Adaptations and history.

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