Final answer:
The beaks of finches changed rapidly in 1976-1977 due to a drought that favored larger beaks, but reverted when the environmental conditions changed, demonstrating evolution by natural selection.
Step-by-step explanation:
The rapid change in the beaks of finches observed in 1976-1977 followed by a reversion can be primarily attributed to environmental conditions that temporarily favored certain physical traits. This scenario is an example of evolution by natural selection, where the beak size of Darwin's finches changed with the availability of different-sized seeds. During the drought in the Galápagos, fewer seeds were available, giving birds with larger beaks a survival advantage as they could consume a wider range of seed sizes. However, after conditions improved and smaller seeds became more plentiful due to an El Niño event, birds with smaller beaks had a reproductive advantage, leading to a decrease in average beak size.