Adaptive radiation

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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, 14 species of Darwin's finches on the Galápagos Islands developed from a single species of finch that reached the islands. Other examples include anoles of the Caribbean islands, Hawaiian silverswords and picture-winged pomace flies, the development of the first birds (which suddenly were able to expand their territory into the air), and the development of lung fish during the Devonian period about 300 million years ago. Another example of adaptive radiation is the diversity within Phylum Mollusca.

The dynamics of adaptive radiation are such that, within a relatively short time, many species derive from a single or a few ancestor species. The rise and fall of new species then continues very slowly, compared to the initial outburst of species.

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

General adaptation. A species that develops a radical new ability can reach new parts of its environment. An example of general adaptation is bird flight.
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.
Geographic isolation. Isolated ecosystems, such as archipelagos and mountain areas, can be colonized by a species which upon establishing itself undergoes rapid divergent evolution. Monotremes and marsupials are examples of geographic isolation: having evolved from placental mammals, they are found only in Australia, an island isolated by ocean.

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

References

Wilson, E. et al. Life on Earth, by Wilson,E.; Eisner,T.; Briggs,W.; Dickerson,R.; Metzenberg,R.; O'brien,R.; Susman,M.; Boggs,W.; (Sinauer Associates, Inc., Publishers, Stamford, Connecticut), c 1974. Chapters: The Multiplication of Species; Biogeography, pp 824-877. 40 Graphs, w species pictures, also Tables, Photos, etc. Includes Galápagos Islands, Hawaii, and Australia subcontinent, (plus St. Helena Island, etc.).

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