 The multiple realizability of higher level kinds is typically viewed as an obstacle to reduction (e.g., Kincaid 1997), and many have interpreted the many-one relation between molecular and higher level phenomena in a similar way. Rosenberg’s earlier discussions that criticized theory reduction in light of multiple realization (1978, 1985, 1994) used  Donald Davidson’s  (1970) notion of  supervenience,  which is another way to spell out an idea of ontological without epistemic reduction  (non-reductive physicalism).  Higher level facts supervene on physical facts if any difference between two higher level situations implies a difference in the physical basis. Yet this is consistent with two distinct physical situations corresponding to the same higher level state (many-one). Since biological properties such as fitness supervene on physical properties, fitness can be nothing more than a combination of physical properties, even though no such property complex is necessary for having a particular fitness value. Higher level theories abstract away from irrelevant variation in lower level features and thereby arrive at generalizations that are explanatory precisely because they do not include features irrelevant to explaining the phenomenon (Putnam 1975, Strevens 2008). From this perspective, theory reduction of classical to  molecular genetics  is impossible as the many–many relations between classical and molecular kinds make any reductive account radically disjunctive. At the same time, the notion of supervenience implies that classical and molecular genetics are not incommensurable, and that molecular genetics can shed light on the exceptions of classical genetics.