5.2 - C - Finches of Daphne Major
When Charles Darwin visited the Galapagos Islands, he collected many specimens of small birds called finches. He observed that the size and shape of their beaks varied from island to island and were likely related to their diets.
Since Darwin, there has a lot of research into the finches he observed, particularly those located on Daphne Major. On the island there is sizable population of medium ground finches, but hardly any small ground finches. In the absence of small ground finches, the medium ground finches have a smaller body and beak size useful for eating the small seeds.
In 1977, though, there was a drought on the island which caused a shortage of small seeds. So, the medium ground finch had to feed on larger, harder seeds which needed a larger beak to break. As a result, the members with larger beaks survived while small-beaked members began to die out. As shown in the graph to the right, this resulted in a larger average beak size for the population.
During subsequent rainy years, the supply of small seeds returned and the population of small-beaked finches increased, bringing down the average population beak size. However, during dry years in 1980 and 1982, small seeds were replaced by larger seeds and the average beak size increased again and then decreased when rain returned.
This change in beak size of this species is a good example of how characteristics of a population can vary in response to changes in the environment. This change is also one of few examples of natural selection we have been able to directly observe.