Despite the potential for mechanical, developmental and/or chemical mechanisms to prevent self-fertilization, some amount of incidental self-fertilization is inevitable in some predominantly outcrossing species. In such cases, inbreeding (by self-fertilization or biparental inbreeding) may severely compromise individual fitness by exposing the recessive genetic load, which is otherwise hidden in the heterozygous state. Unquestionably, much of this load is an inevitable consequence of the mutational process and the masking of deleterious recessive alleles in predominantly outcrossing populations. However, we show that when back-up embryos efficiently compensate for inviable embryos lost early in development, natural selection can favor highly deleterious recessive variants that functionally induce self-sacrificial death of inbred embryos. Our work provides numerous testable hypotheses, and suggests that the distinction between late self-incompatibility and early acting inbreeding depression may be somewhat artificial in some taxa.

Altruism is defined as when an individual increases the fitness of another at a cost to their own fitness. Much research on the evolution of altruism focuses on hypotheses based on the role of a single locus or multilocus quantitative trait in kin selection, group selection, and reciprocity. Very few studies of altruism have explored how linked selection, or selection at loci neighboring an altruism locus, impacts the evolution of altruism. While linked selection can decrease the efficacy of selection at neighboring loci, it is also possible it might have other effects including promoting selection for altruism by increasing relatedness in regions of low recombination. Here, we used population genetic simulations to study how negative selection at linked loci, or background selection, affects the evolution of altruism. When altruism occurs between full siblings, we found that background selection interfered with selection on the altruistic allele, increasing its fixation when the altruistic allele was disfavored and reducing its fixation when the allele was favored. Background selection has the same effect on other, nonsocial, genes, suggesting that altruistic genes may not be affected by linked selection differently from other types of genes in family-structured populations.