  We study invasion fronts and spreading speeds in two component
reaction-diffusion systems. Using a variation of Lin's method, we construct
traveling front solutions and show the existence of a bifurcation to locked
fronts where both components invade at the same speed. Expansions of the wave
speed as a function of the diffusion constant of one species are obtained. The
bifurcation can be sub or super-critical depending on whether the locked fronts
exist for parameter values above or below the bifurcation value. Interestingly,
in the sub-critical case numerical simulations reveal that the spreading speed
of the PDE system does not depend continuously on the coefficient of diffusion.
