 A full resolution of this issue cannot be attempted here; however, it is worth making one observation about the idea that mutation, selection, migration and drift should be regarded as ‘causes’ of gene frequency change. There is an important difference between drift on the one hand and the other three factors on the other. This is because mutation, selection and migration are directional; they typically lead to a non-zero expected change in gene frequencies (Rice 2004 p. 132). Random drift on the other hand is non-directional; the expected change due to drift is by definition zero. As Rice (2004) points out, this means that mutation, selection and migration can each be represented by a vector field on the space of gene frequencies; their combined effects on the overall evolutionary change is then represented by ordinary vector addition (which arguably lends substance of the “force” metaphor). But drift cannot be treated this way, for it has a magnitude but not a direction. In so far as proponents of the ‘non-causal’ view are motivated by the oddity of regarding drift, or chance, as a force, they have a point. However this line of argument is specific to random drift; it does not generalize to all the factors that affect gene frequency change. And it does not alter the fact that, in attributing the spread of a gene to random drift, we have given a bona fide scientific explanation that is potentially falsifiable; for we have said that the gene’s spread was not due to any systematic advantage that the gene conferred on organisms that had it. 