Adaptive radiation is when one species branches out to occupy multiple niches within an environment and eventually evolves into multiple species. Species are most successful at adaptive radiation after a mass extinction or when colonizing a previously uninhabited island. In both of these cases, niches that normally would be occupied by competing species are left free for a new species to exploit.
An ecosystem is comprised of many species of plants and animals that compete for survival. In a typical ecosystem, a species will evolve to occupy a specific ecological niche, which is defined as the role that a life form has in its environment, including the habitat it lives in and the resources it consumes. In most ecosystems, multiple species occupy the same niches and must continue to evolve in order to gain an advantage over competing species in the face of limited resources. All species in a system continually evolve, so most of these competitions end in a stalemate, and only occasionally will one species develop to dominate the others.
In a new environment, such as on an island that has been recently formed, many ecological niches are unoccupied. Adaptive radiation occurs when one species arrives in a new environment and evolves to utilize a number of ecological niches. Eventually, one species will become multiple species that can no longer interbreed. This evolutionary pattern is common throughout the world and can be traced through the fossil record of many plants and animals.
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One of the most famous examples of adaptive radiation is the evolution of finches on the Galapagos Islands. It is believed that these small, seed-eating birds arrived a few million years ago on the relatively uninhabited islands that lie 600 miles to the west of Ecuador. In the absence of other small birds, the parent species evolved different beak shapes in order to eat the different foods available on the islands. Finches evolved to have beaks that helped them eat insects, grubs and flowers, and one species even evolved to have a beak that made it possible to fashion and use a tool to catch bugs. After visiting the Galapagos Islands in 1835, Charles Darwin brought many of these birds home with him, and the later study of their beaks led him to develop his theory of evolution.
Another example of adaptive radiation is the evolution of marsupials on the Australian continent. Marsupials started out as small, possum-like creatures, but they evolved into many species when they arrived in Australia about 55 million years ago. There were no other mammals on the continent, so marsupials radiated out to fill the roles that non-marsupials filled in the rest of the world. Kangaroos evolved into large, grazing animals and marsupial lions evolved into large predators.