Evaluating the Impact of Purifying Selection on Species-level Molecular Dating

Abstract

The neutral theory of molecular evolution suggests that the constancy of the molecular clock relies on the neutral condition. Thus, purifying selection, the most common type of natural selection, could influence the constancy of the molecular clock, and the use of genes/sites under purifying selection may produce less reliable molecular dating results. However, in current practices of species-level molecular dating, some researchers prefer to select slowly evolving genes/sites to avoid the potential impact of substitution saturation. These genes/sites are generally under a strong influence of purifying selection. Here, from the data of 23 published mammal genomes, we constructed datasets under various selective constraints. We compared the differences in branch lengths and time estimates among these datasets to investigate the impact of purifying selection on species-level molecular dating. We found that as the selective constraint increases, terminal branches are extended, which introduces biases into the result of species-level molecular dating. This result suggests that in species-level molecular dating, the impact of purifying selection should be taken into consideration, and researchers should be more cautious with the use of genes/sites under purifying selection.