The finches have had their ups and downs. In 1977 a drought left them with nothing to eat but big, tough seeds left from the year before: the next generation evolved bigger, blunter beaks. In 1983 heavy rains produced a surfeit of seeds; the birds responded by breeding madly. A 1985 drought left only small seeds on the ground, which favored smaller, quicker birds with thin beaks. The Grants’ conclusion: foreseeing how a species is going to evolve is as difficult as predicting the weather. The work also suggests that small changes in environment–like the earth’s rising temperatures–can have a big and unforeseeable impact on the course of evolution.
