Question

A study by Solloch and et. al., in 2017, gives the map above which shows the frequency of alleles with a CCR5-delta32 mutations over 87 different countries. This mutation deletes the presence of a co-receptor (CCR5) on the outside of human T-cells (lymphocytes). Some viruses, such as the one responsible for the Black Death and Human Immunodeficiency Virus (HIV), require this receptor for attachment to host cells during the infection process. The Black Death was an epidemic that passed over northern Europe during the 14th century killing nearly 60% of Europeans. According to this information, which explanation best explains why northern Europeans show a greater immunity for HIV than some other parts of the world?

Answer 1

People who had this mutation back in the 14th century had better chance of survival (better fitness) then people who did not have this mutation. They were majority of people who survived the Black Death because of natural selection. Their offspring inherited this mutation too, and their offspring after.

There was no natural selection for this mutation in some other parts of the world because they were not hit by the Black Death.

Since the same mutation is responsible for immunity to HIV and the Black Death, offspring of the people who have survived the Black Death are immune to HIV too. Part of the world that was hit by the Black Death, now have more people with this mutation.

Answer 2

It's Directional Selection.