Sexual Size Dimorphism as a Correlated Response to Selection on Body Size: An Empirical Test of the Quantitative Genetic Model

Abstract

Abstract.-We artificially selected for body size in Drosophila melanogaster to test Lande's quantitative genetic model for the evolution of sexual size dimorphism. Thorax width was used as an estimator of body size. Selection was maintained for 21 generations in both directions on males only, females only, or both sexes simultaneously. The correlated response of sexual size dimorphism in each selection regime was compared to the response predicted by four variants of the model, each of which differed only in assumptions about input parameters. Body size responded well to selection, but the correlated response of sexual size dimorphism was weaker than that predicted by any of the variants. Dimorphism decreased in most selection lines, contrary to the model predictions. We suggest that selection on body size acts primarily on growth trajectories. Changes in dimorphism are caused by the fact that male and female growth trajectories are not parallel and termination of growth at different points along the curves results in dimorphism levels that are difficult to predict without detailed knowledge of growth parameters. This may also explain many of the inconsistent results in dimorphism changes seen in earlier selection experiments.Key words.-Artificial selection, body size, Drosophila melanogaster, growth trajectories, sexual size dimorphism.Received June 27, 1995. Accepted March 19, 1996.The total response of each sex is due to the direct response to selection on that sex and the correlated response to selection on the other sex. Therefore, where the subscripts m and where h 2 is the narrow-sense heritability and S is the selection differential. The selection differential is often standardized by dividing by the phenotypic standard deviation Up to give the selection intensity i, so that(Leutenegger and Cheverud 1985; based on Falconer 1981). Dimorphism can then be calculated by comparing the relative responses in each sex. For the remainder of this paper, SSD r will refer to the size ratio of the larger sex to the smaller, and SSD d to the size of the larger sex minus that of the smaller. Where dimorphism in general is being discussed, SSD will be used.If the genetic correlation between the sexes is high, selection for a change in body size in one sex will result in a strong correlated response in the other and SSD will be difficult to alter. This idea was first expressed qualitatively by Fisher (1958) and later quantified by Lande (1980). Lande converted the standard quantitative genetics formulae (3a,b) into a set of recursion equations that can be used to follow the trajectories of the two sexes under a given set of parameter values. His model also incorporated the effect of stabilizing natural selection favoring some optimum body size value in each sex, which provides a means of producing a response plateau as the extrinsic selective force (e.g., sexual selection) moves the organisms away from their original optimal size. The standard response equation (1) has no built-in constraints, so it cannot predict respon...

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