How Darwin's finches got their beaks

Darwin's finches are the emblems of evolution. The birds he saw on the Galapagos Islands during his famous voyage around the world in 1831-1836 changed his thinking about the origin of new species and, eventually, that of the world's biologists.

Darwin wondered about the changes in shape of bird beaks from island to island. So-called cactus finches boast longer, more pointed beaks than their relatives the ground finches. Beaks of warbler finches are thinner and more pointed than both. These adaptations make them more fit to survive on available food.

Researchers at Harvard Medical School have taken the story one step further. Using modern genetic analyses, they found a molecule that regulates genes involved in shaping the beaks of Darwin finches. "Calmodulin is a protein that binds and activates certain enzymes, which triggers a signal that eventually turns specific genes on or off," explains Arkhat Abzhanov, an evolutionary biologist at Harvard. These signals alter the behavior of cells responsible for beak sculpturing.

Members of the research team received permission to collect finch eggs from the Galapagos National Park, a group of rocky islands in the Pacific Ocean, about 600 miles west of Ecuador. Female finches lay clutches of four to five eggs, one per day. To avoid disruption and abandonment of the nests, the researchers took only the third eggs laid.

In the Department of Genetics at Harvard Medical School, 26 bird embryos were examined, using gene chips that reveal which genes are most active in the heads of the developing finches. This activity was then matched with the size and shapes of adult beaks.

The investigation soon focused on calmodulin as the switch that can turn on genes involved in increasing beak length. This protein had never before been implicated in the development of the skulls and faces of any birds.

"We found that calmodulin was indeed expressed at detectably higher levels in cactus finches compared to ground finches, and thus associated with their longer beaks," says Clifford Tabin, professor of genetics. "This higher level is both biologically relevant and functionally important for shaping of elongated beaks, which are used in a specialized manner to probe cactus flowers and fruit for pollen, nectar, and seeds." The same surge of calmodulin was not found in more blunt-beaked ground finches.