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

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.

Genetic Constraints and the Evolution of Display Trait Sexual Dimorphism by Natural and Sexual Selection

“…For example, sex-specific quantitative trait locus (QTL) effects have been detected in Drosophila melanogaster for sexually dimorphic quantitative traits such as life span (Nuzhdin et al 1997;Wilson et al 2006), abdominal pigmentation (Kopp et al 2003), sensory-bristle number (Dilda and Mackay 2002), and CHCs (Foley et al 2007). Sexual dimorphism has also been shown to respond to artificial sexual selection (Wilkinson 1993;Reeve and Fairbairn 1996), and insex # genotype teractions have been reported in analyses of genome-wide transcription in D. melanogaster (Jin et al 2001).…”

The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage

“…This presents a conundrum for evolutionary biologists: how does SD evolve given the evolutionary constraints imposed initially by the shared genomes of the two sexual morphs (Lande, 1980(Lande, , 1987Reeve and Fairbairn, 1996, 1999Fairbairn, 1997;Badyaev, 2002)? In some cases, the sexes can be so disparate as to be unrecognizable as the same species (Darwin, 1871) and yet these highly distinct phenotypes can arise from substantively identical genomes.…”

Sexual Dimorphism in Relation to Current Selection in the House Finch

“…First, sexual dimorphism in a species may be ancestral to ecological divergence or speciation (e.g., Bjö rklund 1991a; Schluter and Price 1993;Price 1998) and thus variation in sexual dimorphism in response to changes in selection may be reduced by patterns set over evolutionary time. Second, sexual selection favoring dimorphism may be opposed by natural selection on the same traits (e.g., Howard 1981;Price 1984a,b;Weatherhead et al 1987;Fairbairn and Preziosi 1996;Wikelski and Trillmich 1997), selection on closely correlated traits in the opposite sex (Lande 1980;Reeve and Fairbairn 1996), or selection during different life stages (reviewed in Schluter et al 1991). Alternatively, no relationship is expected when sexually dimorphic traits lack appropriate genetic variability or are highly genetically correlated (e.g., by linkage, epistasis, or pleiotropy) or when phenotypic plasticity (such as behavioral modification of displays) reduces selection pressures on sexually dimorphic traits (Badyaev and Hill 1999).…”