ACCOUNTING FOR BIOLOGICAL DIVERSITY 1

The concept of adaptive radiation (or at least the term) was fathered by D. Lack, G. G. Simpson, and other contributors to the Evolutionary Synthesis, and reflected their conviction that most evolution, including differences among species, was caused by environmental agents of natural selection. If that were true, and if cladogenesis often occurs in bursts of almost simultaneous diversification (as inferred by paleontologists and supported by the prolific polytomies in many modern molecular phylogenies), then adaptive radiation, which Dolph Schluter defines as ‘‘the evolution of ecological diversity within a rapidly multiplying lineage,’’ would account for much of the boundless diversity of species and their characteristics. In assessing the prevalence and causes of adaptive radiation for the first time since the Synthesis, Schluter addresses as great, as encompassing, a theme as biology sounds. The ‘‘ecological theory’’ of adaptive radiation that developed during the Synthesis, Schluter notes, was tripartite: divergence between species is caused by different selection owing to differences in the species’ habitats and resources; evolution of differences in habitat and resource use is driven by competition or permitted by alleviation of competition; speciation (the evolution of reproductive isolation) is a consequence of the divergent natural selection that drives phenotypic and ecological differentiation. At least through the 1950s, the theory was based mostly on patterns described by paleontologists and systematists, and quantitative studies were few. Since then, population genetic models, phylogenetic analyses, experimental studies of form and function, ecological studies, and quantitative genetics have been brought to bear on the elements of the theory, as well as on more recent ideas pertaining to adaptive radiation. Schluter sets out to reevaluate the theory in the light of modern studies, concentrating on diversification at low taxonomic levels (e.g., within genera). He concludes that ‘‘[o]n the whole, it should be regarded as one of the most highly successful theories of evolution ever advanced’’ (p. 242). But as he ably describes, much of the theory’s support is based on only a few wellstudied cases and we do not know how far they can be generalized. The role of factors other than divergent ecological selection and competition is poorly understood, and some profoundly important questions are still virtually without answer. Schluter’s treatment of this grand subject is marked by the same consciousness of hypothesis-testing and experimental