Anomalous evolution of nonsinglet nucleon structure functions

We calculate the scale dependence of nonsinglet nucleon structure functions. Due to anomalous axial symmetry breaking a large flavour asymmetry of the quark--antiquark sea is generated nonperturbatively. This produces a strong scale dependence of the nonsinglet structure function in an intermediate range of Q^2. Evolving nonperturbatively a pure valence distribution from an infrared scale we can thus compute F_2^p-F_2^n as measured by the NMC, and give detailed predictions for its Q^2 dependence at fixed x. We also compare our results with Drell--Yan data.