We introduce a simple model which shows non-trivial self organized cri
tical properties. The model describes a system of interacting units, m
odelled by Polya urns, subject to perturbations and which occasionally
break down. Three equivalent formulations - stochastic, quenched and
deterministic - are shown to reproduce the same dynamics. Among the no
vel features of the model are a non-homogeneous stationary state, the
presence of a non-stationary critical phase and non-trivial exponents
even in mean field. We discuss simple interpretations in term of biolo
gical evolution and earthquake dynamics and we report on extensive num
erical simulations in dimensions d = 1, 2 as well as in the random nei
ghbors limit.