Figure 1.

The effect of genetic linkage on the effectiveness of selection. Consider two closely linked single nucleotide polymorphisms (SNPs A and B) with one of the 'alleles' at each site favored by selection (denoted by a superscript +). Selection acts more weakly on the 'interfering' haplotypes (A⁺B^- and A^-B⁺), where positive selection on one SNP is counterbalanced by negative selection on the other, compared with the 'reinforcing' haplotypes (A⁺B⁺ and A^-B^-), where selection on the two SNPs is complementary. This disparity causes interfering haplotypes to persist longer after they have accumulated by chance in finite populations. See Box 1 for further details.