Interdemic selection and the sex ratio: a social spider perspective

Computer simulations of a metapopulation model that mimics social spider populations show that highly female-biased sex ratios evolve when continuously inbreeding lineages (colonies) proliferate differentially to replace other lineages that have become extinct. When lineages of any size proliferated, that is, when the advantage to the colony of producing more females was removed, biased sex ratios did not evolve despite extreme conditions of inbreeding. Selection at the colony level is therefore necessary to counteract Fisherian selection within colonies, which here is shown to be acting regardless of the degree of population subdivision. Smaller size of the founding groups and lower migration rates resulted in more female-biased sex ratios because they reduced the within-group genetic variance and increased the variance among groups and therefore made selection among groups more effective. The outcome of the selective process, however, depended not only on the degree of population subdivision (i.e., the ratio of the variances), but also on the relative rate of turnover of the groups and the individuals within them: the lower the rate of colony turnover, the less biased the equilibrium sex ratio. The threshold size for proliferation, brood size, and conditional control over the production of at least one male also affected the equilibrium sex ratio under some of the conditions investigated.