Figure 2.

Schematic of HR interference under alternative selection regimes: (a) advantageous mutations arise on different backgrounds (haplotypes), interfere with each other and prevent each other's fixation. Linked neutral and slightly deleterious variants will increase in frequency until recombination generates new haplotypes, which drive beneficial mutations (now in the same haplotype) quickly to fixation while purging slightly deleterious alleles. (b) Deleterious alleles enter the population on different haplotypes. Owing to drift and/or to interference with selective advantageous mutations, they remain at low frequencies in the population until recombination generates a new haplotype resulting from the combination of the two deleterious alleles. Selection will remove this new haplotype more efficiently. However, an advantageous mutation will be lost, given that there was not enough time for recombination to break its association with a deleterious background. (c) Negative selection on multiple linked slightly deleterious mutations (referred to as a weak HR effect in Charlesworth et al. [148])—owing to the limited burden carried by such mutations, slightly deleterious variants tend to remain and accumulate in populations. Haplotypes that carry a larger mutational burden can be successively removed from the population. Interference occurs when there is little to no recombination, and selection at other loci on different haplotypes reduces the effective population size, impacting the rate at which they are lost from the population by making it more difficult to remove haplotypes that carry these deleterious mutations. Recombination combines chromosomes to create haplotypes that are free of or are loaded with deleterious mutations, increasing the efficacy of selection. Examples of mutations along the chromosomes (grey lines) are represented by different colours.