Modularity in Evolution: Some Low-Level Questions

Intuitive notions about the advantages of modularity for evolvability run into the problem of how we parse the organism into traits. In order to resolve the “question of multiplicity”, there needs to be a way to get the human observer out of the way, and define modularity in terms of physical processes. I will offer two candidate ideas towards this resolution: • the dimensionality of phenotypic variation, and • the causal screening off of phenotypic variables by other phenotypic variables. With this framework, the evolutionary advantages that have been attributed to modularity do not derive from modularity per se. Rather, they require that there be an “alignment ” between the spaces of phenotypic variation, and the selection gradients that are available to the organism. Modularity may facilitate such alignment, but it is not sufficient; the appropriate phenotype-fitness map in conjunction with the genotypephenotype map is also necessary for evolvability. 1 The Question of Multiplicity A good deal of work in recent years has shown that the structure of the genotype-phenotype map is of fundamental importance to the process of evolution. The variational properties