This assumption can lead to indefinite improvement or deterioration of protein function. Alternatively, the later “fixed model” fixes the distribution of mutations’ effect on protein function, but allows the mean fitness of population to evolve. This allows the distribution of  to change with the mean fitness of population.
This assumption can lead to perpetual improvement or decline of protein performance. Alternatively, a later "stable model" stabilizes the impact of mutations on protein functionality, while allowing for evolutionary changes in the population's average fitness.