The ratio between nonsynonymous and synonymous substitutions per site in a protein-coding sequence is often used to detect genes that are under positive or negative selection. If the the ratio is approximately 1.0, selection is essentially neutral; a ratio of much less than 1.0 indicates that synonymous substitutions (amino acid conserving) are much more prevalent than nonsynonymous substitutions (amino acid changing), and implies the gene is under positive selection.

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