Common-garden studies on adaptive radiation of photosynthetic physiology among Hawaiian lobeliads.

Species in an adaptive radiation often occupy different habitats so that individuals of each species develop under different conditions. Showing that a radiation is adaptive thus requires evidence that taxa have diverged genetically and that each has an ecological advantage in using particular habitats or resources, taking into account both phenotypic plasticity and phylogenetic relationships among species. Here, we use a common-garden experiment to show that representative species of Hawaiian lobeliads have diverged adaptively in their leaf-level photosynthetic light responses. Across species, plants genetically shifted their photosynthetic physiology with native light regime in accord with theoretical predictions and exhibited adaptive crossover in net carbon gain-that is, species native to a given light regime outperformed others only under conditions similar to those they occupy in the field, with the rank order of species based on photosynthesis per unit leaf mass changing with light level. These findings make a powerful case for adaptation of photosynthetic light responses to native light regimes and, combined with our earlier field studies, provide the strongest demonstration to date for the evolution of divergent adaptations for energy capture in any group of closely related plants.