Shapes in the shadow: evolutionary dynamics of morphogenesis.

This article investigates the evolutionary dynamics of morphogenesis. In this study, morphogenesis arises as a side-effect of maximization of number of cell types. Thus, it investigates the evolutionary dynamics of side-effects. Morphogenesis is governed by the interplay between differential cell adhesion, gene-regulation, and intercellular signaling. Thus, it investigates the potential to generate complex behavior by entanglement of relatively "boring" processes, and the (automatic) coordination between these processes. The evolutionary dynamics shows all the hallmarks of evolutionary dynamics governed by nonlinear genotype phenotype mapping: for example, punctuated equilibria and diffusion on neutral paths. More striking is the result that interesting, complex morphogenesis occurs mainly in the "shadow" of neutral paths which preserve cell differentiation, that is, the interesting morphologies arise as mutants of the fittest individuals. Characteristics of the evolution of such side-effects in the shadow appear to be the following: (1) The specific complex morphologies are unique (or at least very rare) among the set of de novo initiated evolutionary histories. (2) Similar morphologies are reinvented at large temporal distances during one evolutionary history and also when evolution is restarted after the main cell differentiation pattern has been established. (3) A mosaic-like evolution at the morphological level, where different morphological features occur in many combinations, while at the genotypic level recombination is not implemented and genotypes diverge linearly and at a constant rate.