Monogamy promotes worker sterility in insect societies

18 Inclusive-fitness theory highlights monogamy as a key driver of altruistic sib-rearing. Accordingly, 19 monogamy should promote the evolution of worker sterility in social insects when sterile workers 20 make for better helpers. However, a recent population-genetics analysis (Olejarz et al. 2015) found 21 no clear effect of monogamy on worker sterility. Here, we revisit this analysis. First, we relax 22 genetic assumptions, considering not only alleles of extreme effect—encoding either no sterility or 23 complete sterility—but also alleles with intermediate worker-sterility effects. Second, we broaden 24 the stability analysis—which focused on the invasibility of populations where either all work25 ers are fully-sterile or all workers are fully-reproductive—to identify where intermediate pure or 26 mixed evolutionarily-stable states may occur. Finally, we consider additional, demographically27 explicit ecological scenarios relevant to worker non-reproduction. This extended analysis demon28 strates that an exact population-genetics approach strongly supports the prediction of inclusive29 fitness theory that monogamy promotes sib-directed altruism in social insects. 30