Split sex ratios and the evolutionary origins of eusociality
The influence of the genetic system of haplodiploidy on the evolution of eusociality has been a central topic in evolutionary biology since it was first discussed by Hamilton (1964) in the context of his invention of inclusive fitness theory. Since then, work on that influence had outstripped the development of inclusive fitness theory itself. Recently, Taylor (1986) has provided a deeper understanding of inclusive fitness theory that allows us to use it to derive a quite general expression for the selective advantage of sib rearing. Three forces can be identified in this advantage: an average relatedness effect, a parental skewed sex ratio effect, and a sex ratio manipulation effect. A simple guiding principle for each sex can be derived on the assumption of maternal sex ratio equilibrium. "Arbitrary ploidy" models explain three apparent "coincidences of cancelling" between the genetic systems of diploidy and haplodiploidy. The concept of reproductive value is central to the new development of inclusive fitness theory, to sex ratio theory, and to the advantage of sib rearing. Reproductive values of the different categories of individual provide theoretical results about equilibrium sex ratios in generalizations of the sphecid and halictid patterns of Seger (1983). The literature on the advantage of sib rearing is illuminated by the analysis of previous sections, which also suggests the potential importance of "split" sex ratios. Sex ratios are split when groups of parents contribute systematically different sex ratios to the same offspring generation. Variability between parents in cost ratio would lead us to expect strongly split sex ratios, but this expectation is countered to the extent that individual plasticity in the sex ratio produced among offspring can lead to unpredictable population sex ratios. © 1986 Academic Press Inc. (London) Ltd All rights reserved.