DIRECTIONAL ASYMMETRY AND THE MEASUREMENT OF DEVELOPMENTAL INSTABILITY

Three widely used methods of estimating fluctuating asymmetry may yiel d serious overestimates if directional asymmetry is present. When two sides of a bilateral trait grow at different rates, then the asymmetry variance (Var[l-r]) increases with size, even when developmental nois e is nil. But the residual variance around a population's mean develop mental trajectory is invariant with respect to size. Thus, it can be u sed as a measure of developmental instability. We introduce a measure of developmental instability, the residual variance (s(delta)(2)), obt ainable from either a major axis regression: which is equivalent to a principal component analysis on l and r, or a general structural model . This residual variance can be estimated from directionally asymmetri c or even antisymmetric traits. We present examples of developmental i nstability estimated from directionally asymmetric mandibles (house mo use) and leaves (soybean), and antisymmetric flaws (fiddler crab). (C) 1998 The Linnean Society of London.