Some Techniques for the Measurement of Complexity in Tierra

Abstract

Recently, Adami and coworkers have been able to measure the information content of digital organisms living in their Avida artificial life system. They show that over time, the organisms behave like Maxwell’s demon, accreting information (or complexity) as they evolve. In Avida the organisms don’t interact with each other, merely reproduce at a particular rate (their fitness), and attempt to evaluate an externally given arithmetic function in order win bonus fitness points. Measuring the information content of a digital organism is essentially a process of counting the number of genotypes that give rise to the same phenotype.

Whilst Avidan organisms have a particularly simple phenotype, Tierran organisms interact with each other, giving rise to an ecology of phenotypes. In this paper, I discuss techniques for comparing pairs of Tierran organisms to determine if they are phenotypically equivalent. I then discuss a method for computing an estimate of the number of phenotypically equivalent genotypes that is more accurate than the “hot site” estimate used by Adami’s group. Finally, I report on an experimental analysis of a Tierra run.