On the slopes of the carolina appalachian mountains, ecologists have studied some closely related plethodontid salamander species for decades. many of these salamander species, when they occur in separate valleys from each other, tend to have very similar food size choices. however, when these species occur together in a mountain valley, their food choices tend to differ, with some species selecting small insects and others feeding exclusively on larger insects. this may be an example of

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?