  We considered a stochastic version of the Bak-Sneppen model (SBSM) of
ecological evolution where the the number $M$ of sites mutated in a mutation
event is restricted to only two. Here the mutation zone consists of only one
site and this site is randomly selected from the neighboring sites at every
mutation event in an annealed fashion. The critical behavior of the SBSM is
found to be the same as the BS model in dimensions $d$ =1 and 2. However on the
scale-free graphs the critical fitness value is non-zero even in the
thermodynamic limit but the critical behavior is mean-field like. Finally $<M>$
has been made even smaller than two by probabilistically updating the mutation
zone which also shows the original BS model behavior. We conjecture that a SBSM
on any arbitrary graph with any small branching factor greater than unity will
lead to a self-organized critical state.
