Abstract
It is well known that random fluctuations in gene frequencies are due to the process of sampling with which zygotes are produced at every generation, that they are among the causes of evolution by determining what is somewhat inappropriately called ‘genetic drift’, and that this was the centre of an early and substantially unsettled controversy between Sir Ronald Fisher and Sewall Wright. It is, perhaps, less widely appreciated that Sir Ronald, who was against the importance of random fixation, was also the first to consider it theoretically under the name of ‘Hagedoorn effect’ (Fisher 1922). The reason that led Fisher to a negative conclusion is summarized in the relationship between the chance of success of the rare mutant and its selective advantage. This relationship is given in detail in his 1930 paper on the distribution of gene ratios for rare mutations, in which it is shown that the chance of success rises almost proportionately to the selective advantage, in such a way that even small changes in the selective coefficient affect deeply the chance for success. Clearly, the overall chance with which mutants showing selective disadvantage will be fixed depends also on the relative frequency with which such mutants arise, that is, on the distribution of selective coefficients for mutations.