A multi-level evolution model where forced extinctions occur throughout the system based on a species fitness value (or survivability) is developed that is essentially the fusion of the evolution model of Bak and Sneppen and the food-web model of Amaral and Meyer. This model is found to describe the fossil record and behave as a self-organized critical system with a power law exponent of approximately 2, but is also found to be remarkably similar to a model that causes the forced extinctions randomly throughout the system. To explain this result we show that fitness is nearly randomly distributed with a slight peak in forced extinction (due to fitness) in the middle levels. These findings lend strong support to the hypothesis that coextinction effects (propagated through a food-web) provide a robust explanation of the fossil record, independent of the mechanism for species competition.
- Self-organized criticality