The degree to which a new mutation is neutral affects how important a role genetic drift plays in the evolutionary process. As Chapter 4 explains, mutations, which add genetic diversity to populations and come in three categories (advantageous, deleterious, and neutral), are always occurring.
For mutations that are advantageous or deleterious, and when population size is large, natural selection is the primary driving force that determines whether the frequency of particular alleles increase or decrease in subsequent generations. If mutations are deleterious, natural selection removes them; if they're advantageous, natural selection favors them, and they increase in frequency.
But if the particular new mutant is neutral, neither advantageous nor disadvantageous compared to the original, natural selection can't be responsible for changes in its frequency through time. In this case, genetic drift is the driving force.
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