Yes, the ratio of non-synonymous to synonymous substitution,

ω = dN/dS is the evolutionary rate of protein coding genes.

But your statement is incomplete. It is not only for the positive selection. The type of selection is dependent on the values of ω. If it is > 1 then only you can say that the selection was positive. If it is 0, the selection would be neutral and negative value would indicate towards the negative selection pressure on the protein coding genes.

It is even a bit more complex than Abhijeet Singh stated above, because for almost all protein-coding genes, there are some codons that are very highly conserved and invariant across huge evolutionary distances while other codons in the same gene are more variable. Studying the average dN:dS ratio of a whole gene can be useful, but looking at the rate variations at individual sites or regions of a gene/protein can also be useful. In general the catalytic core of enzymes tends to be highly conserved, so finding conserved sites can give clues as to which amino acids are involved in the function of the enzyme. Almost all protein-coding genes are under a lot of negative selection pressure to remain functional, and the sites that do change over time are not really under positive selection but only under less extreme negative selection pressure.

This rate Ѡ=dN/dS is used as a measure of functional constraint. It is used to study evolutionary pressure on protein coding genes, and dN/dS=1 under neutrality, with values >1 showing positive selection and values

PFA this paper, it will be really helpful for u to understand the population genetics of dN/dS ratio.

Thank you Tabsum Akhter

dN/dS is not a rate of evolution, but yes dN and dS. dN/dS is a relationship between two different rates of evolution (dN and dS). For example, one can have dN/dS=1 for different amounts of dN and dS (different rates of evolution) just being dN=dS. Example, Article Slow but not low: Genomic comparisons reveal slower evolutio...