Suppose that gene A does not affect fitness in a small population of earthworms contained in a glass enclosure. For 20 worm generations, a pair of curious scientists study the frequencies of the A gene’s two alleles (A and a). In generation 1, 75% of the alleles are A. By generation 5, the a allele has become more common than A. Then, by generation 20, the A allele is most common again. What is the most likely explanation for this oscillation in allele frequencies of the A gene?