1. Introduction
Evolution is the interplay of mutation and selection.
Mutation means that genomes are passed on from
parent to offspring with some changes. Selection means
that fitter individuals reproduce faster than others.
Fitness can be frequency dependent or not. Frequency
dependence emerges when the fitness of a phenotype
depends on the frequency of itself and other phenotypes
in the population. Here we study constant fitness.
Genomes are given by sequences, and each sequence
has a constant fitness value. Genomes are arranged in
sequence space and adapt on a fitness landscape.
Wright (1932) introduced fitness landscapes, while
Maynard Smith (1970) invented sequence space. Eigen
and Schuster (1977) combined these two ideas. In the
formalism of quasispecies theory, there is a replicating
population of RNA or DNA genomes. Let us consider
binary sequences of length n. There are 2n possible
sequences. Sequences replicate subject to mutation. Let
us only consider point mutations. Denote by u the