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