‘The ecology of adaptive radiation’ revisited: A 25-year reflection

Abstract Twenty-five years ago, the book The Ecology of Adaptive Radiation by Dolph Schluter presented a testable framework that integrated ecology, genetics, and natural history to explain the mechanisms driving adaptive radiation, the rapid diversification of a single lineage into multiple species occupying distinct ecological niches. One important contribution was to re-orientate the study of adaptive radiation from macro- to microevolution, emphasizing processes and mechanisms rather than patterns, reshaping how adaptive radiation is conceptualized and studied. In this review, we revisit questions raised in Schluter’s foundational work to evaluate progress over the past quarter-century, focusing on eight key questions, grouped under four main themes, that remain unresolved. We first discuss the role of opportunity and diversification in radiations; whether there are consistent stages of adaptive radiation, whether they undergo early bursts and how can we measure and identify ecological opportunity that may initiate radiation. Turning to traits, we then discuss the genetic basis of adaptation before shifting to questions of phenotypic evolution. These include the role of evolvability in adaptive radiation; the stability of adaptive landscapes and how lineages cross adaptive valleys. Finally, we focus on the important role of speciation in the context of adaptive radiation. In our suggestions for future directions in the field over the next 25 years, we highlight the need to deepen the taxonomic scope of adaptive radiation and to improve our knowledge of the molecular basis of adaptive radiation. We argue that we are only at the beginning of our understanding of the role of gene flow in generating novel genetic combinations and driving the formation of new species. We suggest that there is a need to shift from a retrospective focus to a predictive eco-evolutionary dynamics framework. Finally we appeal for the need to further develop our understanding the role of plasticity in adaptive radiation and the role that subtle differences in organismal traits might play in improving our comparative analyses.