Final answer:
The maintenance of CF alleles in a population is likely due to balancing selection, where the heterozygous genotype provides a survival advantage, or genetic drift, especially in smaller populations. Positive selection and lack of mutation are less plausible reasons for the maintenance of these alleles.
Step-by-step explanation:
The maintenance of CF (cystic fibrosis) alleles in a population can be a complex process influenced by various evolutionary forces. In this context, balancing selection could be a potential reason for the maintenance of CF alleles.
Balancing selection occurs when multiple alleles are actively maintained in the gene pool of a population at frequencies above those expected from mutation alone. This can happen when the heterozygous genotype provides a survival advantage, as is the case with sickle cell anemia and malaria resistance, which is an example of heterozygote advantage.
Another possibility is genetic drift, which refers to changes in the frequency of an allele in a population due to random sampling. This is more likely to have an effect in smaller populations.
On the other hand, positive selection refers to a process where beneficial traits increase an organism's chances of survival and reproduction, leading to an increase in the frequency of the beneficial alleles. However, regarding CF alleles, if those were significantly beneficial, they would likely become predominant rather than just maintained in the population.
Lastly, the option that suggests lack of mutation is not a valid reason for allele maintenance, as mutations are the source of new genetic variations, and the presence of the allele implies that mutation has already occurred. Moreover, without mutation, an allele's frequency would not be maintained through genetic mechanisms - it would be subject to change through the aforementioned forces.