This is the quasispecies equation. It describes constant
selection with mutation. The average fitness of the
population is given by f ¼
P
i aixi : Quasispecies dynamics
have been studied in stochastic and deterministic
frameworks (Fontana and Schuster, 1987; Swetina and
Schuster, 1988; Eigen et al., 1989; Nowakand Schuster
1989)