Geographic isolation refers to the separation of populations due to a physical barrier such as rivers, oceans, and mountains. Due to the isolation of the smaller population among the original and larger population, a natural selection or a genetic drift may happen.

When the finches arrived in the different islands of Galápagos, they were isolated and they do not share the common gene pool anymore because they rarely flew over the open water. Even when some flew across from one island to another, their differences in mating rituals and other behavioral patterns have made interbreeding and coexisting in one habitat difficult. In this case, finches choose their mates by looking for individuals that look like them. Finches that have small beaks do not breed with finches that have a large beak. Because of this, the gene pools of the two populations remained isolated and they evolved separately. Eventually, they become separate species.

An observation on the finches’ beak structure and feeding behavior were conducted to determine how the seed types affected the evolution of the finches. According to this study, the size of the bird’s beak is related to the way it feeds itself. The birds with smaller beaks have a higher chance of surviving when the vegetation is scarce during the hot and dry season since they can feed on smaller seeds.

On the other hand, birds that have a large beak are favored in habitats that have plants with larger seeds. As a result, those organisms which are not able to adapt will eventually die, whereas organisms which can survive will be able to reproduce and evolve.