A New Model of Genetic Variation and Evolution Evaluates Relative Impacts of Background Selections and Selective Sweeps

Abstract

Intra-population genetic variation and interspecies divergence in chromosomeregions can be considerably affected by different local recombination rates. There are two models: (i) the selective sweeps that reduces the genetic diversity at linked sites and elevates the divergence rate; and (ii) the background selection that reduces the genetic diversity at linked sites and divergence rate. An intriguing question, yet highly controversial, is which one is dominant. In this paper, I develop a framework of generalize background selection, formulated by a diffusion model with two killing functions: the one associated with (negative) background selection is the rate to stop a fixation process of a mutation randomly, and the other associated with positive background selection (selective sweep) is the rate to stop a loss process of a mutation randomly. A simple relationship between the level of reduced diversity and the rate of divergence is derived, depending on the strength of generalized background selection (G) and the proportion of positive background selection (β). We analyzed the interspecies divergence and intra-population diversity in low-recombination regions of three organisms (fruitfly, soybean and human). Strikingly, all datasets demonstrated the dominance of (negative) background selection, and the positive background selection (selective sweeps) only has a small contribution (β~10%). However, our analysis rejects the notion of β =0, namely, a complete negative background selection is unlikely. These findings may shed some lights on the long-term debates around Neutral Theory.