Twin Beaks

Darwin's theory of evolution is widely accepted as valid, but it is usually difficult to see the process in action. A 30-year study of finches on a Galapagos island has changed all that, says Steve Connor

For the past 30 years, on a remote and uninhabited speck of land sticking out of the vast Pacific Ocean, a unique experiment in Darwinian evolution has taken place. Since 1973, scientists have monitored two of the commonest species of finch that eke out a precarious living on Daphne Major, a rocky outcrop which forms part of the Galapagos archipelago. The findings, published in the journal Science, are a remarkable testament to the highly unpredictable and wildly chaotic nature of evolutionary change.

The heroes of this story are Peter and Rosemary Grant, a husband-and-wife team who made the volcanic crater of Daphne Major their home in order to study its finches in detail. With their daughters and collaborators, the Grants photographed each member of the two species of finch, weighed every adult bird, ringed every newborn chick and measured the length and angle of every beak of thousands of birds from a dozen generations.

Such an exhaustive collation of data has produced perhaps the finest evidence yet assembled to explain exactly how evolution works in reality – and how it is subject to the vagaries of environmental change. It is the best field study to date documenting evolution as it happens – a difficult task given the usually long timescales involved in the generation of physical changes leading to the splitting of one species into two. "There are similar studies," says Peter Grant, a professor at Princeton University in New Jersey. "But in several respects ours is the most intensive and extensive of its kind."

Darwinian evolution is not just a theory, he says, especially in the light of what happened to these two species of finch over the past 30 years. "There is evidence from laboratory and field studies that demonstrate the reality of evolution. Yes, indeed, many scientists regard evolution as a fact. Evolutionary theories, on the other hand, attempt to explain the whys and wherefores of evolution," Grant says.

The two species in the study are the medium ground finch, Geospiza fortis, and the cactus finch, G. scandens. The ground finch has a wide angled, blunt beak which is well suited to cracking open the seeds of the perennial plants living on Daphne Major. Larger ground finches can even break open the shells of the harder and bigger seeds of the caltrop plant. The narrow-angled, finer beaks of the cactus finch, in contrast, are better suited for a softer diet of the fruits and pollen of the cactus plant. What makes body size, and beak size and shape, so important is that they are all highly heritable and, because they are largely determined by genes, they should provide a perfect marker for evolutionary change. Indeed, the Grants' "null expectation" at the outcome of the experiment was that the means (averages) for each of these traits should not change significantly during the course of the study.

"This expectation of no change is not supported by the data," they write in Science. "Mean body size and beak shape were markedly different at the end of the period (2001) than at the beginning (1973) in both species. Between these times, mean body and beak size for G. fortis decreased, then increased sharply and decreased again more slowly. Beak shape abruptly became more pointed in the mid-1980s and remained so for the next 15 years. G. scandens, a larger species, displayed more gradual and uniform trends towards smaller size and blunter beaks, thereby converging towards G. fortis in morphology," they say.

They found that the study unequivocally demonstrated the unpredictable nature of evolution, which can result in changes occurring over decades. "Mean body size and beak shape of both species at the end of the study could not have been predicted at the beginning. Moreover, sampling at only the beginning and at the end would have missed beak size changes in G. fortis in the middle," they say.

What the findings showed so clearly was that natural selection was far from a static force and that these two species of finch constantly have to cope with environmental change. The first crisis occurred in 1977 when Daphne Major was hit by a severe and prolonged drought, caused by a Pacific Ocean phenomenon called La Niña – a sort of climatic opposite to El Niño.

The drought wiped out some of the plants with small seeds, which killed off most of the ground finches, although some of the larger ones with bigger beaks survived by eating caltrop seeds. As a result, from one generation to the next, the average beak size of the ground finch increased by 4 per cent. But then, in 1983, the island was hit by a severe El Niño, with heavy rains triggering the growth of small-seed plants. Smaller ground finches thrived, resulting in a shrinking of average beak size by about 2.5 per cent within a few years.

But what about the cactus finches? The rains of 1983 were not good for cactus plants, which were overrun by vines. As a result, the cactus finch suffered badly from El Niño. But it was the smaller, socially subordinate females who suffered the most as males monopolised the remaining cactus plants.

What was odd was that the beak of the cactus finch during this period began to get blunter. After the heavy rains, male cactus finches outnumbered females by five to one because they took most of the food. In desperation, some of the males who failed to find cactus finch females, mated with ground finch females. Their hybrid offspring were healthy and fertile and, furthermore, mated only with other cactus finches because they had imprinted on the song of their cactus finch father.

Because of this one-way flow of genes from ground finch to cactus finch ("introgressive hybridisation" as it is called technically), the beak of the cactus finch grew progressively blunter after 1983.

Many biologists will be surprised to hear that two closely related, but fully separated species can interbreed to produce fertile hybrids. Yet the finch study shows how important hybridisation is to the continual evolution of two related species living side by side in the same environment. Indeed, the findings may suggest that one day the two species could become one again, although Grant thinks this is unlikely.

"Speciation is the process of one species splitting into two. Our study does not reveal how these particular two species originated. Instead it shows that well after two species have been formed they are capable of interbreeding," he says. "Eventually they will lose that capability, but our view is that it will take a very long time, possibly hundreds of years."

How species originate is at the heart of Darwinist discussion. It was Charles Darwin, who visited the Galapagos in 1835, who first made a study of these and the other finches of the archipelago. David Lack, the Oxford ornithologist, made the association famous more than a century later by popularising the christening of the Geospizinae as "Darwin's finches". This gave rise to the legend that on first seeing the wide diversity of the finches, Darwin's theory of evolution came to him in a blinding flash.

In fact, Darwin failed to categorise his finch collection properly and, back in England, he was not sure what to make of the wide diversity of finches and their beaks. At one stage he thought each finch came from a different island, but much later it dawned on him that the variation was because each finch was adapted to a particular ecological niche.

In 1845, Darwin wrote a prescient comment on the range of beak shapes and finch species that he knew to exist among the islands. "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends," he wrote in the 1845 edition of the Journal of Researches.

The accepted explanation for the many species of finches that inhabit the Galapagos is thata breeding pair, or several pairs of ancestors were blown from the mainland of South America. In a classic example of "adaptive radiation", the evolutionary process whereby a single species gives rise to several species each occupying a different ecological niche, the many species of finches seen today evolved. Studying just how this adaptive radiation came about, however, has been difficult, which is why the Grants' research has proved so useful. Asked to explain what his study has told us about evolution, Peter Grant says: "It occurs more often than you might think. Very small evolutionary changes may be taking place all around us but remain undetected until someone attempts to measure them," he says. "Global warming is probably having evolutionary as well as ecological effects on living things in our environment."

So if the beak of the finch has taught us anything, it is that evolution is going on here and now. We just need to look long enough to see it happening.