Heavy rains and mudslides cause a river to change course, isolating two groups of lizards from one another. over a long period of time, " one group will probably become an endemic species. both groups will probably become native species. "the groups will probably diverge genetically, and speciation may occur." one or both groups will probably emigrate. one or both groups will probably become invasive species.

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?