By Monastersky, Richard
Science News , Vol. 131
Natural selection: Bird seeds of change
A legacy of the 1983 El Nino is grantingevolutionary biologists the rare opportunity to test the central tenet of their field: the theory of natural selection. As part of a long-term study that began in 1973, these scientists are monitoring how El Ninos and other climate shifts forced changes within a population of finches on the Galapagos islands.
Two researchers report in the June 11NATURE that the eight months of extraordinarily heavy rainfall during that El Nino led to the differential survival of smaller birds for the two subsequent years. Earlier, parts of this study had demonstrated that periods of drought promoted the survival of larger birds with big bills. These swings in the population not only prove that environmental forces can shape the population of a species, but also show that the direction of evolution can change or reverse, often quite rapidly, says Peter R. Grant of Princeton (N.J.) University.
Grant and H. Lisle Gibbs of the Universityof Michigan in Ann Arbor have been observing the species Geospiza fortis on the island of Daphne Major, which measures roughly 3/4 mile by 1/2 mile in area. Daphne Major and the other Galapagos islands are particularly well-suited to studies of natural selection because they provide isolated populations of birds that live in a variable climate, says Grant. The birds, commonly known as Darwin's finches, also possess physical traits that are highly inheritable, such as weight and bill size.
For most of the year, the finches subsiston seeds of varying size and hardness. During lean years of little rainfall, the birds deplete the supply of small, soft seeds that require more rainfall, and then must turn to the harder, larger seeds that remain. …