The frequency of blood type O is very high in some populations of native Americans. In many North American tribes, for example, the frequency is over 90% and often approaches 100%. A majority of individuals in those populations have blood type O (homozygous for the O allele). [see Theme: ABO Blood Types]Since there's no solid evidence that blood types are adaptive,1 the standard explanation is random genetic drift.
Jerry Coyne explains it in Why Evolution Is True.
One example of evolution by drift may be the unusual frequencies of blood types (as in the ABO system) in the Old Order Amish and Dunker religious communities in America. These are small, isolated, religious groups whose members intermarry—just the right circumstances for rapid evolution by genetic drift.Not everyone agrees with this conclusion. There are three possible explanations.
Accidents of sampling can also happen when a population is founded by just a few immigrants, as occurs when individuals colonize an island or a new area. The almost complete absence of genes producing the B blood type in Native American populations, for example, may reflect the loss of this gene in a small population of humans that colonized North America from Asia around twelve thousand years ago.
Founder effectHalverson and Bolnick (2008) realized that the different scenarios could be distinguished by looking at allele frequencies in ancient populations, before extensive contact with Europeans. They looked at the sequences of the two human ABO genes in samples from ancient burial mounds. The ABO gene is located at 9q34.1-q34.2 on chromosome 9. It encodes the enzyme N-acetylgalactosaminyltransferase that's responsible for the ABO blood types. There are many alleles for each blood type [OMIM 110300]. The O alleles are all null alleles (pseudogenes).
The founding population of the Americas was relatively small so the high frequency of blood type O is just due to chance in a small population. This is classic random genetic drift.
Ancient populations of native Americans had higher frequencies of blood types A and B but they went through a severe population decline in the 1500s and 1600s due to the introduction of diseases from European invaders. More individuals with blood type O survived just by chance. This is similar to the founder effect in that it postulates a connection between allele frequency changes (by drift) and a small population.
The selection model postulates that individuals with blood type O were more likely to survive the diseases introduced by Europeans.
Halverson and Bolnick successfully determined the genotypes of 15 individuals who lived about 1800 years ago. Fourteen of them were homozygous for type O alleles (O1 and O1v). The other individual had genotype AO1.
The results show that the high frequency of blood type O was present in populations long before extensive contact with Europeans. This rules out the bottleneck and selection hypotheses and confirms the standard explanation that the effect is due to a founder effect (genetic drift).
It looks like individuals can survive easily without a functional ABO gene. At some point in the future, the null allele might become fixed in the entire population and humans will have one less gene.
1. You will find lots of papers claiming to have found associations between various blood types and different diseases and conditions. Many studies haven't been replicated. There's no consistent pattern.
Halverson, M.S., and Bolnick, D.A. (2008) An ancient DNA test of a founder effect in Native American ABO blood group frequencies. American journal of physical anthropology, 137:342-347. [doi: 10.1002/ajpa.20887]