Positive vs. Purifying Selection. A mutation that affects the function of a gene (i.e. not neutral) can change the shape and function of the product (here the product is a protein, but it could also be an RNA) or the expression pattern of the product. If this change in function or expression increases organismal fitness, we expect the frequency of the allele to increase over time (Positive Selection). If the mutation decreases organismal fitness, we expect the frequency of the new allele to decrease over time (Purifying Selection).
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selection, mutation, synonymous, nonsynonymous, evodevo, evolution, development, functional mutation, allele, fitness