Final answer:
Non-synonymous and synonymous substitution rates in genetics refer to the rates at which nucleotide substitutions result in changes or no changes in amino acids. These rates can provide evidence of selection. Purifying selection is inferred when the rate ratio is less than 1, neutral evolution when it equals 1, and positive selection when it is greater than 1.
Step-by-step explanation:
In genetics, non-synonymous and synonymous substitution rates refer to the rates at which nucleotide substitutions in protein-coding genes result in changes or no changes in the corresponding amino acids, respectively. These rates can provide evidence of whether a gene or locus is under selection.
When the rate ratio is less than 1, purifying selection is inferred, indicating that non-synonymous substitutions are deleterious. When the rate ratio equals 1, neutral evolution is inferred, suggesting no difference in fitness between non-synonymous and synonymous substitutions. When the rate ratio is greater than 1, positive selection is inferred, indicating that non-synonymous substitutions are favored by natural selection.