On the sign of the average effect of an allele

The concept of the average effect of an allele pervades much of evolutionary population genetics. In this context the average effect of an allele is often considered as the main component of the "fitness" of that allele. It is widely believed that, if this fitness component for an allele is positive, then the frequency of this allele will increase, at least for one generation in discrete-time models. In this note we show that this is not necessarily the case since the average effect of an allele on fitness may be different from its marginal additive fitness even in a one-locus setting in non-random-mating populations.