  Of course, if fitness is a probabilistic propensity, then the fitter among competing organisms will not always leave more offspring, and the theory of natural selection will have to be understood as making the claim that (probabilistic) fitness differences result in reproductive differences not invariably but only with some probability. With the inclusion of such propensities, the tautological nature of the explanation is dissolved since there is no deterministic path between the propensity to leave more offspring and the actual reproductive rates. But putting aside the tautology issue, there is also a scientific reason for favoring propensity approaches: since the theory of natural selection allows for drift, this qualification on its claims will be a welcome one.