Natural selection: bird seeds of change.

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. Because large birds with bigger bills are the only ones able to crack open the hard seeds, a greater number of large birds survived through the dry years of 1977, 1980 and 1982.

The 1983 El Nino allowed Grant andGibbs to observe the reverse of this process. Rains from this climatic upheaval pounded the tropics of the eastern Pacific and dropped 1,359 millimeters of rain on the Galapagos from December 1982 through July 1983. Normal yearly rainfall ranges between 50 and 100 mm.

As is natural with the onset of theannual rainy season, the birds began to build nests and mate during the first weeks of the rain. However, during an El Nino, "they breed like crazy. . . . and keep on going until the rain stops,' says Grant. In 1983, the birds produced eight broods, as opposed to a normal one or two. As well, record numbers of seeds grew on the island.

The researchers propose that smallerbirds were more adept at eating the small seeds that dominated the post-El Nino food supply, and this led to their increased survival rates in 1984 and 1985.

The observation of the relationshipbetween finch size and rainfall is unique, writes biologist Jon Seger in an accompanying editorial. The finch study is the first to demonstrate that the natural ecology has forced a population in two opposing directions at different times. Seger, from the University of Utah in Salt Lake City, also writes that "if such strong and constantly changing patterns of selection should turn out to be the rule for many species . . . then we are probably far from understanding the processes that maintain heritable quantitative variation in natural populations.'

Photo: Large ground finch with big bill.