What was the effect of natural selection on beak size in the Grants' study?

The Grants studied finches on the Galapagos Islands for many years, focusing on the effect of types of food on the size and shape of the finches' beaks. The finches had speciated based on the foods they found available, with some species feeding on large seeds, some on small seeds, and some on other foods. In years with drought in which some foods were scarce, they found that different beak sizes were advantageous than in a wetter year, causing a change in the average beak size seen in the populations. This is important because it is an example of natural selection occurring on a scale that we can easily observe.