The article by Motoo Kimura discusses the probability of fixation of mutant genes in a population, emphasizing that this probability is influenced not only by selection but also by random fluctuations. Kimura extends previous work by Fisher, Haldane, and Wright to include any level of dominance and introduces a more general formula for the probability of fixation, \( u(p) \), which accounts for random fluctuations in selection intensity and small population sizes. The formula is derived using a continuous stochastic process and is expressed in terms of the initial frequency, selection coefficients, and effective population number. Kimura applies this formula to various scenarios, including genic selection and zygotic selection, and explores the effects of random fluctuations in selection intensity on the fixation probability. He demonstrates that even in large populations, an advantageous gene can be lost due to fluctuating selective values. The article also addresses the question of "quasi-fixation" and provides conditions under which an advantageous gene might be lost despite its average advantage.The article by Motoo Kimura discusses the probability of fixation of mutant genes in a population, emphasizing that this probability is influenced not only by selection but also by random fluctuations. Kimura extends previous work by Fisher, Haldane, and Wright to include any level of dominance and introduces a more general formula for the probability of fixation, \( u(p) \), which accounts for random fluctuations in selection intensity and small population sizes. The formula is derived using a continuous stochastic process and is expressed in terms of the initial frequency, selection coefficients, and effective population number. Kimura applies this formula to various scenarios, including genic selection and zygotic selection, and explores the effects of random fluctuations in selection intensity on the fixation probability. He demonstrates that even in large populations, an advantageous gene can be lost due to fluctuating selective values. The article also addresses the question of "quasi-fixation" and provides conditions under which an advantageous gene might be lost despite its average advantage.