Character Displacement and the Adaptive Divergence of Finches on Islands and Continents

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Abstract
I consider the differing amounts of adaptive divergence on continents and remote archipelagoes as a source of evidence for ecological character displacement. A classical example is the accelerated evolution of morphological differences between finch species on the Galapagos and Hawaii [USA], thought to result from an absence of competitor taxa on the islands. I show that the amount of divergence is greater in the Galapagos and Hawaiian finches, even when only beak-size divergence in the granivorous component is considered. Studies of seed availability and use, of biochemical differences between species, and of natural selection and the quantitative genetics of size on islands and continents support the hypothesis of character displacement. Several alternative explanations are considered (greater resource diversity on continents, predation on continents, reduced gene flow, and founder events on islands), but although each may play a role, little available evidence suggests that they are sufficient to account fully for the pattern. Implications of this result are discussed, including the possibility that community-wide character displacement involving many taxa is common.