Your history professor is interested in genealogy and tells your class that, on his mother’s side, he is descended from the original Amish population, a religious and insular group of people in Pennsylvania who tended to marry within their social circle. He also notes that many members of his extended family have the condition polydactyly, an extra finger or toe. The high frequency of polydactyly in your professor’s ancestry is an example of ________ in the Pennsylvania Amish.