Final answer:
Deleterious alleles like those causing cystic fibrosis may persist in a population due to mutation-selection balance and heterozygote advantage. Despite cystic fibrosis being a harmful condition, carriers could have a selective advantage if they show increased resistance to certain diseases.
Step-by-step explanation:
Natural Selection and Allele Frequencies
Natural selection is a process where alleles that confer beneficial traits increase in frequency because individuals with those traits are more likely to have successful reproduction. Conversely, alleles for deleterious traits, such as those causing cystic fibrosis, may remain in the population due to a variety of factors including mutation-selection balance, heterozygote advantage, and genetic drift.
Mutation-Selection Balance and Deleterious Alleles
One reason deleterious alleles remain in the population is the mutation-selection balance. While natural selection works to remove these harmful alleles, new mutations continuously introduce them. The balance between these forces can maintain a certain frequency of the deleterious allele in the population.
Heterozygote Advantage
A well-documented example of this advantage is the persistence of the sickle-cell allele in populations where malaria is prevalent. Carriers of the sickle-cell allele (individuals with one copy of the allele) have an increased resistance to malaria, leading to a higher fitness in those environments, which keeps this allele in the gene pool.
Cystic fibrosis follows a similar pattern; it's hypothesized that carriers of the cystic fibrosis mutation may exhibit resistance to certain diseases. In populations where such resistance provides a significant advantage, the allele may be maintained despite its negative effects when present in homozygous recessive individuals.