Answer:
founder effect
Step-by-step explanation:
When a few individuals from a population migrate to some new habitat and start a new population, the gene pool of the new population may not have the same allele frequencies as that of the original population. This change in the allele frequency of the new population is called the founder effect. In the given population, the increased frequency of the AB+ phenotype might have resulted from the founder effect. This is a chance event that occurs when a few individuals with a small fraction of genetic variations found a new population.