Prerequisites and mechanisms of the evolutionary process
For natural selection to begin operating, three key prerequisites must be present in a population:
- Hereditary variability. This is the basis for any evolutionary changes. Without primary genetic diversity, selection would have nothing to "choose" from.
- Overproduction of offspring. Organisms in nature always produce significantly more offspring than the resources of their habitat can sustain.
- Struggle for existence. A logical consequence of the overproduction of offspring. Individuals are forced to constantly compete with each other for limited food resources, as well as withstand the severe pressure of abiotic factors.
The interaction between mutations and selection determines the direction of evolution. The mutation process acts as a raw material supplier — it generates genetic material. Natural selection, in turn, performs the function of a strict directional filter that fixes traits beneficial for survival and eliminates those that hinder the organism from the gene pool.
Forms of natural selection
Depending on how environmental conditions change, three main forms of selection are distinguished, affecting traits in a population in different ways.
- Directional selection. Activated when environmental conditions change. In this case, the average norm of a trait begins to shift in a direction more advantageous for survival.
Examples: reduction of vision organs in subterranean dwellers (mole), elongation of the neck in giraffe ancestors, industrial melanism (darkening) of butterflies in industrial areas. A classic modern example is the development of antibiotic resistance. Bacteria with accidental resistance mutations survive therapy and reproduce, completely displacing the original sensitive strains.
- Stabilizing selection. Operates in stable, constant conditions. Its task is to preserve the established average norm of a trait and ruthlessly cut off any extreme, atypical deviations.
Examples: the survival to the present day of ancient relict species (the tuatara reptile); infant survival statistics, which are always higher in babies with average weight compared to extremely low or excessive weight.
- Disruptive selection. Favors individuals with extreme trait variants while actively acting against the mean value. Often leads to population polymorphism.
Example: insect populations on open oceanic islands with constant strong winds. Either wingless forms survive (do not take to the air) or long-winged ones (capable of actively resisting the air current), while insects with medium-sized wings are blown into the ocean and perish.
Formation of adaptations and genetic load
The result of prolonged natural selection is the emergence of adaptations — a set of genetic, behavioral, and morphophysiological features that ensure a species survives and maintains a specific lifestyle in a particular environment.
Adaptations are usually divided into several levels:
- Morphological: anatomical structural features, such as the ideal streamlined body shape in fish or protective camouflage coloration in animals.
- Physiological: reorganization of internal life processes. For example, the unique ability of the camel to extremely conserve water in the desert or the development of precise echolocation in bats.
- Behavioral: complex instincts, including parental care and regular seasonal bird migrations.
Despite the perfection of many adaptations, populations are not ideal. The gene pool inevitably accumulates a genetic load — lethal and sublethal mutations. They reduce the overall fitness of the population. In the human population, this load is maintained through the continuous mutation process and the phenomenon of heterozygote advantage. A striking example is sickle cell anemia: in malaria-endemic zones, this dangerous gene is preserved because heterozygous carriers acquire resistance to the malaria parasite.
Briefly: answers to topical questions
What types of the struggle for existence exist?
Within the struggle for existence, three main types of competition and antagonism are distinguished.
- Intraspecific — competition between individuals of the same species.
- Interspecific — competition between individuals of different species.
- Struggle against unfavorable abiotic environmental factors — confronting the conditions of inanimate nature.
What evolutionary consequences does disruptive selection lead to, besides polymorphism?
Disruptive selection leads to the elimination of intermediate forms. At the same time, two peaks of the trait distribution curve are formed.
The forms of selection are easy to remember by the fate of the "norm": Directional — the norm shifts following environmental changes, Stabilizing — the norm is preserved in a stable environment, Disruptive — the norm is split into two opposite extremes.
Class and exam questions
To which form of selection does bacterial resistance to antibiotics belong?
This is a classic example of the directional form of natural selection. Environmental change (the introduction of an antibiotic) shifts the survival norm towards bacteria possessing resistance mutations.
Why is genetic load maintained in populations if selection is supposed to weed out harmful genes?
The genetic load is constantly replenished by new mutations. In addition, harmful genes are often preserved in a hidden form in heterozygotes, and in some cases (as with sickle cell anemia in malaria zones), such heterozygotes even gain a survival advantage.
What else to study on this topic
- Mechanisms of antibiotic resistance formation at the cellular level
- Forms of the struggle for existence: intraspecific, interspecific, and struggle against environmental conditions
- Population polymorphism as a consequence of disruptive selection
- Heterozygote advantage using the example of sickle cell anemia and malaria
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Natural selection and adaptations" and 3,793 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the "Epigenetics and evolution" section
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.