Continental and oceanic islands
In the past, some islands (continental islands) formed part of the mainland (or of the continental shelf), from which they either became separated when the sea level rose (British Isles) or crumbled off the edges when the continental blocks rifted (some of the Seychelles). Some islands are so large and geologically old that they basically form small continents (New Guinea). Distinctive fauna and flora occur on all types of islands, frequently including species whose relatives have become extinct on the parent mainland. From the standpoint of study of evolutionary plasticity, however, oceanic islands located far away from the mainland are important; these were formed, e.g., as a result of volcanic activity or a combination of volcanoes and corral reefs, and were colonized in the past only by individual “shipwrecked” species arriving from the distant mainland (Hawaiian Islands). Only these species underwent a dramatic decrease in the population size that could renew their evolutionary plasticity.