The action of natural selection consists in selecting individuals more adapted to a given ecological condition, eliminating those disadvantaged for that same condition.
The expression more adapted refers to the higher probability of a given individual surviving and leaving offspring in a given environment.
Natural selection acts permanently on all populations. Even in stable and constant environments, natural selection acts in a stabilizing manner, is present, eliminating deviant phenotypes.
However, the environment does not represent a constant and stable system, either over time or over space, which determines different interactions between organisms and the environment.
This heterogeneity provides different selective pressures on the gene set of the population, avoiding the elimination of certain alleles that would not be maintained in a constant and stable environment. Thus, genetic variability is reduced less.
This is the case with the maintenance in the human population of certain alleles that would normally be eliminated because they are poorly adaptive. An example is the allele that causes a disease called sickle cell anemia or sycemia.
This disease is caused by an allele that conditions the formation of abnormal hemoglobin molecules with poor oxygen carrying capacity. Because of this, the red blood cells that contain them become sickle shaped when the oxygen concentration decreases. For this reason they are called sickle cells.
The heterozygous cells have both normal red blood cells and sickle cell red blood cells.. Although slightly anemic, they survive, although less viable than normal homozygotes.
Under normal environmental conditions, the allele for sickle cell anemia suffers a strong negative selective effect, occurring with low frequency in the populations. However, a high frequency of this allele was observed in large regions of Africa, where there is a high incidence of malaria.
This high frequency is due to the advantage of heterozygous individuals for sickle cell anemia because they are more resistant to malaria. "Normal homozygous individuals" are at high risk of death from malaria while "homozygous individuals for the anomaly" die of anemia. The heterozygotes, however, have, under these environmental conditions, adaptive advantage, providing the high rate of a lethal allele in the population.