Before revision: In this article, a biallelic reversible mutation model with directional and quadratic selection is analyzed that reconnects to an approach proposed by Kimura 20 . Kimura starts from a diffusion model and derives its equilibrium distribution up to a constant.
After revision: In this article, a biallelic reversible mutation model with directional and quadratic selection is analyzed that reconnects to an approach proposed by Kimura 20 ( Possibility of extensive neutral evolution under stabilizing selection with special reference to nonrandom use of codons (PNAS,1981)), who starts from a diffusion model and derives its equilibrium distribution up to a constant.