On his visit to the Galapagos Islands, Charles Darwin discovered several species of finches that varied from island to island, which helped him to develop his theory of natural selection.

Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species - in other words, the occurrence of more than one form or morph. Polymorphism is common in nature; it is related to biodiversity, genetic variation and adaptation; it usually functions to retain variety of form in a population living in a varied environment. ... Polymorphism results from evolutionary processes, as does any aspect of a species. It is heritable and is modified by natural selection. Polymorphism as described here involves morphs of the phenotype.

In the case of the finches on the Galápagos Islands, there is no evolution from one species into another that has taken place. Normal variations in genetics and changes in environment are the causes of the varying beak sizes among the finches. It has been observed in times of drought that the size of the finches' beaks changed. As the supply of edible seeds diminished, only tough, hard-to-open seeds were left, and only birds with larger, deeper beaks could eat them. The generations to follow showed a dramatic increase in overall beak size in the population because only the finches with larger beaks would survive the drought and be able to mate. However, as the rains returned the reversal of beak size was observed. The size of finch beaks tend to oscillate between large and small depending on climate changes, but the finches have never shown any signs of changing into anything other than a finch.