Nei M
Center for Demographic and Population Genetics, University of Texas Health Science Center, Houston 77225.
Philos Trans R Soc Lond B Biol Sci. 1988 Jul 6;319(1196):615-29. doi: 10.1098/rstb.1988.0069.
The extent and pattern of protein and DNA polymorphisms are discussed with emphasis on the mechanism of maintenance of the polymorphisms. Statistical studies suggest that a large proportion of genetic variability at the molecular level is maintained by a mutation-drift balance. At some loci, such as those for histocompatibility in mammals, however, a form of overdominant selection seems to be involved. In the presence of overdominant selection, polymorphic alleles may be maintained for tens of millions of years, so that the number of nucleotide differences between alleles is often very large, as in the case of self-incompatibility alleles in plants. There are also an increasing number of examples in which an adaptive change of a morphological or physiological character is caused by a single nucleotide substitution. Nevertheless, these mutations seem to be a small proportion of the total nucleotide changes that contribute to genetic variability and evolution. Although there are many examples of frequency-dependent selection, this form of selection is apparently unimportant for the maintenance of genetic variability except in some special cases. Observations on the evolutionary change of DNA suggest that the driving force of evolution is mutation rather than selection.
本文讨论了蛋白质和DNA多态性的程度和模式，重点关注多态性维持的机制。统计研究表明，分子水平上的大部分遗传变异是由突变-漂变平衡维持的。然而，在某些基因座，如哺乳动物的组织相容性基因座，似乎涉及一种超显性选择形式。在超显性选择存在的情况下，多态等位基因可能会维持数千万年，因此等位基因之间的核苷酸差异数量通常非常大，就像植物中的自交不亲和等位基因一样。也有越来越多的例子表明，形态或生理特征的适应性变化是由单个核苷酸替换引起的。然而，这些突变似乎只占导致遗传变异和进化的总核苷酸变化的一小部分。虽然有许多频率依赖性选择的例子，但这种选择形式除了在某些特殊情况下，对维持遗传变异显然并不重要。对DNA进化变化的观察表明，进化的驱动力是突变而不是选择。
