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Are All Mutations Harmful?

Most mutations are harmful or neutral, because protein sequences have been highly optimized through evolution. Studies in fruit flies show that about 70% of nonsynonymous mutations—those that change amino acids—are harmful (Griffiths et al., 2019).

Beneficial mutations are rare, but they play a key role in evolution. A real example is lactose tolerance in humans. Mutations that allow adults to digest milk became beneficial after humans began domesticating cattle. However, whether a mutation is useful often depends on the environment. If conditions change, a once-beneficial mutation may no longer provide an advantage (NHGRI, 2023).