Beaks of Darwin' Finches

In 1835, before he had developed his theory of evolution, Charles Darwin collected specimens of 13 previously unknown species of finches from the isolated Galapagos Islands. The Galapagos finches closely resembled a species of finches living on the mainland of South America, but each of the Galapagos species of finches had a differently shaped beak unique to it. His observations led Darwin to speculate that "from an original paucity of birds in this archipelago , one species has been taken and modified for different ends." This is the essence of Darwin's theory of evolution by natural selection: birds with a particular beak shape survived and reproduced because their beak made them well adapted for using a particular food source. In this way one original species that came to Galapagos from the mainland ultimately evolved into 13 new species.

The correspondence between the beaks of the 13 finch species and their food source immediately suggested to Darwin that evolution had shaped them. If his suggestion that the beak of an ancestral finch had been shaped by evolution is correct,then it ought to be possible to see the different species of finches acting out their evolutionary roles, each using their beaks to acquire their particular food specialty. The four species that crush seeds within their beaks, for example, should feed on different seeds, those with stouter beaks specializing in harder-to- crush seeds.

Many biologists visited the Galapagos after Darwin, but it was 100 years before any tried this key test of his hypothesis when the great naturalist David Lack finally set out to do this in 1938. Observing the birds closely for a full five months, his observations seemed to contradict Darwin's proposal. Lack often observed many different species of finch feeding together on the same seeds. We now know that it was Lack's misfortune to study the birds during a wet year, when food was plentiful. The finch's beak is of little importance in such flush times; small seeds are so abundant that birds of all species are able to get enough to eat.

The key to successfully testing Darwin's proposal that the beaks of Galapagos finches are adaptations to different food sources proved to be patience. Starting in 1973, Peter and Rosemary Grant of Princeton University and generations of their students have studied the medium ground finch Geospiza fortis on a tiny island in the center of the Galapagos called Daphne Major. These finches feed preferentially on small, tender seeds produced in abundance by plants in wet years. The birds resort to larger, drier seeds, which are harder to crush, only when small seeds become depleted during long periods of dry weather and plants produce few seeds.

The Grants quantified beak shape among the medium ground finches of Daphne Major by carefully measuring beak depth(width of beak, from top to bottom, at its base) on individual birds. Measuring many birds every year, they were able to assemble a detailed portrait of evolution in action. The Grants found that beak depth changed from one year to the next in a predictable fashion. During droughts, plants produced few seeds and all available small seeds were quickly eaten, leaving large seeds as the remaining source of food. As a result, birds with large beaks survived better, because they were better able to break open these large seeds. Consequently, the average beak depth of birds in the population increased the next year, only to decrease again when wet seasons returned.

]Could these changes in beak dimension reflect the action of natural selection? An alternative possibility might be that the changes in beak depth do not reflect changes in gene frequencies, but rather are simply a response to diet - perhaps during lean times the birds become malnourished and then grow stouter beaks, for example. If this were the case, it would not be genetics but environment alone that influences beak size. To rule out this possibility, the Grants measured the relation of parent bill size to offspring beak size, examining many broods over several years. The depth of the beak was passed down faithfully from one generation to the next, regardless of environmental conditions, suggesting that the differences in beak size indeed reflected genetic differences.