关于选择极限问题的讨论

关于选择是否有极限的问题,在国内外都有所争议。例如,根据数量遗传学的原理和方法进行选种,从长远的观点来看是否还能继续有效的问题,是许多读者很感兴趣的。下面刊出李明定和吴常信两位先生撰写的《关于选择极限问题的讨论》一文,我们希望对这一问题的讨论能有所帮助。

Discussion on the Limit of Selection

In Long-term artificial selection experiments, there are two main theories available.One is the limited theory developed by Robertson(1960) and another was developed by Hill(1982a, 1982b), termly unlimited theory .Robertson's theory shows that the additive g'enetic variance will be completely exhausted after selected on a quantitative trait for about 20 or more generations and at that time the response to selection will reach a limit.While Hill's theory dees think that the additive genetic variance will reach a value of 2NeVm after selected for 25 to 35 generations initiated from a homozygous line and it retains this value in the subsequent generations. There is no limit for the response to directional selection from Hill's theory. The main difference between these two theories is that possible contributions of mutations occurring after the selection has started were ignored by Robertson's theory. Many experiments suggested that mutations should not be ignored in artificial selection experiments. Mutation variance is a very important parameter in measuring the importance of mutation in a specifically selected population. It cannot, however, be used to compare different traits and species because of different dimension. But mutation heritability can do because it is expressed as a ratio of mutation variance to the environmental variance. The mutation heritabilities for most traits of varied species were estiamted to be 10~(-8) or so.At present, most animal and plant species are viewed as a stage in that all the useful initial variation has been used up or almost and that being utilized has arisen by mutation in the population under selection.