Meneely Philip M
Department of Biology, Haverford College, Haverford, Pennsylvania 19041
Genetics. 2016 Feb;202(2):371-5. doi: 10.1534/genetics.115.184564.
The origin of beneficial mutations is fundamentally important in understanding the processes by which natural selection works. Using phage-resistant mutants in Escherichia coli as their model for identifying the origin of beneficial mutations, Luria and Delbrück distinguished between two different hypotheses. Under the first hypothesis, which they termed "acquired immunity," the phages induced bacteria to mutate to immunity; this predicts that none of the resistant mutants were present before infection by the phages. Under the second hypothesis, termed "mutation to immunity," resistant bacteria arose from random mutations independent of the presence of the phages; this predicts that resistant bacteria were present in the population before infection by the phages. These two hypotheses could be distinguished by calculating the frequencies at which resistant mutants arose in separate cultures infected at the same time and comparing these frequencies to the theoretical results under each model. The data clearly show that mutations arise at a frequency that is independent of the presence of the phages. By inference, natural selection reveals the genetic variation that is present in a population rather than inducing or causing this variation.
有益突变的起源对于理解自然选择发挥作用的过程至关重要。卢里亚和德尔布吕克以大肠杆菌中的噬菌体抗性突变体为模型来识别有益突变的起源，区分了两种不同的假说。在他们称为“获得性免疫”的第一种假说下，噬菌体诱导细菌突变为具有抗性；这预测在噬菌体感染之前不存在任何抗性突变体。在称为“突变为抗性”的第二种假说下，抗性细菌源自与噬菌体存在无关的随机突变；这预测在噬菌体感染之前群体中就存在抗性细菌。通过计算在同时感染的不同培养物中抗性突变体出现的频率，并将这些频率与每个模型下的理论结果进行比较，可以区分这两种假说。数据清楚地表明，突变出现的频率与噬菌体的存在无关。由此推断，自然选择揭示的是群体中已存在的遗传变异，而非诱导或产生这种变异。
