The relationship between robustness and evolvability is complex because robust populations harbour a large diversity of neutral genotypes that may be important in adaptation. Although neutral mutations do not change an organism’s phenotype, they may nevertheless have epistatic consequences for the phenotypic effects of subsequent mutations. In particular, a neutral mutation can alter an individual’s ‘phenotypic neighbourhood’, that is, the set of distinct phenotypes that the individual can access through a further mutation. Pioneering studies based on RNA folding and network dynamics suggest that genotypes expressing a particular phenotype are often linked by neutral mutations into a large neutral network, and that members of a neutral network differ widely in their phenotypic neighbourhoods. Numerous studies have documented the importance of neutral variation in allowing a
population to access adaptive phenotypes, and neutral networks have consequently been proposed to facilitate adaptation. (Draghi et al. Nature Vol. 463, 21 January 2010).