A Model

We outline here a very preliminary result from exploring this issue in the Tierra system. The system was seeded with an ancestor program, designed to reproduce genetically rather than by template copying. The genotype to phenotype mapping consisted of a simple recoding of each allowed machine word by a different word, via a lookup table. The lookup table itself was explicitly coded for in the genotype. Thus, mutations in the section of the genotype coding for the lookup table would result in a different table in the offspring, and thus a different mapping from genotype to phenotype in successive offspring in such a lineage.

A number of experiments have been performed on the subsequent evolutionary behaviour in this system. These will not be presented in detail here: however, we have indeed detected the emergence of new programs with mutated genotype-phenotype mappings (in the sense of mutated translation tables) which, nonetheless, subsequently breed true. To test the degree of change in mapping, we have artificially transplanted the genotype from such a (remote) descendant back into the original ancestor phenotype and verified that it cannot recreate the descendant (precisely because this genotype does not represent the descendant phenotype relative to the mapping implemented by the ancestor).

Now the nature of the very simple genotype to phenotype mappings used in these particular experiments means that we do not expect that the changes in mapping reported above would actually amount to evolutionarily significant changes in the mutational connectivity of the phenotype space; but they do concretely demonstrate that evolution in the genotype-phenotype mapping is, at the very least, possible.

Barry.McMullin@rince.ie