Ecological Causes of Adaptive Radiation

According to the naturalists of the first half of this century, adaptive radiation is the outcome of three ecological processes: phenotypic differentiation of populations by resource-based divergent natural selection, phenotypic differentiation through resource competition (ecological opportunity and divergent character displacement), and ecological speciation (speciation as a consequence of adaptation to different resource environments). Despite a recent surge of interest in the phenomenon, especially in phylogenetic histories of radiations, we know too little about the ecology of most radiations to assess the roles of the three processes. I summarize our own efforts to test the theory with a radiation of three-spined sticklebacks apparently still in its early stages. The role of divergent selection is supported by a strong relationship among populations and species in mean morphology, feeding performance, habitat use, and growth rate. Trade-offs in feeding performance and growth rate between habitats are steep. Ecological character displacement is indicated by the large differences between sympatric species and the intermediate features of solitary species inhabiting lakes of similar size. A pond experiment showed that natural selection on a solitary species is altered following introduction of a competitor and favors divergence. Evidence for ecological speciation in sticklebacks is weakest, but there are several hints of its importance: speciation was rapid and accompanied by divergence into different ecological niches; selection against hybrids is stronger in the wild than in the laboratory, which suggests that hybrid fitness depends on ecological context; premating isolation depends in part on traits that diverged in association with the exploitation of different resources; and reproductive isolation may have evolved in parallel in different populations experiencing similar environmental conditions. Comparison with other studies suggests that these findings apply broadly.