What caused the appearance of different types of Galapagos finches?

An excellent example of how a supposedly single population can diverge in the process of adapting to new habitats (a process called adaptive radiation) is a group of finches living in the Galapagos Islands, west of the coast of Ecuador. These birds are called “Darwin reels,” since Darwin studied them during his Beagle journey. Currently, 14 species of finch live on the Galapagos Islands. As a result of radiation, they formed a number of very different forms from each other. The diversity of these birds is most pronounced in the nature of their nutrition and in the form of a beak. In some species, the beak is adapted for crushing seeds of different sizes; in others – for the seizure of insects and plant buds; thirdly, to extract insects from under the bark of trees. It is also possible that the appearance of the beak plays a role in demonstrations during courtship and thereby contributes to reproductive isolation.
According to Lack, the first passerine birds that arrived in the Galapagos Islands were probably a flock of ancestral finches. These reels found here large and varied food resources in the absence of competitors and predators. The result was adaptive radiation, during which representatives of the original group were isolated on different islands, adapted to local conditions and gave rise to 14 independent species. Due to the special conditions characteristic of the Galapagos Islands, many of these finches occupied ecological niches, usually unusual for finches. The geographical isolation from each other and from the birds inhabiting the mainland, as well as the opportunities for rapid divergence as a result of adaptation to new habitats, probably played a decisive role in the adaptive radiation of Darwin finches