 Other accounts of drift are not so easily classified. For example, Timothy Shanahan (1992) argues that conceptually, random drift and natural selection are the ends of a continuum. However, to reach this conclusion, Shanahan must reject heritability as a necessary condition for natural selection. As evolutionary biologist John Endler has argued, this has the effect of trivializing natural selection to the claim that “there are differences among different phenotypes” (Endler 1986: 13).