Sexual size dimorphism in a Drosophila clade, the D. obscura group

The Drosophila obscura clade consists of about 41 species, of which 20 were used for analyses of wing and thorax length. Our primary goal was to investigate the magnitude of sexual size dimorphism (SSD) of these traits within this clade and to test Rensch's Rule [when females are larger than males, SSD (e.g., female/male ratio) should decrease with body size]. Our secondary goal was methodological and involved evaluating for these flies alternative measures of SSD (female/male ratio, female/male absolute difference, female/male relative difference), developing a bootstrap method to estimate the magnitude of intraspecific variation in SSD, and applying a new method of estimating allometric relationships that is phylogenetically based and incorporates error variance in both traits. All indices of SSD were strongly correlated for both size traits. Nevertheless, female/male ratio is the best index here: it is easily interpretable and essentially independent of size. For both traits, SSD (F/M) varied interspecifically, showed a strong phylogenetic signal, but did not differ for the main phylogenetic subgroups or correlate with latitude. Factors underlying variation in SSD in this clade are elusive and might include genetic drift. SSD (wing) tended to decrease with increasing size, as predicted by Rensch's Rule, though not consistently so. SSD (thorax) was unrelated to size. However, analysis of published data for thorax length of Drosophila spp. (N=42) with a larger size range showed that SSD decreased significantly with increasing size (consistent with Rensch's Rule), suggesting our ability to detect SSD-size relations in the D. obscura data may be limited by low statistical power.