(7) is a generalisation of the Ornstein–Uhlenbeck process for phenotypic DZNep described by Bedford and Hartl (2009), but for an arbitrary free fitness landscape and including the correct population size dependence of the strength of the drift term via the Einstein relation Eq. (5). This set of stochastic differential equations is similar to those used by Nei et al. (1983), but includes the effects of sequence entropy. Eq.