Natural selection

When a population of single species increases compared to the other population, this reproduction capability is referred as natural selection.  The mechanism by which evolution mechanism occurs is natural selection. Natural selection can be an outcome if an individual is mutated or genetically changed in its expression. It retards the growth of new allele frequency. George William states that evolution is a result of natural selection.

Balancing natural selection

Process of evolutionary changes through natural selection may either result in change of genetic variations by maintaining the same or depleting the genetic variations. Natural selection helps in maintenance when heterozygote fits comfortably compared to homozygote. This maintenance and depletion of natural selection are balanced and are referred as balanced selection. For instance when alleles of sickle cell anemia is maintained in human body system, then it causes malaria. By a single variation in red cell shaped, it is easy to predict whether cells are normal or sickle shaped. When an individual contains alleles for sickle cell, then it may lead to anemia formation. This is because sickle shaped cells prevent the cells to carry the oxygen. If an individual has heterozygote alleles may develop some resistance to anemia as the malarial cells cannot invade. Homozygous alleles are in danger compared to heterozygous alleles. When these two are balanced individual can be safe.

Types of natural selection

Major types of natural selection are as follows.

• Stabilizing selection
• Directional selection
• Disruptive selection

Stabilizing selection: H. Bumpus was the first to propose stabilizing selection in the year 1899. He studied stabilizing selection in wings of Passer domesticus, house sparrows. Both long and short winged sparrows were being killed. He stated that stabilizing selection will not help in evolutionary changes. This cannot be considered as evolution as most of the times it never lead to evolution.

Directional selection: Phenotypes are favored by strong natural selection. Hence the genetic variation ultimately reduces. For instance antibiotic resistance induced by microorganisms like bacteria and insecticide resistance induced by insects. Population of resistant individuals will never allow genetic variations. It may cause displaced characters when two species start competing. When birds of long and short beak compete it may lead to development of intermediate beak in birds.

Disruptive selection: When heterozygous alleles are at danger whereas homozygous are selected. This type of selection increases the diversity among population. At times there are chances for single organism to evolve as two separate organisms. For instance African swallowtail butterfly of two different morphologies mimic one another with aposematic coloration that is Batesian mimicry.

Questions:

• How can natural selection be balanced in a better way?
• Describe types of natural selection in detail.
• Give an instance for each type of selection.
• How natural selections help in evolution of population?