Two resident populations of finches of Daphne Major have been the subject of careful scrutiny by Grant, his wife, Rosemary, and numerous graduate students and assistants. The fieldwork has involved documenting the life history of every finch on the island in meticulous detail. The parents are known, and measurements are made of body dimensions, particularly body size and beak depth. The goal has been to study natural selection in action. If changes were to occur as a result of environmental pressures, the researchers had the opportunity to detect them and to investigate cause and effect relationships.
Selection pressures come from drought years and rainy years.
When there is plenty of moisture, grasses grow profusely, herbs are abundant and shrubs produce copious foliage and seeds. In a drought year, everything is in short supply. In particular, the ratio of small seeds to large seeds tends to drop. This last point is relevant to the ground finches, whose beaks are adapted for eating seeds of a certain size. In 1977 and 1982, Daphne Major had a very poor rainy season.