highly significantP = 4.33 × 10 −42
Furthermore, there was a weak but highly significant negative correlation between the number of paralogs for a gene and its d N /d S ratio compared with orthologs (Spearman’s rank: r s = −0.131, P = 4.33 × 10 −42 ), suggesting that genes that are retained as paralogs tend to be under stronger purifying selection.