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Back to predicting genetic response to selection. In the figure 2 below you see the process of selection and response in a schematic overview. The two normal distributions represent two generations. The top distribution is the generation of the parents. The best animals are selected for breeding. The other animals are not reproducing within the breeding program. The selected parents perform better than the population average. The size of the difference in performance between the parents and the total population (i.e. the superiority of the parents) is called the Selection differential, abbreviated as S. Note that ‘performance’ in this case indicates the selection criterion. That could be the phenotype (mass selection) or the EBV. The selected parents produce the next generation (lower distribution). This generation on average will perform better than the previous generation. The difference in average performance between both generations is called the selection Response, abbreviated as R. In general, the offspring generation will not perform as well as the average of the parents. Why is that? Because selection was not based on the true genetic potential (TBV) of the animals, but on an estimate of that (EBV). It depends on the quality of the estimate how big the difference is between the selection differential and the selection response. The better the estimate, the closer the performance of the offspring will be to the performance of the selected parents. However, also the poorer the estimate the more often wrong selection decisions will be made because the genetically best animals have not been recognised as such. This results in less genetic gain. Note that lack of accuracy of the EBV (almost) never results in a selection response that is larger than the selection differential. The only situation in which this may happen is in case of cross breeding (see that chapter for further explanation). With within population selection the aim is to select the very best animals as parents. Any inaccuracies will lead to a result (offspring) that perform less, not better, than expected.
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Thus:
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Thus: predicting genetic gain is about prediction the future: how much better will the offspring perform compared to the current generation. The superiority of the selected parents in comparison to their generation is called the selection differential (S). The superiority of the offspring in comparison to their parents is called the selection response (R). |