The human HLA-DRBI and the chimpanzee Patr-DRB1 gene are orthologs. At the protein level, the human HLA-DRB1*0701 and HLA-DRB1*0302 alleles show 31 amino acid differences out of 270 amino acid positions. But human HLA-DRB1*0702 and the chimpanzee Patr- DRB1*0702 proteins are so closely related that they differ at only 2 positions out of the 270. What does the difference between these two pairwise comparisons tell us about the origins of HLA polymorphism?