Individual differences in developmental precision and fluctuating asymmetry: a model and its implications

In many studies, fluctuating asymmetry (FA) has been used as a measure of i ndividual differences in developmental imprecision. A model of how variatio n in developmental imprecision is associated with variation in asymmetry is described and applied to important issues about FA. If individual differen ces in developmental imprecision exist, asymmetry due to developmental erro r should be leptokurtically distributed. Moreover, the greater the magnitud e of individual differences, the greater the leptokurtosis. Asymmetry purpo rtedly due to developmental error in a variety of species is indeed leptoku rtically distributed. The level of leptokurtosis suggests that the CV in in dividual differences in underlying developmental imprecision is generally 2 0-25, consistent with it being a fitness trait. In addition, data suggest t hat: (1) the individual differences that underlie the developmental impreci sion of different traits are largely shared across traits and not trait-spe cific; (2) the heritability of these individual differences may average bet ween 35 and 55%, despite small heritabilities of individual trait FAs; and (3) correlations between FA and fitness traits or components suggest high c orrelations between underlying variation in developmental precision and fit ness in many species. Theoretical implications are discussed.