Their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population.

随着新等位基因在每个群体中独立突变产生，它们在许多遗传位点上的等位基因频率逐渐变得越来越不同。

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So even though a drug might work well for patients whose tumors have a particular mutation, when the drug is used for a broader population, it shows only a small effect.

所以，即使一种药物对有同一变异的肿瘤患者有效，但当应用于更广泛的患者时就可能收效甚微。

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The accepted scenario was a simple case of evolutionary selection. In a bacterial population exposed to a killer drug, a few lucky individuals might have a genetic mutation that kept them alive.

我们本以为这是再简单不过的进化选择了：当药物杀向全体细菌时，总有那么几个幸运儿，基因突变了，小命保住了，它们可以活下来继续繁殖下一代，而剩下的那些倒霉蛋就从此命归黄泉了。

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This, some scientists believe, leads to new illnesses formed by genetic mutation crossing the species barrier and entering the human population.

ECONOMIST: Still spreading | The

This, some scientists believe, leads to new diseases formed by genetic mutation crossing the species barrier and entering the human population.

ECONOMIST: Getting better, or worse? | The

There, they say, the mutation that existed in under a tenth of the population spread to nearly nine-tenths because those with it survived far better than those without.

ECONOMIST: Banyan: Splittism on the roof of the world | The