The fitness of manipulating phenotypes: Implications for studies of fluctuating asymmetry and multivariate selection

Phenotypic manipulation (or phenotypic engineering) that alters trait distr ibutions provides a way to increase the statistical power of detecting rela tionships between traits and fitness. Manipulations relying on plastic resp onses, however, assume a specific relationship between the perturbation and the alteration of the traits when multiple traits are involved. We measure d several traits, including condition measured as fluctuating asymmetry, in the ladybird beetle Harmonia axyridis under six different diets to examine how altered environments affected multiple traits and their distributions. Although diet affected fluctuating asymmetry, we found no consistent relat ionship between degree of asymmetry and other phenotypic measures. As expec ted, individual traits were altered by our treatments. Contrary to expectat ion, relationships among traits were not constant among diets. Our results suggest that assumptions about the relationship between condition and trait values, especially fluctuating asymmetry, cannot be made. Further, studies that use manipulated phenotypes to statistically determine the form of sel ection must first demonstrate that the pattern of the phenotypic correlatio n matrix is not itself altered by the manipulation. If the phenotypic corre lation matrix is not constant, then experimental estimates of selection coe fficients may not reflect selection that occurs in the wild.