  A punctuated equilibrium model of biological evolution with relative fitness
between different species being the fundamental driving force of evolution is
introduced. Mutation is modeled as a fitness updating cellular automaton
process where the change in fitness after mutation follows a Gaussian
distribution with mean $x>0$ and standard deviation $\sigma$. Scaling behaviors
are observed in our numerical simulation, indicating that the model is
self-organized critical. Besides, the numerical experiment suggests that models
with different $x$ and $\sigma$ belong to the same universality class. PACS
numbers: 87.10.+e, 05.40.+j
