In a small group of people living in a remote area, there is a high incidence of "blue skin" a condition that results from a variartion in the structure of hemoglobin. All of the "blue-skinned" resident can trace their ancestry to one couple, e who were among the original settlers of this region. The unusually high frequency of "blue skin" in the area is an example of:
a- genetic drift
b- heterozygote advantage
c- natural selection
d- mutation