Adaptive radiation is one of the most common evolutionary mechanisms. It occurs when a founding population expands into a variety of different ecological niches. Each subgroup gradually adapts to the new environment by the process of natural selection, eventually evolving into different "species," or populations that do not interbreed with one another.
One of the best illustrations of the process of adaptive radiation comes from the so-called "Darwin's finches" of the Galapagos Islands, off the northwest coast of South America. When Darwin visited this volcanic island group during the voyage of the Beagle in the mid-1830s, he collected a number of finches found on different islands.
Upon his return to England, he was surprised to learn from the ornithologist John Gould that the finches were new species, and indeed Gould classified them as separate species. They included two different groups: finches that were ground feeders and those that were tree feeders, and the beaks of the different species were accordingly different. Many historians believe that it was this information that sparked the young naturalist's interest in the problem of species creation. The problem Darwin encountered was why would there be different species on islands that were so close together. Perhaps species had not remained "fixed" and unchangeable since the moment of Creation? In 1859, Darwin published the results of his two-decades long investigation of this problem in On the Origin of Species.
The following chart showing a new genealogical tree for Darwin's finches using DNA analysis was published in Nicholas Wade, "Finch DNA shows Darwin was right," New York Times, 11 May 1999. Wade describes the recent evidence that Darwin was right: the finches were indeed "all descended from a single ancestral species." The chart shows the evolutionary relationship among the 14 species of Darwin's finches: