From the article: This distribution is determined by the occurrence of the sickle cell mutation and its selection by falciparum malaria.
Andrew: "Then why is the Hybrid frequency only 15-25% in a malarial area?"
Only 15-25%?!!!! That is a very high incidence of an otherwise deleterious gene. The article explains that falciparum malaria selects (makes more prevalent) for people that are more immune to malaria. The incidence of malaria is proportional and tandem to the incidence of sickle cell.
If we had rates approaching 25% sickle cell here in America, it would be considered a huge public health crisis.
Oh, so now you agree with me. Remember, you are arguing that it is a beneficial gene when compared to the normal allele in the malarial regions. In the malarial regions it is the normal allele that is deleterious.