Diversification on islands: bulimulid land snails of Galápagos

Adaptive radiation is one of the main processes involved in the formation of the world’s organic diversity. Thus, understanding the processes involved in adaptive radiation becomes a key component of the study of evolutionary biology. The study of adaptive radiation entails a wide range of questions involving various aspects of phylogeny, biogeography, adaptation, and speciation. One of many approaches to a problem as broad as this is to seek an understanding of a small and simplified part of it. Islands provide this simplicity, naturally. The aim of the present work is to reach a better understanding of diversification in adaptive radiations, using Galápagos endemic bulimulid land snails as model system. First, I combined phylogenetics and biogeographical analyses to partition island species diversity by its originating processes: between-island colonization and within-island speciation. The results show that diversification of Galápagos bulimulid land snails has been driven by a combination of geographic factors (island age, size, and location), which affect colonization patterns, and ecological factors, such as plant species diversity, that foster within-island speciation. I then tested the classic evolutionary theory of ecological opportunity, which proposes that the combination of increase in wealth of resources and decrease of potential enemies promotes diversification in lineage colonizing previously empty adaptive zones. I show that ecological opportunity predicts the extent of intraspecific phenotypic variation, explicitly linking the role of competition from congeners and the heterogeneity of resources to the extent of intraspecific phenotypic divergence as adaptive radiation proceeds. Finally, I tested for a fit between the phenotypes of diversifying species and their environments. I tested for such a phenotype-environment association (1) across the adaptive radiation of Galápagos bulimulid land snail species, and (2) within one species, Bulimulus reibischi. I found that shell morphology has evolved across species repeatedly on different islands in response to moisture gradient in the same direction as within-species morphological variation among populations of B. reibischi. The parallel association between environment and phenotype both above and below the species level suggests that ecologically-based selection is involved in diversification at both population and species levels in this adaptive radiation.