PHENOTYPIC SIMILARITY AND THE EVOLUTIONARY SIGNIFICANCE OF COUNTERGRADIENT VARIATION

Countergradient variation is a geographical pattern of genotypes (with respect to environments) in which genetic influences on a trait oppos e environmental influences, thereby minimizing phenotypic change along the gradient. Phenotypic similarity across changing environments ough t to be of intense interest because it belies considerable genotypic c hange. When it occurs in characters that are positively associated wit h fitness, countergradient variation conflicts with the hypothesis tha t local adaptation to one environment trades off against performance i n another environment. Cases of countergradient variation therefore of fer unique insight into the mechanisms that produce and maintain pheno typic similarity and/or differences along environmental gradients.