Entrenchment as a Theoretical Tool in Evolutionary Developmental Biology

A long-standing challenge to evolutionary theory has been whether it can explain the origin of complex organismal features. We examined this issue using digital organisms--computer programs that self-replicate, mutate, compete and evolve. Populations of digital organisms often evolved the ability to perform complex logic functions requiring the coordinated execution of many genomic instructions. Complex functions evolved by building on simpler functions that had evolved earlier, provided that these were also selectively favoured. However, no particular intermediate stage was essential for evolving complex functions. The first genotypes able to perform complex functions differed from their non-performing parents by only one or two mutations, but differed from the ancestor by many mutations that were also crucial to the new functions. In some cases, mutations that were deleterious when they appeared served as stepping-stones in the evolution of complex features. These findings show how complex functions can originate by random mutation and natural selection.

Comparative developmental evidence indicates that reorganizations in developmental gene regulatory networks (GRNs) underlie evolutionary changes in animal morphology, including body plans. We argue here that the nature of the evolutionary alterations that arise from regulatory changes depends on the hierarchical position of the change within a GRN. This concept cannot be accomodated by microevolutionary nor macroevolutionary theory. It will soon be possible to investigate these ideas experimentally, by assessing the effects of GRN changes on morphological evolution.