Punctuated equilibria or gradual evolution: fluctuating asymmetry and variation in the rate of evolution.

The debate on long-term rates of evolution is polarized around two models: the punctuated equilibrium hypothesis (long periods of very little evolution interrupted by periods of relatively rapid evolutionary change) and the gradual view (the rate of evolution is gradual, albeit with variable rate). Here we suggest that the pace of evolutionary change can be independently determined by estimating the degree of trait asymmetry. Fluctuating asymmetry occurs as a result of random deviations in the development of bilaterally symmetric traits. The degree of fluctuating asymmetry closely reflects the ability of individuals to cope with stress. Traits subject to strong directional selection become less developmentally canalized and more susceptible to the influence of stress. Strong stabilizing selection has the opposite effect. Characters of individuals from periods of rapid evolutionary change should hence be characterized by high variability, high fluctuating asymmetry and fluctuating asymmetry should increase with trait size in the direction of selection. In contrast, characters of individuals from periods of little evolutionary change should demonstrate low variability, low asymmetry and a flat or U-shaped relationship between size and asymmetry.