Feed-backs among inbreeding, inbreeding depression in sperm traits and sperm competition can drive evolution of costly polyandry

Ongoing ambitions are to understand the evolution of costly polyandry and its consequences for species ecology and evolution. Emerging patterns could stem from feed-back dynamics between the evolving mating system and its genetic environment, defined by interactions among kin including inbreeding. However, such feed-backs are rarely considered in non-selfing systems. We use a genetically-explicit model to demonstrate a mechanism by which inbreeding depression can select for polyandry to mitigate the negative consequences of
mating with inbred males rather than avoiding inbreeding. Specifically, given inbreeding depression in sperm traits, costly polyandry evolved to ensure female fertility, without requiring explicit inbreeding avoidance. We show the mechanisms behind two main feed-backs. First, resulting sperm competition caused evolution of sperm traits and further mitigated the negative effect of inbreeding depression on female fertility. Second, the evolving mating system fed back to decrease population-wide homozygosity, and hence inbreeding. However, net overall reduction was small due to compound effects on the variances in sex-specific reproductive success and paternity skew. Purging of deleterious mutations did not eliminate inbreeding depression in sperm traits and hence selection for polyandry. Polyandry evolution, both directly and through sperm competition, might consequently facilitate evolutionary rescue for populations experiencing sudden increases in inbreeding.