 Accounts of biological function which refer to natural selection typically have the form that a trait's functions causally explain the existence or maintenance of that trait in a given population via the mechanism of natural selection. William Wimsatt (1972), Ruth Millikan (1984), and Karen Neander (1991a), all treat the past history of natural selection as the selection process that legitimizes the notion of a biological function. Within such approaches there is a dispute about the exact role of natural selection, whether as a source of variation (sometimes referred to as the “creative” role of natural selection, e.g., Neander 1988; see also Ayala 1970, 1977), or only as a filter on variations that arise independently (Sober 1984). 