Final answer:
The high prevalence of type O blood among pureblood Native Americans is best explained by the Founder effect, wherein a small group originating from a larger population carries a genetic trait that becomes prevalent in the new population.
Step-by-step explanation:
The observation that most pureblood Native Americans have type O blood can be best explained by the Founder effect. This genetic principle occurs when a new population is established by a very small number of individuals from a larger population. Their gene pool may not be representative of the source population. Due to the Founder effect, genetic drift could lead to the new population having a higher frequency of certain alleles, including those related to blood type. In the case of Native Americans, it is believed that the original populations that migrated to the Americas had a higher frequency of type O blood, and this trait became predominant through genetic drift and the Founder effect.