Answer
If an amino acid has several alternatives wit the same protein function, then a replacement is neutral with respect to the organism's fitness. In this case, the rates of synonymous and nonsynonymous substitutions in the corresponding DNA sequences are very similar, so the ratio of the rates should be around 1.
If an amino acid's position is under positive selection, the rate of nonsynonymous substitutions will be greater than the rate of synonymous substitutions in the corresponding DNA.
If an amino acid position is under purifying selection, then the rate of synonymous substitutions should be much higher than the rate of nonsynonymous substitutions in the corresponding DNA sequences.
Work Step by Step
If an amino acid has several alternatives wit the same protein function, then a replacement is neutral with respect to the organism's fitness. In this case, the rates of synonymous and nonsynonymous substitutions in the corresponding DNA sequences are very similar, so the ratio of the rates should be around 1.
If an amino acid's position is under positive selection, the rate of nonsynonymous substitutions will be greater than the rate of synonymous substitutions in the corresponding DNA.
If an amino acid position is under purifying selection, then the rate of synonymous substitutions should be much higher than the rate of nonsynonymous substitutions in the corresponding DNA sequences.
