Neutral evolution of mutational robustness
In evolution of ribozymes in vitro, mutations responsible for an increase in fitness are only a small minority of the total number of accepted mutations (see Continuous in vitro evolution of catalytic function.). This fact indicates that, even in adaptive evolution, the majority of point mutations is neutral. This is the basis of Kimura's neutral theory of evolution, see the paper.
A neutral network is a collection of mutually neutral genotypes (i.e. producing the same phenotype, whether structure or function), which are connected via single mutational steps; they sometimes form extended networks that permeate large regions of genotype space. A population is mutationally robust (insensitive against mutations) when it inhabits a highly interconnected region of the network so that most mutations lead to the same neutral network, thus leaving the phenotype unchanged.
In Neutral evolution of mutational robustness, the authors found analytically, that for a range of population sizes and mutation rates of biological interest, the population's distribution over a neutral network is determined solely by the network's topology.