Stabilizing selection

Mechanisms of action and genetic effects

To understand the essence of this evolutionary process, it is necessary to examine how exactly it affects a population. The key condition for triggering this mechanism is the long-term existence of a population in an unchanging, absolutely stable habitat. Under such conditions, organisms have already achieved optimal adaptation, and any changes are more likely to harm than to benefit.

The primary mechanism consists in increasing the frequency of well-adapted phenotypes. This occurs due to the strict elimination of individuals that exhibit extreme deviations from the established norm. It is precisely these individuals with atypical traits that prove to be the least adapted to stable conditions.

As a result of this constant elimination pressure, the following genetic effects are observed:

• Narrowing of the norm of reaction. The population becomes more homogeneous, leading to a significant reduction in overall genotype variability.
• Preservation of successful combinations. Allele combinations that were successfully formed during past stages of evolutionary development are reliably fixed and preserved in the gene pool.
• Protection against mutational pressure. Selection acts as a kind of filter. It safeguards the optimal adaptive norm from the destructive influence of the spontaneous mutational process, methodically weeding out any departures from the standard.

Biological significance and conservative role

The overarching biological significance of stabilizing selection lies in the fact that it plays a pronounced conservative role. It is critically necessary for maintaining stability in living nature. Without this mechanism, the results previously achieved through directional selection would be quickly eroded and lost.

One of the most impressive consequences of this conservative role is the preservation of the total biochemical unity of life on our planet. This selection acts continuously at all levels of organization of living matter, preventing basic molecular mechanisms from changing.

Striking examples of biochemical conservatism:

1. The universality of the set consisting of exactly 20 amino acids used to build all proteins.
2. The striking similarity of the fine molecular mechanisms of DNA replication and repair across completely different organisms.
3. Strict conservatism in the structure of essential, vital genes. These include genes encoding ribosomal RNA (rRNA), structural histone proteins, and cytochrome enzymes.

Classic examples of selection in action

The effect of stabilizing pressure can be easily traced through specific examples from zoology and medicine, where survival directly depends on matching the average phenotypic norm.

• Human population: Newborn survival statistics serve as a classic example. The highest survival rate is consistently recorded in infants with an average body weight varying within a narrow range — from 3.2 to 3.6 kg. Infants born with marked hypotrophy (critically low weight) or, conversely, hypertrophy (excessive weight) die significantly more often due to non-compliance with the optimal adaptive norm.
• Birds (domestic and wild): A similar pattern is observed during incubation. In birds such as ducks or chickens, the maximum hatchability and viability of chicks is noted exclusively from eggs of average weight. Eggs with extreme weight values yield a much lower percentage of successful offspring.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of elimination of individuals with extreme phenotypic deviations
• Relationship between genetic variability and the narrowing of the reaction norm
• DNA replication and repair processes as an example of biochemical conservatism
• Structure and evolutionary stability of conservative genes (histones, cytochromes, rRNA)

Other topics in the section "Forms of natural selection"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.