The efficacy of selection may increase or decrease with selfing depending upon the recombination environment

Much theory has focused on how a population’s selfing rate affects the ability of natural selection to remove deleterious mutations from a population  However, most such theory has focused on mutations of a given dominance and fitness effect in isolation. It remains unclear how selfing affects the purging of deleterious mutations in a genome-wide context where mutations with different selection and dominance coefficients co-segregate. Here, we use computer simulations to investigate how mutation, selection and recombination interact with selfing rate to shape genome-wide patterns of mutation accumulation and fitness.  In addition to recovering previously described results for how the selfing rate affects the efficacy of selection against mutations of a given dominance class, we also find that the interaction of purifying selection against mutations of different dominance classes changes with selfing rate. In particular, as outcrossing populations transition from purifying selection to pseudo-overdominance, they experience a dramatic increase in the genetic load caused by additive, mildly deleterious mutations. We describe the threshold selfing rate that prevents pseudo-overdominance.

Methods
This study is based on individual-based forward simulations performed in SLiM and on analytic modeling. 

Analytic models are fully described in the Appendix associated with the manuscript.

Raw outputs from SLiM were post-processed in R with scripts that are in this repository.