Final answer:
Directional selection is exerted on the malaria population as the malaria-causing parasites evolve resistance to quinine. This leads to an increase in the frequency of resistant individuals over time, making quinine less effective.
Step-by-step explanation:
The observation that quinine is becoming less effective over time implies that malaria-causing parasites are evolving resistance to the drug. The type of selection being exerted on the malaria population is most likely to be directional selection. This type of selection favors alleles that contribute to resistance to quinine, leading to an increase in the frequency of resistant individuals within the population. As susceptible parasites are killed off by quinine, resistant parasites survive and reproduce, passing on the resistance alleles to subsequent generations.
Directional selection typically occurs when environmental pressure causes a shift in allele frequency in a consistent direction, such as an increase in drug resistance. In this case, it is driven by the selective pressure of quinine treatment. Over time, the population evolves to a state where the drug becomes less effective, necessitating the need to develop new antimalarial drugs or alternative treatment strategies to combat the evolving resistance.