Fig. 1.

Allele frequency dynamics under negative frequency-dependent selection (NFDS). A new allele of a parasite-associated host gene enters the population at low frequency, originating either by mutation/recombination of existing alleles or by introgression from another population/species. Under the NFDS hypothesis, it is expected to confer higher fitness than common alleles, because these have been the main target of selection by coevolving parasites, and thus rises in frequency (trajectory 1 in A and B). Once common, the allele’s fitness is expected to decrease again, because it now becomes itself the main target of selection by parasites, resulting in two possible trajectories: Once the allele is rare again, it loses its fitness disadvantage, because parasites adapt to other, more common alleles, and ultimately gains again in frequency, leading to a full cycle in the allele frequency trajectory (trajectory 2). Instead, the allele can also be lost from the population, because of either continuous fitness disadvantage or neutral drift effects (trajectory 3). A and B depict different perspectives of the same process. The blue color shading in A indicates the time dependency of the allele’s frequency trajectory under NFDS. In B, the black arrows depict the passing of evolutionary time, and the dashed arrow exemplifies an introgressed allele.