Maximum principle and mutation thresholds for four-letter sequence evolution

A four-state mutation–selection model for the evolution of populations of DNA sequences is investigated with particular interest taken in the phenomenon of error thresholds. The mutation model considered is the Kimura 3ST mutation scheme; the fitness functions, which determine the selection process, come from the permutation invariant class. Error thresholds can be found for various fitness functions; the phase diagrams are more interesting than for equivalent two-state models. Results for (small finite sequence lengths are compared with those for infinite sequence length, obtained via a maximum principle that is equivalent to the principle of minimal free energy in physics