The Genetics and Evolution of Fluctuating Asymmetry

Variation in the subtle differences between right and left sides of bilateral characters, or fluctuating asymmetry (FA), has long been considered to be primarily environmental in origin, and this has promoted its use as a measure of developmental instability (DI) in populations. There is little evidence for specific genes that govern FA per se. Numerous studies show that FA levels in various characters are influenced by dominance and especially epistatic interactions among genes. An epistatic genetic basis for FA may complicate its primary use in comparisons of DI levels in outbred or wild populations subjected or not subjected to various environmental stressors. Although the heritability of FA typically is very low or zero, epistasis can generate additive genetic variation for FA that may allow it to evolve especially in populations subjected to bottlenecks, hybridizations, or periods of rapid environmental changes caused by various stresses.