Figure 1. Sequence space of RNA genotypes and the neutral networks they form.

Genotypes (spheres) encoding the same phenotype (p₁ to p₂₅) are connected via neutral mutations (with probability q). Non-neutral mutations (with probability 1−q) are only explicitly shown (dashed connections) for one highlighted genotype (black circle). The number of phenotypes available to this genotype is K=4, these are drawn in color (p₁ to p₄). The yellow neutral network (p₁) is under selection and has high fitness (large spheres), all other genotypes are lethal. After an environmental shift the blue phenotype (p₂) might become beneficial so that the highlighted genotype could make a transition with only one mutation (black arrow).