Adaptive Radiation

The ETIB assumes that islands are within the geographic distance into which a species is likely to disperse, thus maintaining genetic populations between source and island populations. On islands that are beyond the range within which populations can maintain genetic contact with source populations, one might predict (based on the theory) that few species should be present. But this tends not to happen. Isolated islands that are formed initially without life are often found to have large numbers of closely related species. When single colonists, isolated genetically from their source population, give rise to a series of species that have diversified ecologically, the phenomenon is termed adaptive radiation. Usually it occurs beyond the so-called radiation zone, or normal range of dispersal of a given organism. Species that form through adaptive radiation are typically neoendemics, formed in situ and found nowhere else. Among arthropods, the Hawaiian Islands hold the record in having the largest number of neoendemics, an extraor dinary 98% of the fauna. Founder effects, behavioral isolation, ecological isolation, and host-associated isolation have all been implicated in the process of adaptive radiation. For insects, particularly noted examples include Drosophila flies, which are well known for their diversity of mating behaviors, as well as lineages of crickets that have diversified in song repertoire, sapfeeding planthoppers that have proliferated by switching between plant hosts, and beetles that have formed new species on different substrates. Diversification may follow a predictable pattern, at least in some groups; for example, among Tetragnatha spiders, similar ecological sets of species have evolved over and over again on each of the different Hawaiian Islands.

Compared with their hypothetical colonizing ancestors, species on remote oceanic islands are often characterized by a reduction in dispersal ability. Indeed, they are often found to have very narrow ranges of dispersal. Moreover, the individual species that colonize remote islands are a small sample of the continental source. They are therefore said to be "disharmonic" and not representative of the biotic diversity on continents, a phenomenon accentuated by the frequent proliferation of successful colonists.

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