In this paper, we wish to present some simpli¯ed cases of discrete Bak-
Sneppen models in which explicit computations via Markov chains are possible, hence
reaching a better understanding of some rather hidden phenomena of the general case:
in particular "avalanches" can be read in terms of mean waiting times and in terms of
transitions between structures. The simple models allow us to introduce new frames
that do not seem to have been considered in the previous literature, namely the case of
partitioned Bak-Sneppen frames, that appear more realistic from the point of view of
speed of evolution and do not present a unique criticality level, but a staircase tending
towards a ¯nal equilibrium level, cadenced by an increasing sequence of footholds. The
introduction summarizes Bak-Sneppen models, starting from the central model due to
Bak and Sneppen, and recalls their use in applied sciences. The ¯rst section gives the ge-
neral frame of models where locality and globality coexist, the second section shows the
simplest case of a matching between locality and globality, that will become exemplar
in the most complex frames of Bak-Sneppen processes. The main quantitative theorems
are stated and proved in the third section and ¯nally the fourth section presents exam-
ples that illustrate the more sophisticated points of our paper and the use (and limits)
of experimental results, while the ¯fth section considers real world situations where Bak
Sneppen partitioned schemes can be tailored to represent the core of their evolution.
