The mind of primitive anthropologists: hemoglobin and HLA, patterns of molecular evolution.

Frank Livingstone played a central role in defining the population genetics of the sickle cell mutation at position 6 of the human beta globin gene, the most famous amino acid substitution in evolutionary biology. Its discovery occurred at a time when traditional, 19th-century principles of natural selection were being joined with the newly discovered mechanics of DNA structure and protein synthesis to produce Neo-Darwinian theory. When combined with the epidemiology of malaria in Africa, differential mortality for both homozygotes, and the resulting advantage of the heterozygote, sickle cell became the classic balanced polymorphism. Human HLA-A has 237 molecular alleles. The histocompatibility system has as its primary function the presentation of peptides to T-cell receptors and plays an essential role in the immune system. Nearly all of the alleles are codominant and fully functional. Despite almost 30 years of disease-association studies with HLA-A, no convincing evidence has been found for differential fertility or mortality at this locus. Yet the dogma in the histocompatibility field is that this extensive human polymorphism is maintained by "balancing selection." Explaining HLA-A polymorphism is what one might call the sickle-cell-effect. This one mutation, coming as it did at the historical convergence of Darwinian theory and modern genetics, and carrying with it the strong relationship between mutation, disease, and allele frequency, has conditioned our discussion of human genetic variation and population genetics. Has the strength of this early idea made evolutionary biologists uncritical of systems like HLA-A and retarded the search for new mechanisms of molecular evolution? Is it now time to move away from a focus on mutation and polymorphism in evolutionary genetics and toward a systems theory that would explain the origin and evolution of hemoglobin and HLA-A and the biochemical pathways that surround them?