... protein.¹

Obviously it happens sometimes. If it didn't, there wouldn't be any adaptive evolution. It's just that, on average, mutations are more likely to decrease fitness than to increase it.

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... code.²

By the way, the ``universal'' genetic code is not universal. There are at least eight, but all of them have similar redundancy properties.

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... approximation,³

We'll see that they may not be completely neutral a little later, but at least it's reasonable to believe that the intensity of selection to which they are subject is less than that to which non-synonymous substitutions are subject.

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