve species and their environments, we must keep them both capable of further change.”

For all their revealing discoveries on Daphne, the Grants are a reclusive pair, illuminating natural selection while toiling for months at a stretch in the relative obscurity of a volcanic island in the Pacific Ocean. Yet their contributions to evolutionary biology have garnered them many laurels, notable among which is the 1998 E. O. Wilson Naturalist Award. But it was the 2002 Darwin Medal from the Royal Society of London that fittingly crowned their accomplishments in evolutionary biology. “I was astonished to get the award—not so much for Peter but for myself. I wasn't even a fellow of the society then,” Grant says. “We are treading in Darwin's footsteps, so the award was wonderfully gratifying.”

Full gratification for the Grants is yet to come. Their best work, Grant says, is hardly behind them. She looks forward to documenting the fate of the new lineage of finches on Daphne. “We're very excited about this lineage. Will it survive intact for many generations with its genetic variation augmented through introgression, or will it die out thanks to inbreeding?,” she wonders. Either way, the Grants plan to return to Daphne to find out.