Before revision:  Notwithstanding this , a high mutation rate, in combination with a low aspiration level, has positive nonlinear effects, and a heterogeneous system achieves a higher level of co-operation relative to a homogeneous system .
After revision:  Notwithstanding this , a high mutation rate, in combination with a low aspiration level, has positive nonlinear effects, and a heterogeneous system achieves a higher level of cooperation than the weighted average of homogeneous populations .