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Trends in the insular radiation of a lizard

abstract1 (full description below): The American Naturalist 100:47-64.
Insular Radiation. PDF/Acrobat file     SoulĂ©, M.E.

In 1835 the phenomenon of insular radiation was brought to the attention of Charles Darwin when the then Vice-Governor of the Galapagos Islands pointed out to him the distinctness of the giant tortoises on the different islands of the archipelago. Since then, study of the often remarkable diversification of a stock on a group of islands has contributed considerably to the development of evolutionary theory although episodes of controversy have always punctuated discussion of the subject. Currently, much of the debate centers on the causes of the diversification. It is often difficult to assign adaptive significance to some of the more bizarre of the morphological characteristics of island animals. Consequently, some workers have abandoned natural selection as an immediate, exclusive explanation of island evolution, and have concluded that at least some of the modifications are random and unpredictable (Mayr, 1963, p. 538; Hartman, 1953; Radovanovic", 1959). Genetic drift concomitant with the colonizing event, or during later population bottlenecks, is suggested as one randomizing factor in the evolution of island animals. A related explanation is the founder principle, the putative effects of which begin when a population bottleneck has occurred (Mayr, 1954, 1963). According to Mayr, the resuk of a sudden isolation of a few individuals in a small area is a f'genetic revolution," which, in turn, can lead to profound and unpredictable changes in the genetic composition of the population (see Dobzhansky and Pavlovsky, 1957, and Dobzhansky and Spassky, 1962, for experimental support of this hypothesis). Other workers (e.g., Ford, I960) have argued that natural selection accounts for the patterns of insular evolution, at least for poly genie characters. Neither side claims the sufficiency of one explanation to the complete exclusion of the other, but the difference in emphasis is real.