The CCR5 gene codes for a protein on the surface of T cells. The CCR5 protein is used by Human Immunodeficiency Virus (HIV) to enter T cells, which ultimately leads to Acquired Immunodeficiency Syndrome (AIDS). AIDS was first reported in the European population in the early 1980s.
About 3,000 years ago in Central-Western Europe, a mutation occurred that prevents normal expression of the surface protein. People with one copy of the mutated allele have delayed onset of AIDS. Homozygotes for the mutation are resistant to HIV infection.
Evidence suggests that the trait also protects individuals from the smallpox virus. Smallpox was common in the European population from the Middle Ages until the early 1900s, causing several major epidemics with high death rates. It was eradicated through widespread vaccination.
The frequency of the allele is 16% in Scandinavia and 4% in Greece.
What are the expected frequencies of HIV-resistant individuals in each population and which evolutionary explanation for the difference is MOST plausible?