How can genetic drift change blood group frequencies in small populations?

Genetic drift refers to changes in the gene pool of a small population that occur strictly by chance, rather than through natural selection or reproductive advantage. In the context of blood groups, this phenomenon can significantly alter the frequency of specific alleles within a community.12

In small, isolated populations, the gene pool is limited, making it susceptible to sampling error. If individuals carrying a particular blood group allele fail to reproduce or are absent from the breeding group, that allele can be lost entirely from the population. Conversely, an allele may increase in frequency simply due to the random composition of the surviving or reproducing members.12

For example, it has been suggested that B alleles were lost by chance from early Native American and Australian Aboriginal populations when those communities were small. Because these changes are random, they can result in pronounced discrepancies in blood group frequencies between different populations, regardless of the biological or immunological value of the blood types involved.21