Abstract

For neutral additive genes, the quantitative index of population divergence (Q_ST) is equivalent to Wright’s fixation index (F_ST). Thus, divergent or convergent selection is usually invoked, respectively, as a cause of the observed increase (Q_ST > F_ST) or decrease (Q_ST < F_ST) of Q_ST from its neutral expectation (Q_ST = F_ST). However, neutral nonadditive gene action can mimic the additive expectations under selection. We have studied theoretically the effect of consecutive population bottlenecks on the difference F_ST - Q_ST for two neutral biallelic epistatic loci, covering all types of marginal gene action. With simple dominance, Q_ST < F_ST for only low to moderate frequencies of the recessive alleles; otherwise, Q_ST > F_ST. Additional epistasis extends the condition Q_ST < F_ST to a broader range of frequencies. Irrespective of the type of nonadditive action, Q_ST < F_ST generally implies an increase of both the within-line additive variance after bottlenecks over its ancestral value (V_A) and the between-line variance over its additive expectation (2F_STV_A). Thus, both the redistribution of the genetic variance after bottlenecks and the F_ST - Q_ST value are governed largely by the marginal properties of single loci. The results indicate that the use of the F_ST - Q_ST criterion to investigate the relative importance of drift and selection in population differentiation should be restricted to pure additive traits.