General characteristics and number of forms
Although evolutionary biology records more than 30 different manifestations of natural selection, to understand the fundamental processes of adaptation, it is sufficient to examine three main forms. These include stabilizing, directional, and disruptive selection. The main difference between them lies in the mechanisms of conservation or elimination (death) of individuals with certain trait values, which is clearly demonstrated by the graphical data of the distribution curve.
Directional selection
This form of selection operates under conditions where individuals with a deviation from typical characteristics gain an advantage.
- Mechanism: Selection occurs for the survival of those organisms whose trait deviates in one specific direction. A characteristic example is the survival exclusively of larger individuals compared to the rest of the population mass.
- Result: On the distribution graph, this process looks like a clear shift of the curve's peak. This means that the mean value of the trait systematically shifts in the direction that ensures the best survival.
Stabilizing selection
Unlike directional change, this form strives to preserve the already existing optimal state.
- Mechanism: The main factor is selection for the survival of exclusively average-sized individuals. Those organisms that possess typical, averaged characteristics get maximum chances for successful reproduction.
- Result: There is a strict elimination (culling) of all extreme variants. Graphically, this is expressed by the fact that the distribution curve becomes narrower and higher. Biologically, this process is described as the narrowing of the norm of reaction.
Disruptive (diversifying) selection
This form represents a process that literally "tears" a population into several separate groups by phenotype.
- Mechanism: The vector of selection is aimed at preserving extreme variants of trait manifestation. For example, only the smallest and largest individuals may predominantly survive in a population.
- Result: Average, intermediate forms undergo elimination. When analyzing graphical data, one can see that the initial trait distribution curve changes its shape: instead of a single peak, two isolated vertices are formed.
Briefly: answers to topical questions
Under what environmental changes does directional selection operate?
Directional selection operates when environmental conditions change, and the old norm of reaction of the genotype ceases to correspond to them. In such situations, a shift in the mean value of the trait occurs, and a new norm of reaction is established. This form of selection can lead to the development of a trait (enhancement of a function or structure) or to its reduction—the loss of organs that have become useless. As a result, the accumulation of new adaptations is ensured.
What are the classical examples of stabilizing selection in nature?
Classical examples of stabilizing selection include the preservation of the average norm of a trait in constant conditions and the trimming of extreme deviations.
- Relict species — preservation of the tuatara.
- Birds — maximum hatching rate in ducks and chickens is observed from eggs of average weight.
- Humans — the highest survival rate is noted in newborns with an average body mass (3.2–3.6 kg), whereas infants with hypotrophy or hypertrophy die more frequently.
What specific examples of disruptive selection are given in biology textbooks?
As examples of disruptive selection, cases are cited where selection favors extreme variants of a trait and disfavors the average value, which leads to polymorphism.
- Insects on oceanic islands — either wingless individuals survive or insects with very powerful wings; medium-winged ones are blown away by the wind.
- Anthropogenesis — disruptive selection played a decisive role in the emergence of human races, favoring carriers of different adaptive traits in various conditions.
It is easy to remember the results of graphical changes by their names: Directional — "moves" the apex sideways; Stabilizing — "squeezes" (narrows the norm of reaction), preserving the center; Disruptive (diversifying) — "tears" one peak into two extremes.
Class and exam questions
How many forms of natural selection exist in total?
More than 30 different forms are known in nature, but classical theory distinguishes three main ones: directional, stabilizing, and disruptive.
What happens to average individuals under disruptive selection?
Intermediate, average forms undergo elimination. Only holders of extreme trait variants survive (e.g., only small and large ones).
Which form leads to the narrowing of the norm of reaction?
The narrowing of the norm of reaction is a direct result of stabilizing selection, in which all extreme deviations are culled and only typical representatives are preserved.
What does the result of directional selection look like graphically?
On the trait distribution graph, a shift of the curve's apex is observed, which reflects the change (shift) of the mean value in one direction.
What else to study on this topic
- Graphical data: plotting trait distribution curves
- Mechanisms of elimination of extreme and intermediate forms
- The concept of the norm of reaction and its narrowing
- Examples of individuals with trait deviations (large and small variants)
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