Final answer:
Humans have evolved genetic resistance to malaria, such as the sickle-cell trait in heterozygotes and resistance in blood type O individuals, reflecting natural selection.
Step-by-step explanation:
Evidence of natural selection adapting human populations against malaria includes heterozygote advantage and blood type O resistance.
Individuals carrying one sickle-cell gene (hemoglobin S allele) — heterozygotes (AS) — have a resistance to malaria, leading to improved survival and reproductive advantages in malaria-endemic regions such as Africa. This persistence of the sickle-cell trait despite its drawbacks in non-malarial regions demonstrates natural selection in human evolution. Additionally, individuals with blood type O are less susceptible to severe malaria because their blood does not react as severely with proteins from the Plasmodium parasite, showing another adaptation against the disease.
As malaria-causing parasites, such as Plasmodium falciparum, have developed drug resistance, human populations in affected areas have concurrently evolved genetic traits that confer malaria resistance. This evolutionary arms race exemplifies how human populations can undergo natural selection due to the pressures exerted by a parasitic disease.