Artificial Life VI

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

Tim Taylor
Department of Artificial Intelligence, University of Edinburgh, UK

John Hallam
Department of Artificial Intelligence, University of Edinburgh, UK


The role of contingency (random events) in an artificial 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 different measures were used to track the course of evolution in each run, and ``activity wave diagrams'' were also produced. 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 randomization version of the paired-sample t test). When comparing absolute values of the measures between the runs, some significant differences were found. However, looking at the *difference* in values between adjacent sample points for a run, no run was significantly different to any other for any of the measures. This suggests that the general behaviour is the same in all runs, but the accumulation of differences results in significantly different 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 different seeds for the random number generator, to be confident 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.

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