Final answer:
The lower genetic diversity in Native American populations compared to those in Africa is most likely due to founder effects/genetic drift, where small populations and chance events lead to significant changes in genetic structure.
Step-by-step explanation:
The lower genetic diversity in Native American populations compared to African populations is best explained by (B) founder effects/genetic drift. Small populations are more vulnerable to genetic drift because the gene pool is smaller and less stable. An example of founder effects influencing genetic diversity can be seen in the Afrikaner population of South Africa, where a small group of Dutch settlers exhibited a higher-than-normal proportion of certain mutations, leading to a population with high incidences of diseases like Huntington's disease (HD) and Fanconi anemia (FA).
Genetic drift is impactful in small populations because there are fewer alleles overall. If there's a chance event or catastrophe, such as a bottleneck effect, the genetic variability is greatly reduced. The founding individuals of a population may carry a set of alleles that is not representative of the original population, which also contributes to genetic drift, especially if they start a new population in a new location.