NOTE 22M: Interestingly, this is independent of the strength of selection against A^PB^P. For a single locus under migration and selection, Δp_A ~ sp_Aq_A – mp_A, and so polymorphism is possible only if m < s (Box 28.3). So, Dobzhansky–Muller divergence can be maintained even with gene flow, but the threshold rate of gene flow is one-quarter that with divergence at a single gene. (See Problems 18.7 and 22.7.)