Disruptive selection

Essence and mechanism of action of disruptive selection

Disruptive selection works on the principle of "rupturing" a single population. Unlike other forms of selection, it is directed strictly against average, typical characteristics that may have previously been the norm. When environmental conditions change, it is precisely the carriers of intermediate forms that lose their advantages, whereas survival is ensured by the presence of sharply expressed, extreme traits.

From the perspective of population genetics, the mechanism of this process represents a complex combination. It is the joint and alternating action of two other forms of selection — directional and stabilizing.

How this happens in practice:

• First, environmental conditions change in such a way that diverging phenotypic groups gain an advantage. At this stage, directional selection actively operates, shifting the norm of reaction in opposite directions from the initial mean value.
• When these new, extreme forms become fixed in their specific ecological niches or conditions, stabilizing selection kicks in. It begins to prune any deviations from these new optima, finally destroying the remnants of intermediate variants.

As a result, a single population is essentially "ruptured" into several isolated groups, each possessing its own set of adaptive traits.

Polymorphism as the main outcome

The prolonged action of disruptive selection inevitably leads to a change in population structure. The main result of this process is the emergence of polymorphism (from Greek poly — many, morphe — form).

Polymorphism within a population means the long-term coexistence of two or more different phenotypic forms within the same habitat range. According to the mechanisms under consideration, the outcome of diversifying selection is the formation of heterozygous polymorphism specifically. This is a state where genetic diversity is maintained at a high level due to constant interbreeding and the survival of carriers of different gene variants that provide those very extreme phenotypes. The population becomes heterogeneous while maintaining its integrity as a biological species at the given stage of evolution.

Classic examples and role in evolution

The action of disruptive selection is best illustrated using specific biological models and historical processes.

Insects on oceanic islands This is a textbook example in evolutionary biology. On open oceanic islands with specific conditions, only two extreme forms of insects survive:

1. Individuals with very powerful wings, capable of actively resisting air currents.
2. Absolutely wingless individuals that physically cannot take flight.

Any intermediate forms (with average, insufficiently strong wings) are culled by selection, as they inevitably perish, unable to cope with environmental conditions.

Impact on anthropogenesis Disruptive selection occupies a special place in the history of our own species. It is believed that this very form of selection played a defining role in the emergence of human races. During the migration of ancient humans across the planet, populations found themselves in radically different climatic and geographical conditions. Selection favored carriers of completely different adaptive traits (e.g., skin pigmentation, facial structure, body proportions) in various regions, which led to the fixation of extreme adaptation variants and the formation of racial diversity.

Class and exam questions

What else to study on this topic

• Directional selection and its difference from disruptive selection
• Stabilizing selection: preservation of mean values
• Heterozygous polymorphism: genetic maintenance mechanisms
• Divergence of traits in evolution
• Adaptive traits of human races in anthropogenesis

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.