The Galapagos islands have often been called a "laboratory of evolution". There are few places in the world were it has been possible to find such a variety of species, both plant and animal, which show so many degrees of evolutionary changes, in such a restricted area.
Once organisms rech oceanic island they are isolated from other landmasses.  If the islands are distant enough from a source to make colonization a rare event, then they may be  thought of as almost independent biological units. Oceanic islands can have species which, though related to mainland forms, have evolved in ways different from their mainland relatives as a result of isolation in a different environment. This a key factor in island evolution.  
It is not surprising that Charles Darwin was so struck by the life he found in these islands. The crucial prize of 450 years of human contact with the Galapagos is the Theory of Natural Selection, the mechanism by which evolution proceeds.  Formulated by Darwin, Natural Selection is the process by which propagation becomes change, and species diverge one from another.
A classic example of adaptive radiation in birds, which has served generations of evolutionary biologists, is Darwin's finches. A total of 13 species evolved within the Galapagos archipelago from a common ancestor whose founding type and American continental source have not yet been identified. A single fourteenth species occurs on Cocos Island in Costa Rica, about five hundred miles northeast of Galapagos. That all the finches are closely related, and presumably evolved from the same progenitor stock, is indicated by a complement of characteristics common to all.
Endemic refers to those living animals that evolved and remained isolated on a given area and, because of its new characteristics, are found nowhere else on the world. In Galapagos you will find a series of species that fall into this classification