Adaptive radiation—an evolutionary process through which a multitude of species emerge rapidly from a single lineage to colonize various ecological opportunities—constitutes a major source of biodiversity. Many ecological factors have been known to modulate adaptive radiation. For example, ample evidence has suggested that competition—one of the most ubiquitous species interactions in ecological communities—could strongly influence adaptive radiation. The findings of the existing studies on the competitive effect on adaptive radiation, however, are mixed. It remains unclear what factors determine the directionality (positive, negative, or non-significant) of the effect of competition on adaptive radiation.
Here we present a meta-analysis and an experiment to examine the roles of the ecological context of competition in adaptive radiation. In the meta-analysis, we extracted data from 17 existing studies and classified them according to the focal traits, competitor relatedness to diversifying lineages, study organisms, and colonization history of competing and diversifying lineages. We note that the top-down and bottom-up effects may also affect the importance of competition for adaptive radiation but has rarely been studied. To examine these effects, we further conducted an experiment with the fast-evolving bacterium Pseudomonas fluorescens SBW25, with the direct manipulation of competition, predation, and productivity.
Results/Conclusions
Our meta-analysis extracted data from 17 existing articles with 53 data entries. When considering all the studies, competition had an overall negative effect on adaptive radiation. The focal traits, competitor relatedness, and colonization history, not study organisms (micro- versus macroorganisms), however, altered the effect of competition on adaptive radiation. In summary, competition suppressed the divergence in discrete traits but favored the variations in continuous traits. Conspecific competitors tended to have non-significant effects on adaptive radiation, while heterospecific competitors tended to strongly impede adaptive radiation. Competition prohibited adaptive radiation when competitors colonized before or simultaneously with diversifying species, not when competitors colonized after diversifying species.
Our experiment indicated that competition explained more variance in SBW25 phenotypic diversification than predation and productivity, suggesting the paramount role of competition on adaptive radiation. However, the effect size of competition was greatest when SBW25 diversified in the high productivity environment in the absence of ciliate predators.
Together, our results demonstrate the important role of competition on adaptive radiation and underscore the necessity of considering the ecological context for better understanding of the effect of competition on adaptive radiation.