WAEC Biology 2009 Question 52

The allele responsible for sickle-cell anaemia first appeared in the tropical and subtropical regions of Asia, the Middle East and Africa. It entered the United States of America when individuals were forcibly brought over from Africa. In micro-evolutionary terms this is an example of

1. A.mutation.
2. B.gene flow.
3. C.genetic drift.
4. D.natural selection.