Adaptation reviewed: A phylogenetic methodology for studying character macroevolution

We present a phylogenetic methodology to test hypotheses of adaptation and to discriminate adaptation from alternative causal explanations of character evolution. To constitute an adaptation, a character must be shown to provide current utility to the organism and to have been generated historically through the action of natural selection for its current biological role. The criterion of current utility is applied by comparing the performance of a derived trait to that of its phylogenetically antecedent state. If the performance of a trait exceeds that of its antecedent state, it constitutes an "aptation" for its observed biological role. Alternatively, if the performance of a trait equals or falls below that of its antecedent state, it constitutes a "nonaptation" or "disaptation," respectively. The criterion of historical genesis is applied using the concept of "selective regime," the aggregation of organismal and environmental factors that combine to determine the expected action of natural selection on actual and potential character variation. An aptation that arose on a lineage having the same selective regime as the focal taxon constitutes an "adaptation," whereas one that arose on a lineage having a different selective regime constitutes an "exaptation." For nonaptations and disaptations, the criterion of historical genesis serves to determine whether the nonaptive or disaptive status arose coincident with the trait itself or secondarily through a change of selective regime. We suggest that this phylogenetic approach will help to overcome general criticisms of adaptationist studies and will serve to bridge the gap between microevolutionary and macroevolutionary studies of adaptation.