Fluctuating asymmetry and developmental stability: heritability of observable variation vs. heritability of inferred cause

Fluctuating asymmetry subtle random deviations from perfect bilateral symmetry ~ is an appealing measure of developmental precision because of the apparent ease with which it may be measured and because its developmental origins seem so straightforward (Palmer, 1996). This appeal has led to its wide application as a measure of developmental stability in studies of inbreeding and outbreeding depression (reviewed in Palmer and Strobeck, 1986; Graham, 1992), as a measure of genetic or environmental stress in biomonitoring studies (Leary and Allendorf, 1989; Graham et al., 1993), and most recently as a measure of fitness or mate quality in studies of sexual selection (e.g., see Moller, 1994; Tomkins and Simmons, 1995). Criticisms of the uses of fluctuating asymmetry (FA) have focused mainly on methodological issues (Palmer and Strobeck, 1992; Fields et al., 1995) but doubts have also been raised about the developmental origins of subtle bilateral variation (reviewed in Markow, 1994; Palmer, 1996). If proponents of FA are not more reflective about these methodological and conceptual issues, the whole approach may become tarnished. This point was made emphatically by Phil Hedrick in his concluding comments to the symposium on developmental instability at which both Moller and Thornhill were present (Markow, 1994, pp. 4344435). For bilateral variation to serve as a credible measure of developmental precision, three essential statistical tests of procedures must be applied (Palmer, 1994, 1996; Swaddle et al., 1994). First, because true FA (the result of real differences between