Why are deleterious mutations maintained in selfing populations? An analysis of the effects of early- and late-acting mutations by a two-locus two-allele model

•High inbreeding depression is observed in plant populations with high selfing rates.•Sakai (2020) proposed the two-stage effects to explain high inbreeding depression.•I developed a two-locus two-allele model to clarify the mechanism of the two-stage effect.•If mutations occur in both early- and late-acting, they are maintained in highly selfing populations.•The detailed mechanisms underlying the two-stage effect were also analyzed.