Final answer:
The higher frequency of the Hbs gene in malarial regions is an example of balancing selection, where the sickle cell trait confers a survival advantage against malaria, thus maintaining the allele's presence in the population.
Step-by-step explanation:
The higher than expected frequency of the Hbs (sickle cell) gene in malarial regions is an example of balancing selection. This is because the presence of one sickle cell allele (Hbs) offers a selective advantage in terms of resistance to malaria, a deadly tropical disease. Heterozygotes (AS), who inherit one normal hemoglobin allele and one sickle cell allele, are resistant to malaria and thus have increased chances of survival and reproduction in regions where malaria is prevalent, like sub-Saharan Africa, the Middle East, and India. This selective pressure upholds the frequency of the sickle cell allele in the population. However, individuals homozygous for the allele (SS) suffer from sickle cell anemia, which can be debilitating and lethal.
Variants such as hemoglobin C and hemoglobin E are also genetic adaptations providing malaria resistance in different geographic regions. If malaria were eliminated, the Hbs allele's frequency might decrease over time because the associated health risks would outweigh the benefits of resistance to a disease that no longer poses a threat.
Ultimately, this scenario underlines the critical role of the environment in determining the relative fitness of different genotypes, illustrating balancing selection, where two or more alleles are maintained in a population due to selective advantages under certain environmental conditions.