Strong selection in moving populations mirrors new challenges to immune systems
Historically changing patterns of infectious disease transmission have usually accompanied large scale movements of human populations. Chronic infections with high mortality, including tuberculosis, malaria, and syphilis probably existed in pre-agricultural societies, but redistribution of populations today and transmission of pathogens by biting insects, especially the arboviruses associated with changes in human activity patterns, makes consideration of the evidence for recent selection in the human genome an important topic. Comparative studies of SNP density and polymorphism across geographically distinct populations have identified regions of chromosome 1, 2, 4, 8, 9,12, and 20 as containing some of the highest signals in the human genome for recent positive selection. In addition, analysis of the human genome compared with that of chimpanzee identify specifically chromosomes 2, 4, 9, and 12 as heavily remodeled in humans, compared to what likely existed in the shared common ancestor. These studies have stimulated the search for appropriate candidate genes that might identify the particular sites where recent adaptive evolution seems to be taking place, and researchers have so far focused on the particular metabolic, apoptotic, neurological, or gametogenic functions of these candidates. However, all these regions are united by one common attribute: they contain important candidate loci known to influence the function of the immune system in response to infectious agents. Some immune function loci can be eliminated from this group because the common ancestor of human and chimp would likely have faced the same disease agents (some retroviruses, some soil pathogens, some water-borne bacteria). Other agents would depend on very large populations to sustain themselves, and could only have reached epidemic proportions after the adoption of agriculture and animal husbandry (measles, mumps, smallpox, dengue, influenza). Based on patterns of recent human population spread, population size, adoption of particular agricultural practices, and proximity of residence in relation to reservoirs, other candidate loci might be predicted to show distinct polymorphism patterns that reflect the most recent outbreaks of infectious agents and the surviving genotypes.