1. If dN/dS = 1, amino-acid substitutions may be largely neutral. However, there is also the
possibility that positive selection just cancels purifying selection, so that some amino-
acid substitutions were driven by natural selection. This situation is thus ambiguous.
2. If dN/dS < 1, purifying selection (selection against deleterious non-synonymous
substitutions) has definitely operated. Some amino-acid substitutions have been may
caused by selection, just not enough to overcome the effects of purifying selection.
3. If dN/dS > 1, selection has caused some amino-acid substitutions. Some substitutions
also have been caused by genetic drift. Purifying selection also likely operates, may
but is not strong enough to overcome the effects of positive selection.
