Final answer:
The statement is false; natural selection, not hybridization, has acted on the high frequency of the S allele in regions with malaria. If malaria were eliminated, the selective advantage of carrying the S allele would vanish, possibly leading to a decrease in its frequency due to the detrimental effects of sickle cell anemia.
Step-by-step explanation:
The statement that hybridization is the most important evolutionary force acting on the high frequency of the S allele for sickle cell disease in human populations living in malarial regions is false. Instead, the primary evolutionary force is natural selection. This is due to the fact that individuals who are heterozygous for the sickle cell trait (AS genotype) have a selective advantage in regions where malaria is prevalent; they are resistant to this deadly disease. Consequently, this trait is maintained in the population even though the homozygous condition (SS genotype) results in sickle cell anemia, a severe disorder.
If malaria were eliminated in these regions, the fitness landscape would change drastically. The protection heterozygotes enjoy against malaria would no longer be advantageous, and thus the selective pressure maintaining the high frequency of the S allele would be reduced. Over time, the frequency of the S alleles would likely decrease, as the negative effects of sickle cell anemia would become more pronounced without the counterbalancing benefit of malaria resistance.