¹School of Biological Sciences, Washington State University, ²Department of Anthropology, Washington State University

Friday 4:15-4:30, 200ABC

The ABO blood group locus in humans is characterized by elevated heterozygosity and apparently stable allele frequencies within most populations. One exception to this rule occurs in Native Americans, since A and B alleles are uncommon in North America and rare in South America. While the maintenance of A, B, and O alleles suggests some form of balancing selection, the loss of diversity in Native Americans supports recent population bottlenecks of uncertain magnitude. Using knowledge of ABO protein function, we modeled asymmetric negative frequency dependent selection and genetic drift to determine the range of N_e values that produce patterns of ABO polymorphism observed in the Americas. Populations must be moderately small to lose either the A or B allele (N_e ≤ 100) and much smaller (N_e ≤ 35) for the complete loss of diversity, which nearly always involved the fixation of the O allele. Declines in heterozygosity at the ABO locus in Native Americans are also consistent with populations sizes as small as N_e = 10. These results imply that although the ancestral Native American population was large enough to maintain A and B alleles, populations expanding into the continent were likely smaller than is appreciated (N_e ≤ 10-35).