I believe the answer is C. genetic drift. Genetic drift is variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce. Since you did not give the the examples listed above it makes it hard to be 100% sure about my answer but this would be my best guess.
ANSWER:
The four examples listed above could all be responsible in the change in a gene pool, also called genetic drift.
~batmans wife dun dun dun...aka ~serenitybella
