Types of Selection

A-D. Diagrammatic representation of the operation of natural selection on different traits: A. phenotypes favoured by natural selection; B. stabilising; C. directional; D. disruptive

• Conditions: Operates in all populations when phenotypic features align with optimal environmental conditions and competition is not severe.
• Mechanism: It tends to eliminate extreme traits from the population.
• Evolutionary Impact: It does not promote evolutionary change; instead, it maintains phenotypic stability within the population from generation to generation.

• Conditions: Operates in response to gradual changes in environmental conditions.
• Mechanism: It exerts selection pressure that shifts the mean phenotype toward one specific phenotypic extreme. Once the new optimum is reached, stabilising selection takes over.
• Evolutionary Impact: It drives evolutionary change by increasing the frequency of new alleles in a specific direction.
• Application: It forms the basis of artificial selection, where selective breeding for desirable traits continuously increases their frequency in the population's gene pool.

• Conditions: The rarest form of selection, driven by fluctuating environmental conditions (such as varying seasons and climates) that favour more than one phenotype.
• Mechanism: Increased internal competition pushes phenotypes away from the population mean and towards the extreme ends.
• Evolutionary Impact: It can split a population into two distinct sub-populations. If gene flow between these groups is prevented, each sub-population may ultimately evolve into a new species.