Adaptive radiation describes the rapid speciation of a single or a few species to fill many ecological niches. This is an evolutionary process driven by mutation and natural selection.

Adaptive radiation often occurs when a species is introduced to a new ecosystem, or when a species can survive in an environment that was unreachable before. For example, the Darwin's finches on the Galapagos islands developed from a single species of finches that reached the islands. Other examples include the introduction of predatory mammals to Australia by humans, the development of the first birds, which suddenly were able to expand their territory into the air, or the development of lung fish during the Devonian period, about 300 million years ago.

The dynamics of adaptive radiation is such that, within a relatively short time, many species derive from a single or a few ancestor species. From this large number of genetic combinations, only a few can survive on long term. After the rapid development of many new species, lots or most of them die out as quickly as they appeared. The surviving species are almost perfectly adapted to the new environment. The rise and fall of new species is now progressing very slowly, compared to the initial outburst of species.

There are three basic types of adaptive radiation. They are :

1. General adaptation. A species that develops a radically new ability can reach new parts of its environment. An example of general adaptation is bird flight.
2. Environmental change. A species that can, in contrast to the other species in the ecosystem, successfully survive in a radically changed environment will probably branch into new species that cover the new ecological niches created by the environmental change. An example of adaptive radiation as the result of an environmental change is the rapid spread and development of mammalian species after the extinction of the dinosaurs.
3. Archipelagoes. Isolated ecosystems, such as islands and mountain areas, can be colonized by a new species which upon establishing itself undergoes rapid divergent evolution. Darwin's finches are examples of adaptive radiation occurring in an archipelago.

In science fiction sometimes adaptive radiation of humans is imagined. This often makes for interesting multi-species worlds.