A selection-mutation balance occurs when

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In a selection-mutation balance, the rate at which deleterious mutations are introduced through mutation is balanced by the rate at which they are removed by selection. This can occur when a mutation confers a slight advantage that is counteracted by the deleterious effects of the same mutation, or when there is no selective advantage or disadvantage to the mutation. As a result, the frequency of the deleterious allele remains stable over time, with some individuals carrying the allele but not exhibiting the deleterious trait. Option A describes a scenario where selection favors a beneficial trait, even if it is coupled with a deleterious trait. Option C describes an unrealistic scenario where numerous deleterious mutations accumulate without any selection against them. Option D describes a scenario where a beneficial mutation becomes deleterious in a new environment, but it is unclear if this leads to a selection-mutation balance.

A Selection-Mutation Balance Occurs When