Other kinds of natural selection

There are two other types of natural selection: stabilizing selection in which the more common allelic traits in a population are favored, and less common are disfavored or eliminated, and destabilizing selection, in which
intermediate traits, which should be most common, are disfavored in a population.A hypothetical example of these effects might use the concept of predation, similar to the peppered moth.

Suppose a moth comes in a range of wing colors from very light to very dark. This range of traits could result from a set of genes that control the degree of coloration of the wing. Individuals in the population with various combinations of alleles might fall somewhere on this continuum, with the intermediate colored wings being most common. Stabilizing selection could select for these intermediate forms if the two extremes are more likely to be prey.

This might be the case if the intermediate form had better
camouflage, or if specific predators targeted the two extreme types.

Destabilizing selection could select for the two extreme types if the intermediate colored moths were more prone to being preyed upon.

Again, since the existence of stabilizing or destabilizing selection depends
on environmental factors (presence of predators, camouflage opportunities) and these factors can change, the selection on the population may also change over time.

Balance polymorphism as a special case

Balanced polymorphism occurs when environmental conditions favor the continued presence of two or more allelic variants. Destabilizing selection can result in such a condition, with the continued presence of the two extreme phenotypes at the expense of the intermediate. The example of the genetic trait causing sickle-cell anemia is an example of a different form of balanced polymorphism.

Polymorphism (polymorphous means literally "having many forms") is a situtation in which there are more than one trait present in a population. The sickle-cell trait is present in relatively high proportion in the population of tropical Africa. This trait is clearly harmful since homozygous individuals often die in their teens or twenties. However, heterozygous individuals have a significant higher resistance to serious effects of malaria, which is endemic to the same area of Africa. Apparently, the beneficial effect on the heterozygotes is enough to ensure the continued presence of the sickle-cell trait, despite the clearly harmful effect of the homozygote.