How can natural selection work to keep a harmful allele in a population?

Answer 1

According to science, natural selection helps suppress harmful changes in species by slowly making adaptations. These adaptations are necessary for the ongoing of the species. Without the changes, the species could become extinct.

Answer 2

One of the post potent factors in keeping deleterious alleles in a population is heterozygote advantage. The classic case is sickle-cell anemia.

In Africans and people of African descent, there is a higher percentage of people with sickle-cell anemia - a genetic blood disorder that causes the red blood cells to become misshapen (in a sickle shape), affecting the way oxygen is transported throughout the body. Sickle-cell anemia definitely decreases life expectancy, but modern advances have lengthened that period to, in some cases, a normal life span. However, before modern technological and medical advances, an individual with sickle-cell anemia wouldn't live long into adult-hood.

So the question becomes: if sickle-cell anemia kills the affected individual and decreases their chance of reproducing, then why does the gene persist in populations? The reason for this is heterozygote advantage. A person that is heterozygous for sickle-cell anemia (or a carrier) has alterations on their blood structure. The blood of a carrier is changed by the latent allele, causing increased surface tension. This tension makes it difficult for malaria to enter the cell. Malaria is another big killer in Africa, but those who are sickle-cell carriers have a partial defense against it. Thus, you can see why the sickle-cell allele is still present. On one hand, sickle-cell fully expressed should decrease reproductive potential of the individual, decreasing the allele frequency. However, the presence of the allele in a carrier contributes to increased fitness in the environment.