Non-synonymous to synonymous substitution rate ratio (dN/dS) for protein domains of different age. Old: 12,076 domains, Vertebrate: 521 domains, Mammalian: 47 domains. dN and dS were calculated for human and mouse orthologous genes, domains with unreliable dN or dS estimates were not considered (domain length < 60 amino acids or dN > 0.5 or dS >2). Differences between each pair of age classes were statistically significant (Kolmogorov-Smirnov test, p-value < 10-5). The area within the box contains 50% of the data; horizontal line is the median; outliers (5%) are represented as small circles.