How have the historic levels of melanin in human populations changed due to migration?
a. Humans that produce less melanin became more common populations migrated toward the equator.
b. Humans that produce more melanin became more common populations migrated toward the equator.
c. Humans that produce more melanin became more common as populations migrated away from the equator.
d. Humans that produce less melanin became more common populations migrated away from the equator.