Genetical processes in the Galapagos

Current understanding of the mode and tempo of evolutionary divergence within the major groups of terrestrial vertebrates of the Galápagos Archipelago is summarized from three perspectives: (1) the number and relative timing of introductions for each group; (2) the influence of historical factors and present-day population demography of patterns and amounts of genetic diversity; and (3) possible mechanisms of adaptive radiation, or macroevolution, within the tortoise and finch groups. Native and introduced rats, lava lizards, geckos, and iguanas most likely had more than one episode of introduction from already differentiated mainland stocks. The finches and tortoises appear to have originated from but a single respective radiation. The influence of these differences in invasion histories is clearly evident in patterns of within-island genetic biochemical diversity and between-island or between-species differentiation. It is argued that much of the pattern of within-species geographic differentiation in genetic characters results from the temporal history of populations, including the amount of migration between islands, as well as differential selection pressures. Major morphological differentiation within both the finch and tortoise radiations has occurred with minimal change at structural gene loci. Studies of, for example, developmental heterochrony and character heritability analyses are needed to understand this apparent paradox. SUMMARY: The Galápagos fauna has played a major role in the development of the evolutionary paradigm, yet we have barely scratched the surface in both understanding the patterns and processes of evolutionary divergence for this fauna and utilizing the examples provided by the Galápagos fauna to test major questions and issues of current evolutionary biology. The fundamentally important issues of the relationships among the Galápagos taxa of given faunal elements and of these to mainland relatives still needs to be addressed for the majority of taxa. Commonality of patterns will help to distinguish between alternative hypotheses of island biogeography (Endler, 1982), evolutionary processes of differentiation (Gould, 1980), and modes of speciation (Bush, 1975). Focus on comparative intertaxon ontogenetic developmental sequences and character heritability estimates will help in examining the issues of adaptive radiation and macroevolution. There is much left to do, and the Galápagos provide a selective set of very tractable organisms to further a more fundamental understanding of evolutionary dynamics. Darwin's evolutionary paradigm was initiated, at least in part, by the Galápagos fauna; it is only appropriate that this fauna and flora continue to be active participants in the further refinement of this paradigm.