A species with polymorphic sites let non-synonymous = a and synonymous = b.
For polymorphic sites among the species, let non-synonymous = c and synonymous = d.
If the difference is due to neutral evolution, then a/b = c/d.
Whereas, if the variation is due to selection, then a/b < c/d.
But, in this case a/b = 20/50 > c/d = 2/18, that matches neither expectations as the ratio of non-synonymous to synonymous polymorphisms (a/b) is comparatively high and the gene being examined can encode a protein tolerant of relatively less species variations. This may possibly suggest that speciation was a new occurrence. Hence, new polymorphisms have been permanent in one species that are not alternates in the further.