THE NUMBER OF ALLELES THAT CAN BE MAINTAINED IN A FINITE POPULATION.
THE NUMBER OF ALLELES THAT CAN BE MAINTAINED IN A FINITE POPULATION.
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作者：
M. Kimura;J. Crow
有时有人提出，野生型等位基因不是一个单一的实体，而是由不同的等等位基因组成的群体，用任何普通程序都无法区分。由于有数百个核苷酸，每个核苷酸都可能进行碱基替换，并且可能通过序列重排、获得和损失进行额外的排列，因此可能的基因状态的数量变成了天文数字。众所周知，单个核苷酸替换会产生最剧烈的后果，但也有一些突变会产生非常微小的影响，而且有可能许多突变非常小，以至于无法检测到。本文的目的不是讨论这种等等位基因系统的合理性，也不是讨论支持或反对这种系统的证据。相反，我们建议检查这样一个系统的一些人口后果，如果它确实存在。这种可能性似乎很大，值得进行这样的调查。如果突变可以产生大量不同的状态，这并不一定意味着这些状态中的很大一部分会在单个种群中共存。有些会因随机漂移而丢失，而另一些则可能选择性地处于不利地位。另一方面，有些基因可能在杂合组合中有益而持续存在。我们将考虑三种可能性：(1)一个由选择性中性等等位基因组成的系统，其在种群中的频率由突变率和随机漂变决定。(2)等位基因互异的系统。(3)杂种和有害突变体的混合物。
T has sometimes been suggested that the wild-type allele is not a single entity, I but rather a population of different isoalleles that are indistinguishable by any ordinary procedure. With hundreds of nucleotides, each presumably capable of base substitutions and with additional permutations possible through sequence rearrangements, gains, and losses, the number of possible gene states becomes astronomical. It is known that a single nucleotide substitution can have the most drastic consequences, but there are also mutations with very minute effects and there is the possibility that many are so small as to be undetectable. It is not the purpose of this article to discuss the plausibility of such a system of isoalleles, or the evidence for and against. Instead, we propose to examine some of the population consequences of such a system if it does exist. The probability seems great enough to warrant such an inquiry. If a large number of different states can arise by mutation, this doesn't necessarily mean that a large fraction of these would coexist in a single population. Some will be lost by random drift and others may be selectively disadvantageous. On the other hand, some may persist by being beneficial in heterozygous combinations. We shall consider three possibilities: ( 1 ) A system of selectively neutral isoalleles whose frequency in the population is determined by the mutation rate and by random drift. (2) A system of mutually heterotic alleles. ( 3 ) A mixture of heterotic and harmful mutants.