Research on evolutionary principles in Galápagos: An overview of the past 50 years

The classical Darwinian theory maintains that evolution is mainly adaptation by natural selection. Over the 70 years following the publication of The Origin of Species in 1859, the Modern Synthesis arose as a fusion between natural selection and Mendelian genetics, and their expansion into population genetics. Two contrasting evolutionary views for adaptive evolution emerged during the development of the Modern Synthesis. Ronald Fisher’s (1930) Fundamental Theorem of Natural Selection holds that simple Darwinian mass selection fully explains all adaptive evolution. On the other hand, Sewall Wright (1931, 1932) asserts that selection alone cannot explain adaptive evolution, which requires natural selection acting together with stochastic processes like random genetic drift. Wright’s Shifting balance theory of evolution uses the metaphor of an adaptive landscape (Figure 2.1) as a surface representing fitness of a species. On the fitness landscape, each locally adapted population occupies an adaptive peak separated from other peaks by valleys of low fitness (more poorly performing combinations of characteristics in that environment). Natural selection, while promoting an increase in the average fitness of each local population, gets them stuck in a local fitness peak that may be lower than the highest adaptive peak possible for that species. Genetic drift allows each local population to genetically “explore” around its fitness peak. The magnitude of the random shift in mean population fitness from one generation to the next depends upon the size of the population: small populations will take much larger random steps than larger populations. When a shift results in a population approaching the slope of a different adaptive peak, natural selection will pull it toward the new adaptive peak. Thus, drift and selection are more likely to produce a shifting of the population across valleys of low fitness towards a new adaptive peak. Both remain largely theoretical but their logic and appeal have made them two of the most influential theoretical perspectives in the study of evolution. Furthermore, modern evolutionary biologists agree that evolution consists of both adaptive and non-adaptive (stochastic) components. However, whether evolution is mainly adaptive remains to be answered. We argue that islands offer excellent opportunities to examine this balance.