15. African Americans develop a form of end-stage kidney disease associated with elevated blood pressure that Europeans do not. Two variants in a gene on chromosome 22, called Apol1, cause the condition. The encoded protein is secreted into the blood, but only the forms in the African Americans who have the kidney disease also kill the parasites that cause African sleeping sickness. The mutations persist because they protect against African sleeping sickness. What phenomenon described in the chapter does this situation illustrate?