  The Bak-Sneppen model is shown to fall into a different universality class
with the introduction of a preferred direction, mirroring the situation in spin
systems. This is first demonstrated by numerical simulations and subsequently
confirmed by analysis of the multi-trait version of the model, which admits
exact solutions in the extremes of zero and maximal anisotropy. For
intermediate anisotropies, we show that the spatiotemporal evolution of the
avalanche has a power law ``tail'' which passes through the system for any
non-zero anisotropy but remains fixed for the isotropic case, thus explaining
the crossover in behaviour. Finally, we identify the maximally anisotropic
model which is more tractable and yet more generally applicable than the
isotropic system.
