The effect of initial reverse selection upon total selection response

A computer has been used to investigate the effect of an initial period of reverse selection on the subsequent response of a population to renewed forward selection with the same population size and selection intensity. As the computer was used to derive gene frequency distributions, there was no random element in the results obtained. A theoretical solution to the problem was obtained for genes with small effects. The process can be adequately described by the duration of the reverse selection (expressed in terms of the population size N ), the product of population size and gene effect, Ns , and the initial gene frequency. If the duration of reverse selection, t , is less than N /2, the loss in selection advance due to the reverse selection is roughly t/N , though slightly greater than this for genes with low frequency. The ‘point of no return’ after which it is impossible, with the same population size and selection intensity, to return even to the starting frequency is 1·4 N generations for genes with small effect and this declines as the gene effect increases. Some extension of results to recessive genes is also given.