Ecological character displacement in adaptive radiation

I give an overview of the observational and experimental evidence for ecolo gical character displacement in adaptive radiation. Sixty-one published cas es of character displacement involving closely related species (congeners) make up the observational data set. All cases involve divergence, even thou gh parallel and convergent displacement are theoretically possible. Charact er ratios in sympatry were greatest when displacement was symmetric (mean 1 .54) and least when asymmetric (mean 1.29), perhaps because the most symmet ric resource distributions are also the broadest. Carnivores are vastly ove rrepresented in the data compared with other trophic groups, with herbivore s the next most common category. I consider five hypotheses to explain this pattern, including the possibility that the likelihood of divergence via c ompetition depends on position in food webs. Overall, the quality and compl eteness of observational data has improved in recent pars, as judged by the extent to which individual cases satisfy six standard criteria. All but on e of the criteria are met in over half the cases. Most often lacking is ind ependent evidence that the species involved compete for resources. For this reason, we cannot be sure that divergence in sympatry is usually the resul t of resource competition rather than some other interaction. Field experim ents on character displacement, which explore how interaction strength chan ges per unit change in phenotypic traits, are only just beginning. I summar ize research on threespine sticklebacks that used experiments in ponds to t est three predictions: that present-day differences between sympatric speci es are a "ghost" of competition past; that adding a competitor alters natur al selection pressures on a species already present, favoring divergence; a nd that divergent natural selection stemming from resource competition is f requency dependent. In total, the evidence suggests that character displace ment occurs frequently in nature, and it probably plays an important role i n the evolution of diversity in many adaptive radiations.