Final answer:
The synonymous substitution rate is indicative of the rate of neutral evolution or genetic drift, as these mutations do not alter protein function and are not typically subject to natural selection.
Step-by-step explanation:
Synonymous substitutions refer to changes in the DNA sequence that do not affect the amino acid sequence of a protein due to the redundancy in the genetic code. As these changes do not affect the protein function, they are usually not influenced by natural selection and thus accumulate at a rate that reflects random mutation and genetic drift.
In contrast, non-synonymous substitutions, which do affect the amino acid sequence, are often subject to selection pressures. The rate of non-synonymous to synonymous substitutions (ω ratio) can indicate the type of selection acting on a gene. When this ratio is less than 1, purifying selection is suggested, as non-synonymous substitutions can be deleterious. A ratio of 1 implies neutral evolution, indicating no selective advantage or disadvantage. Lastly, a predicted ratio greater than 1 is indicative of positive or adaptive evolution, where non-synonymous substitutions confer some advantage and are selected for. However, synonymous substitutions themselves are largely viewed as neutral markers to gauge the evolutionary dynamics where selection is not directly acting.