Limits in recurrent selection. I. Factors affecting the increment in allelic frequencies

Abstract

The limit of intrapopulational recurrent selection and the main factors affecting it were investigated, through the total change in allelic frequency, until total depletion of genetic variability, and was computed through the probability of ultimate fixation of favorable alleles. Half mass selection was considered as reference selection scheme, in a panmitic species. For different combinations of the main factors 360 values of the total change were calculated. Despite the restrictions, it was concluded that selection limit is strongly unfavored when we combine: a low coefficient of heritability (h²), small effective population size (N_e), weak selection intensity, low initial frequency of favorable alleles and a trait controlled by loci with small effects. When one of these factors is at an inadequate level, it can be compensated through changes in the others. High N_e values, during selection cycles, can be chosen for such compensation, provided k is not reduced correspondingly, since, in this case, immediate progress will be reduced. Low experimental precision reduces h², so a high N_e is needed. The product N_eh is a good measure of how much N_e a should be increased if h² is reduced. The model was applied to corn yield data, and the use of local populations for breeding purposes, which have as only advantage its adaptations, was commented. It was also verified that 80% to 85% of the total change in allelic frequency variation is due to factors under the control of the breeder. This indicates that schemes can be chosen not only to assure adequate immediate progress but also appropriate selection limits.