Question

Why are islands often home to species that exist no where else on earth?

Answer 1

In terms of biodiversity, the issue is clearer: islands boast a truly unique assemblage of life. Species become island dwellers either by drifting on islands, like castaways, as they break off from larger landmasses (in the case of continental islands) or by dispersing across the ocean to islands newly emerged from the ocean floor (oceanic islands). Henceforth they are confined to small, isolated areas located some distance from other large landmasses. Over time, this isolation exerts unique evolutionary forces that result in the development of a distinct genetic reservoir and the emergence of highly specialized species with entirely new characteristics and the occurrence of unusual adaptations, such as gigantism, dwarfism, flightlessness, and loss of dispersibility and defence mechanisms. Genetic diversity and population sizes tend to be limited, and species often become concentrated in small confined areas.
Source: http://www.cbd.int/island/intro.shtml
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