Abstract
Adaptationism is often taken to be the thesis that most traits are adaptations. In order to assess this thesis, it seems we must be able to establish either an exhaustive set of all traits or a representative sample of this set. Either task requires a more systematic and principled way of individuating traits than is currently available. Moreover, different criteria of trait individuation can make adaptationism turn out true or false, and criteria based on selection may presuppose adaptationism. In this paper, we show that adaptationism depends on trait individuation and that the latter is an open and unsolved problem.
