A meta-analysis of fluctuating asymmetry in relation to heterozygosity

Fluctuating asymmetry, the random departure from perfect bilateral symmetry , is a common measure of developmental instability that has been hypothesiz ed to be inversely correlated with heterozygosity. Although this claim has been widely repeated, several studies have reported no such association. Th erefore, we test the generality of this association, using meta-analysis, b y converting test statistics for the relationship between heterozygosity (H ) and fluctuating asymmetry (FA) into a common effect size, the Pearson's p roduct-moment correlation coefficient. We have analysed a database containi ng 41 studies with a total of 118 individual samples. Overall we found an u nweighted mean negative effect size; r = -0.09 (i.e. a negative correlation between H and FA). Significant heterogeneity in effect size was mainly cau sed by a difference between ectothermic and endothermic animals, and to a l esser extent by the use of different study designs (i.e. within-population vs. among-populations). Mean effect size for endothermic animals was positi ve and significantly different from the mean effect size for ectothermic an imals. Only for within-population studies of ectothermic animals did we fin d a significantly negative effect size (r= -0.23 +/- 0.09). The distributio n of effect sizes in relation to sample size provided little evidence for p atterns typical of those produced by publication bias. Our analysis suggest s, at best, only a weak association between H and FA, and heterozygosity se ems to explain only a very small amount of the variation in developmental i nstability among individuals and populations (r(2) = 0.01 for the total mat erial).