Indiana jones visits the yanomamo and after testing them finds that only 3 individuals in the small village of 50 people have blood type a. his great granddaughter, elsie jones returns after two generations and finds that no one in the village has blood type a any longer. which process of evolution is likely the cause of these changes?

Genetic drift

Explanation:

Genetic drift is an evolutionary mechanism that in a population it changes allelic frequencies over generations just by chance. For this reason, small populations are more susceptible to be affected by genetic drift than bigger populations.  

In this example, Indiana Jones visited a small village of 50 people and over generations allele A probably was lost just by chance, or by genetic drift.