The earth contains an incredible diversity of life. Literally millions of species are known to man, with more discovered every day. And yet scientists estimate that the species alive today make up less than 0.1% of those that have ever lived. In the last section we discussed the ways in which new species are formed. But all three of those mechanisms of speciation involve the creation of one or two species from another species over a long period of time. This ratio of at most 2 species emerging from 1 original species hardly seems enough to account for the extreme diversity we see today and throughout history.
The mechanism of adaptive radiation helps explain this diversity. An adaptive radiation is a burst of evolution, creating several new species out of a single parent species. As when we discussed species richness, it is useful here to think of uninhabited "islands" of habitat, though in this case, the islands merely need to be uninhabited by the species in question. A population of given species, which we'll imaginatively name species 1, moves into a new habitat and establishes itself in a niche, or role, in the habitat. In so doing, it adapts to its new environment and becomes different from the parent species. If a new population of the parent species, 2, moves into the area, it too will try to occupy the same niche as 1. However, the niche rule states that only one of a group of closely related species may occupy the same niche in a given habitat. Competition between species 1 and 2 ensues, placing pressure on both groups to adapt to separate niches, further distinguishing them from each other and the parent species. As this happens many times in a given habitat, several new species may be formed from a single parent species in a relatively short period of time. Darwin's finches are an excellent example of adaptive radiation.