Geographically-limited selective sweeps occurring anywhere on the linked mtDNA molecule are one possible explanation for a pattern of greater differentiation of mtDNA if the sweep causes polymorphic sites to become fixed within an outcrop for CYTB but does not affect RISP.
However, the mechanism by which a trait becomes fixed within a species, whether through selection or drift, need not be the same as the mechanism by which the trait influences diversification (p.70).
Once a ritual became fixed within a medical compendium or a remedy book collection of recipes, either it could be returned to living practice in oral-aural performance by subsequent selection and use or, alternatively, it might continue to exist in a textual form without being performed.
This suggests that lateral regulators that have introgressed near their target genes have a higher probability to become fixed within the recipient.
Hence there is no evidence that dN/dS declines through time in S. Typhimurium; rather, non-synonymous mutations appear to either be removed rapidly (within a few years, ~50% of non-synonymous SNPs) or become fixed within local subpopulations.
This mutation seems to become fixed within genotypes D and Da, as it was found in strains from five and eight different geographic regions, respectively (Figure 2).
