Final answer:
The sickle cell anemia trait has historically provided resistance to malaria, which maintained its presence in the population through natural selection. However, if a new vaccine eradicated malaria, the evolutionary advantage of carrying the sickle cell trait would diminish, potentially leading to a decrease in the allele's frequency due to the health disadvantages it poses.
Step-by-step explanation:
The presence of the sickle cell anemia trait in the human population is an example of natural selection. Historically, the sickle cell gene provided a survival advantage to heterozygotes (individuals with one normal and one sickle cell allele), because the sickle cell trait provides resistance to malaria, which is prevalent in parts of Africa. In a scenario where a new vaccine eliminates the threat of malaria completely, the evolutionary pressure that maintains the sickle cell allele in the population would diminish.
Without the threat of malaria, the negative effects of the sickle cell disease in homozygous individuals (those with two copies of the sickle cell gene) would likely lead to a decrease in the frequency of the sickle cell allele over time. In other words, without malaria as a selection pressure, the health issues associated with sickle cell anemia become a greater disadvantage, and those without the allele would have higher fitness.
Therefore, in the absence of malaria, it is plausible that the sickle cell trait might eventually become less common in the human population due to natural selection favoring alleles that do not lead to the detrimental health effects associated with sickle cell anemia, especially in homozygous individuals.