Evolution in Peripheral Isolated Populations: Carpodacus Finches on the California Islands

Power, Dennis

The importance of peripherally isolated populations in speciation has long been known. As a prelude to speciation, relatively great phenetic divergence ought to be detectable in geographically isolated populations within a species. House finch populations on the California Islands do show greater phenetic divergence than populations from the adjacent western North American mainland. Canonical variates analysis supported the general finding that there has been greater evolution on fringing islands. It is apparent that the size of a water barrier and distance from the mainland are important. Prevailing winds that have impeded dispersal in certain directions are also important. The most divergent island populations are those that are farthest from the mainland and, because of winds, are in a poor position to receive dispersing birds from other sources. Very small islands also have some of the most divergent populations, but it is not known if there is more rapid evolution in smaller populations. In regard to the evolutionary history of the house finch, there was probably over-water colonization of the northern Channel Islands, followed by dispersal to the southern Channel Islands and marked evolution in situ. Limited immigration from the mainland to the northern islands and to Santa Catalina Island may go on today. Whether colonization of Los Coronados Islands was via the southern Channel Islands or from the mainland remains problematical; phenetically, populations from Los Coronados are intermediate. Separate colonizations and parallel evolution probably took place on islands off Baja California where the most extreme island phenotypes are found on Guadalupe and San Benito islands. Evolution on fringing islands is important in allowing examination of the early stages of adaptive radiation.

Evolution in Peripheral Isolated Populations: Carpodacus Finches on the California Islands

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