How is natural selection related to phenotypes and genotypes?

For example, malarial organisms fail to complete the erythrocytic part of their life cycle in RBCs present in the blood of carriers of sickle cell anemia; in Africa, such heterozygous carriers are getting naturally selected for thousands of years because they are naturally immuned against dreaded malaria. Phenotypes that help an organism survive better and reproduce more frequently are the ones that get selected for.

Consequently, a condition known as the genetic load—the presence of undesirable genes in the gene pool—has resulted in a large number of people still carrying an undesired recessive gene associated with sickle cell anemia.

Natural selection acts on phenotypes, which are the observable characteristics of an organism, but ultimately operates through the underlying genotypes, which are the genetic makeup of an organism. Phenotypes are the traits that affect an organism's fitness and survival in its environment, and natural selection favors those phenotypes that confer advantages for survival and reproduction. These advantageous phenotypes are often associated with specific genotypes that encode for them, and over time, the frequencies of these advantageous genotypes increase in a population due to natural selection. This process leads to evolutionary changes in populations as the frequency of beneficial genotypes increases, resulting in adaptations to the environment.