Replaying the Tape: An Investigation into the Role of Contingency in Evolution

The role of contingency (random events) in an arti-cial evolutionary system is investigated by running the system a number of times under exactly the same conditions except for the seed used to initialize the random number generator at the beginning of each run. Twelve diierent measures were used to track the course of evolution in each run, and \activity wave diagrams" were also produced (Bedau & Brown 1997). The results of 19 runs are presented and analyzed. The performance of every run was compared with each of the others using a non-parametric test (a randomiza-tion version of the paired-sample t test). When comparing absolute values of the measures between the runs, some signiicant diierences were found. However , looking at the diierence in values between adjacent sample points for a run, no run was signiic-antly diierent to any other for any of the measures. This suggests that the general behaviour is the same in all runs, but the accumulation of diierences results in signiicantly diierent outcomes. The results lead us to propose a rule of thumb for future experiments with the system: to check whether the outcome of any particular experiment is robust to contingency in the evolutionary process, at least nine runs should be conducted using diierent seeds for the random number generator, to be conndent of seeing a variety of results. The results are likely to be applicable to other A-Life platforms of self-replicating computer programs, but at this stage can probably tell us little about the role of contingency in biological evolution.