Natural Selection on Synonymous and Nonsynonymous Mutations Shapes Patterns of Polymorphism in Populus tremula

One important goal of population genetics is to understand the relative importance of different evolutionary processes for shaping variation in natural populations. Here, I use multilocus data to show that natural selection on both synonymous and nonsynonymous mutations plays an important role in shaping levels of synonymous polymorphism in European aspen (Populus tremula). Previous studies have documented a preferential fixation of synonymous mutations encoding preferred codons in P. tremula. The results presented here show that this has resulted in an increase in codon bias in P. tremula, consistent with stronger selection acting on synonymous codon usage. In addition, positive selection on nonsynonymous mutations appears to be common in P. tremula, with approximately 30% of all mutations having been fixed by positive selection. In addition, the recurrent fixation of beneficial mutations also reduces standing levels of polymorphism as evidenced by a significantly negative relationship between the rate of protein evolution synonymous site diversity and silent site diversity. Finally, I use approximate Bayesian methods to estimate the strength of selection acting on beneficial substitutions. These calculations show that recurrent hitchhiking reduces polymorphism by, on average, 30%. The product of strength of selection acting on beneficial mutations and the rate by which these occur across the genome (2N(e)lambda s) equals 1.54x10 (- 7), which is in line with estimates from Drosophila where recurrent hitchhiking has also been shown to have significant effects on standing levels of polymorphism.