Incomplete dominance in this gene. Heterozygotes experience slight anemia, homozygotes rarely live to reproduce. Frequency of S in African-American population is about 0.05, suggesting that the allele is being lost. The opposing forces acting on sickle cell polymorphism are an example of antagonistic selection . Antagonistic selection does not have to maintain polymorphisms, that only happens when the heterozygote has the highes fitness.
Antagonistic selection can occur in different life stages.
One genotype may be more fit in terms of the larval stage of an insect, while another genotype is more fit in the pupal or adult stage. This still will not result in long-term polymorphism unless the overall fitness of the heterozygote is higher than either of the homozygotes. The environment is not always constant Fluctuation environments may favor different genotypes at different times, that is, the environment is temporally variable. Different genotypes may be fittest in different microhabitats, that is, the environment is spatially variable. Again, neither condition is sufficient to maintain polymorphism - the heterozygote must still have higher fitness, averaged over time or space.