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