ADAPTATIONS AND HISTORY

The historical definition of adaptations has come into wide use as com parative biologists have applied methods of phylogenetic analysis to a variety of evolutionary problems. Here we point out a number of diffi culties in applying historical methods to the study of adaptation, esp ecially in cases where a trait has arisen but once. In particular, the potential complexity of the genetic correlations among phenotypic tra its, performance variables and fitness makes inferring past patterns o f selection from comparative data difficult. A given pattern of charac ter distribution may support many alternative hypotheses of mechanism. While phylogenetic data are limited in their ability to reveal evolut ionary mechanisms, they have always been an important source of adapti ve hypotheses and will continue to be so.