Prerequisites for selection
For the evolutionary mechanism to start working, a number of mandatory conditions must be met in nature. Without them, directed species change is impossible. There are three main prerequisites:
- Hereditary heterogeneity of individuals. Within any group of organisms, genetic diversity exists. No two individuals are absolutely identical; each organism has its own unique traits fixed in its genotype.
- Excess offspring production. Significantly more organisms are always born in nature than can successfully survive and leave their own offspring. Environmental resources are strictly limited.
- Struggle for existence. Due to resource scarcity and overpopulation, fierce competition arises. It manifests in three forms:
- Intraspecific — the most acute competition between individuals of the same species for food, territory, and mating partners.
- Interspecific — antagonism between different species (e.g., predator and prey).
- Struggle with abiotic factors — survival under conditions of unfavorable environmental changes (temperature, humidity).
How it works: classical examples
The action of selection can be easily traced through historical changes in populations under the pressure of external circumstances.
Industrial melanism (peppered moth) In industrial regions of England (such as Manchester), due to heavy air pollution with soot, birch trunks acquired a dark hue. Previously dominant light-colored butterflies became too noticeable on the darkened bark, and birds began actively preying on them. At the same time, dark-colored forms (melanics), resulting from a random mutation, remained unnoticed by predators. The outcome of such positive selection: dark butterflies survived, reproduced, and became the dominant form in this polluted area.
Poison resistance (rats and warfarin) Selection occurs not only at the level of external traits, but also at the level of variability in enzyme systems. In the 1960s in England, the anticoagulant warfarin began to be used to combat rodents. The dynamics were as follows: initially, the rat population dropped sharply, and the poison worked properly. However, populations completely resistant to the chemical subsequently appeared. Those few individuals that happened to have the necessary enzyme mutation survived and reproduced. As a result, the genetic composition of the population changed, and warfarin became completely ineffective.
Relativity of adaptation
It is important to understand that any adaptation arising during evolution is not absolute. It works ideally only in the specific conditions in which it was formed.
If environmental conditions change abruptly, the old adaptive form may become not just ineffective, but also threaten the survival of the entire population. The spread of warfarin-resistant rats is a striking illustration of how rapidly evolution can proceed under severe selection pressure when old defense mechanisms cease to save the species.
Interaction with the mutation process
Natural selection continuously interacts with mutations, filtering the genetic material of the population. It removes lethal and harmful defects, preventing them from spreading.
A classical example of this interaction in humans is the genetic disorder hemophilia (blood clotting impairment). Statistically, men with severe hemophilia most often die before reaching reproductive age. The result of this brutal yet regular process: the recessive allele causing the disease is gradually eliminated (removed) from the population by natural selection, as carriers do not survive to pass it on to subsequent generations.
Briefly: answers to topical questions
What are the main forms of natural selection?
More than 30 forms of natural selection are known in biology, but three main ones are distinguished.
- Stabilizing — one of the main forms, which can act jointly with directional selection.
- Directional — a form of selection whose alternating action with stabilizing selection shapes polymorphism.
- Disruptive — "diversifying" selection directed against average, intermediate forms and favoring more than one phenotype (usually extreme variants).
OEP: Offspring (excess), Evolution/Variability (heterogeneity), Struggle for resources — three basic conditions that trigger selection.
Class and exam questions
What is the main evolutionary role of natural selection?
It translates random individual organismal variations into beneficial traits, fixing them at the level of the entire population or species.
Why is adaptation called relative?
Any adaptation is useful only in a specific environment. When external conditions change, the very same trait may become useless or even lead to the organism's death.
How does selection affect harmful mutations (using hemophilia as an example)?
Selection eliminates (removes) harmful alleles from the gene pool. Individuals with severe mutations often die before puberty and do not pass the defective gene to descendants.
What else to study on this topic
- Forms of the struggle for existence: intraspecific and interspecific
- Biochemical mechanisms of enzyme system variability
- Influence of abiotic environmental factors on the direction of selection
- Dynamics of the elimination of lethal recessive alleles from the gene pool
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