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The Abert's squirrel is found on the south rim
of the grand canyon and surrounding states.
The Kaibab squirrel can only be found at the
north rim of the canyon.
Scientists believe they were separated by
plateau erosion that separated the forests of
the Grand Canyon, isolating the Kaibab
population of squirrels.

Astudy 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?