Final answer:
The disease sickle cell anemia shows that natural selection does not eliminate recessive alleles; carriers can have an evolutionary advantage such as resistance to malaria, which helps maintain the allele within the gene pool.
Step-by-step explanation:
The presence of the disease sickle cell anemia illustrates that natural selection does not always eliminate recessive alleles. People who are carriers of sickle cell anemia do not express the disease because they only have one copy of the recessive allele. However, these carriers can pass the allele to their offspring. Furthermore, heterozygous carriers have an evolutionary advantage in malaria-prone regions because the presence of one sickle cell allele confers resistance to malaria, a trait that is actually dominant.
This autosomal recessive disorder is an example of how an allele can persist in a population despite having a detrimental effect in homozygous individuals. While the allele can cause serious health problems, such as anemia and blood cell sickling when present in two copies, the heterozygous condition offers a selective advantage in certain environments, which helps maintain the allele within the gene pool.
Natural selection can preserve such harmful alleles if the heterozygous condition results in a survival advantage, as with the sickle cell trait and malaria resistance. This demonstrates the complex interplay between genetics and the environment in the process of evolution.