Ecological character displacement in adaptive radiation

I give an overview of the observational and experimental evidence for ecological character displacement in adaptive radiation. Sixty-one published cases of character displacement involving closely related species (congeners) make up the observational data set. All cases involve divergence, even though parallel and convergent displacement are theoretically possible. Character ratios in sympatry were greatest when displacement was symmetric (mean 1.54) and least when asymmetric (mean 1.29), perhaps because the most symmetric resource distributions are also the broadest. Carnivores are vastly overrepresented in the data compared with other trophic groups, with herbivores the next most common category. I consider five hypotheses to explain this pattern, including the possibility that the likelihood of divergence via competition depends on position in food webs. Overall, the quality and completeness of observational data has improved in recent pars, as judged by the extent to which individual cases satisfy six standard criteria. All but one of the criteria are met in over half the cases. Most often lacking is independent evidence that the species involved compete for resources. For this reason, we cannot be sure that divergence in sympatry is usually the result of resource competition rather than some other interaction. Field experiments on character displacement, which explore how interaction strength changes per unit change in phenotypic traits, are only just beginning. I summarize research on threespine sticklebacks that used experiments in ponds to test three predictions: that present-day differences between sympatric species are a "ghost" of competition past; that adding a competitor alters natural selection pressures on a species already present, favoring divergence; and that divergent natural selection stemming from resource competition is frequency dependent. In total, the evidence suggests that character displacement occurs frequently in nature, and it probably plays an important role in the evolution of diversity in many adaptive radiations.