  We consider the asymptotic behavior of an evolving weakly coupled
Fokker-Planck system of two equations set in a periodic environment. The
magnitudes of the diffusion and the coupling are respectively proportional and
inversely proportional to the size of the period. We prove that, as the period
tends to zero, the solutions of the system either propagate (concentrate) with
a fixed constant velocity (determined by the data) or do not move at all. The
system arises in the modeling of motor proteins which can take two different
states. Our result implies that, in the limit, the molecules either move along
a filament with a fixed direction and constant speed or remain immobile.
