| Abstract: | Fisher proposed that natural selection would adjust the population sex ratio so that parental expenditure on sons equals expenditure on daughters. Thus if two daughters can be produced for every son, the Fisherian equilibrium is ? sons and ? daughters. The relative cost of a son versus a daughter is necessarily manifested in the trade-off between family size and sex ratio, and we offer a method to estimate this trade-off from data on family compositions. Simulation studies indicate that the method works well in some cases but not others. Application of the method to data on a polychaete suggests that sons are much costlier than daughters; the observed sex ratio in fact significantly favored daughters, but not to the extreme predicted by our measure of differential cost. |
