Many of the species on islands are either subtly or profoundly different from those on the nearest comparable area of mainland. Put simply, there are two main reasons for this.
1 The animals and plants on an island are limited to those types having ancestors that managed to disperse there, although the extent of this limitation depends on the isolation of the island and the intrinsic dispersal ability of the animal or plant in question.
2 Because of this isolation, as we saw in the previous section, the rate of evolutionary change on an island may often be fast enough to outweigh the effects of the exchange of genetic material between the island population and related populations elsewhere.
Thus, islands contain many species unique to themselves ('endemics' - species found in only one area), as well as many differentiated 'races' or 'subspecies' that are distinguishable from mainland forms. A few individuals that disperse by chance to a habitable island can form the nucleus of an expanding new species. Its character will have been colored by the particular genes that were represented among the colonists - which are unlikely to be a perfect sample of the parent population. What natural selection can do with this founder population is limited by what is in its limited sample of genes (plus occasional rare mutations). Indeed much of the deviation among populations isolated on islands appears to be due to a founder effect - the chance composition of the pool of founder genes puts limits and constraints on what variation there is for natural selection to act upon.
The Drosophila fruit-flies of Hawaii provide a further spectacular example of species formation on islands. The Hawaiian chain of islands (Figure 1.10) is volcanic in origin, having been formed gradually over the last 40 million years, as the center of the Pacific tectonic plate moved steadily over a 'hot spot' in a southeasterly direction (Niihau is the most ancient of the islands, Hawaii itself the most recent). The richness of the Hawaiian Drosophila is spectacular: there are probably about 1500 Drosophila spp. worldwide, but at least 500 of these are found only in the Hawaiian islands.
Of particular interest are the 100 Hawaiian Drosophila or so species of 'picture-winged' Droso-
phila. The lineages through which these species have evolved can be traced by analyzing the banding patterns on the giant chromosomes in the salivary glands of their larvae. The evolutionary tree that emerges is shown in Figure 1.10, with each species lined up above the island on which it is found (there are only two species found on more than one island). The historical element in 'what lives where' is plainly apparent: the more ancient species live on the more ancient islands, and, as new islands have been formed, rare dispersers have reached them and eventually evolved in to new species. At least some of these species appear to match the same environment as others on different islands. Of the closely related species, for example, D. adiastola (species 8) is only found on Maui and D. setosimentum (species 11) only on Hawaii, but the environments that they live in are apparently indistinguishable (Heed, 1968). What is most noteworthy, of course, is the power and importance of isolation (coupled with natural selection) in generating new species. Thus, island biotas illustrate two important, related points: (i) that there is a historical element in the match between organisms and environments; and (ii) that there is not just one perfect organism for each type of environment.
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