Change in sexual size dimorphism as a correlated response to selection on fecundity.

Fecundity selection is often suggested as the main causal factor underlying the prevalence of female-biased sexual size dimorphism (SSD), but this assumption has not been empirically tested. We selected female Drosophila melanogaster for increased or decreased fecundity (eggs laid over a single 18-h period, between days 5 and 7 posteclosion) for 20 generations, to see what effect this would have on SSD in three morphological traits (thorax width, abdomen width and thorax length). A direct response to fecundity selection was found in the downward direction (16.6%), whereas the response to upward selection (5.7%) was not statistically significant. Significant sex by selection interaction terms in the ANOVAs for thorax width and abdomen width indicate that the two sexes responded differently. Females usually showed a greater correlated response than males. In lines selected for increased fecundity, the correlated response in females for thorax and abdomen width was greater than the direct response in standard deviation units. SSD generally increased with selection for increased fecundity, but showed no consistent trend with selection for decreased fecundity. These results support the general hypothesis that SSD can evolve rapidly in response to fecundity selection. Selection on fecundity also produced correlated responses in life history traits. Downward selection resulted in flies that had lower viability and longevity, and both directions of selection were associated with an increase in development time.