Schematic drawing of a fitness landscape. Circles represent viable sequences, and crosses represent non-viable ones. The sequence at d = 0 serves as the reference sequence. It has two viable and a number of non-viable mutational neighbors. The viable mutants have further viable and non-viable mutational neighbors, and so on. All viable mutants on the lower branch form a single neutral network. On the upper branch, a new neutral network emerges at a mutational distance d = 3 from the reference sequence.