The effect of fluctuating selection on the genealogy at a linked site

The genealogical consequences of temporally fluctuating selection at linked neutrally-evolving sites are studied using coalescent processes structured by genetic backgrounds. Surprisingly, although between-generation fluctuating selection and within-generation fecundity variance polymorphism lead to indistinguishable allele frequency dynamics at the selected site, I show that these two scenarios affect the genealogical structure of the population in distinctive ways. Whereas coalescence times are elevated at the selected site when heterozygotes have lower within-generation fecundity variance than either homozygote, fluctuating selection typically depresses the depth of the genealogy at tightly-linked sites. More importantly, these results indicate that fluctuating selection will have a characteristic signature combining an excess of selected variation at the affected site and a slight reduction in neutral variation at tightly linked sites. This observation suggests that it may be possible to distinguish balanced polymorphisms maintained by environmental variation from those maintained by heterozygote advantage.