Directional asymmetry and the measurement of developmental instability

Abstract

Three widely used methods of estimating fluctuating asymmetry may yield serious overestimates if directional asymmetry is present. When two sides of a bilateral trait grow at different rates, then the asymmetry variance war[/-r]) increases with size, even when developmental noise is nil. But the residual variance around a population's mean developmental trajectory is invariant with respect to size. Thus, it can be used as a measure of developmental instability. We introduce a measure of developmental instability, the residual variance ( f h ) , obtainable from either a major axis regression, which is equivalent to a principal component analysis on 1 and r, or a general structural model. This residual variance can be