Final answer:
The study in the Galapagos Islands in 1973 showed how a drought affected the availability of food for finches. Finches with larger beaks were more likely to survive the drought due to their ability to eat a variety of seed sizes. This study demonstrates evolution by natural selection as the average beak size increased in the surviving population.
Step-by-step explanation:
The 1973 study in the Galapagos Islands documented a drought that caused a change in the availability of food for finches. During this drought, finches with larger beaks had an advantage because they could crack and eat seeds of all sizes, while finches with smaller beaks could only eat smaller seeds. As a result, many of the small-beaked finches died, and the average beak size of the surviving population increased. This study provides evidence for evolution by natural selection as the larger beak size was favored in response to the availability of food during the drought.