Internal Constraints and Ecology in Evolution: A Case Study in Tierra

摘要

We study the evolution of self-replicating assembly language computer programs in T. S. Ray's Tierra system, using a variant mechanism for the replicators to determine their own size. A new ancestor is introduced which uses inherited code for carrying arithmetic operations that give its correct size rather than self-measuring (as in the original Tierra studies). The courses of evolution from Ray's ancestor and our new ancestor are compared. The mechanism for size determination used by replicators proves to be remarkably conserved. Largely similar ecologies evolve in both cases. Parasites, hyperparasites, recurring immunity to parasites and its circumvention are all observed in evolution from either ancestor. But niches for social replicators and social cheating do not appear to arise (apparently due to lack of necessity of end templating in replicators). However a new type of metabolic informational parasitism occurring in evolution from the new ancestor is characterized and exemplified. This new opportunity creates a set of new parasite and hyperparasite niches. Self-measurement vs. self-description for a replicator to determine its own size proved to be an inviolable internal constraint of evolution in these studies. In turn, this affects some aspects of variability (e.g. of size and also in the use of end-marker templating). Altering the size determination mechanism leaves much of the ecological dynamics unchanged, but has an extreme impact on the potential for certain types of niche creation and subsequent ecological dynamics. Finally the smallest known Tierrans using each of these two strategies (self-measuring and genetic encoding of size calculation) are given explicitly.