Molecular Evolution near a Two-locus Balanced Polymorphism

Balancing selection at one locus can increase the amount of selectively neutral variation within neighboring genomic regions. Discrete phenotypic polymorphisms studied in natural populations are frequently determined by sets of interacting genes instead of alternative alleles at single loci. We extend coalescent theory to investigate balancing selection on combinations of linked genes. We find that variation at neutral sites is increased across a much larger genomic region relative to the single-locus models: the entire region lying between the two loci in balanced combination is affected to some degree. Epistatic selection maintains these high levels of neutral variation because it directly opposes the homogenizing effect of recombination. The results of the theory are discussed in relation to published gene sequence data, primarily from Drosophila.