According to Darwin’s theory of evolution and conception of selection, only those individuals survive the «struggle for existence» (struggle for life) that make the best use of the given environmental conditions and can thus reproduce most successfully. The prerequisite is that each species produces more offspring than is required for its maintenance and that the offspring vary among themselves.
Definition and origin of the selection
selection comes from the Latin from selection, which means «choice». Selection describes the success of a population in reproducing and thus ensuring survival.
Today, the selection, taking population genetic principles into account, is called directed displacement of gene or allele frequencies in the gene pool of a population. Since the selection is only based on the characteristics, i.e. the phenotype of an individual, which a certain genotype is based on, those individuals can preferentially pass on their genes that have the hereditary change in the phenotype demonstrate.
Whether an individual is more or less well adapted to its environment, ie the level of its aptitude or fitness, can be determined by its reproductive success read off:
- The greater the life expectancy and the earlier sexual maturity occurs, the more offspring can be sired.
- Reproduction is more successful when mates are found quickly.
- Producing more offspring that reach reproductive age can increase the fitness of the species.
Selection is thus reflected in the fact that carriers of certain genotypes have greater reproductive success than others. It causes a targeted shift in gene or allele frequencies in the gene pool of a population.
selection pressure
selection pressure means the influence, i.e. the strength that a selection factor exerts on a population.
A distinction can be made between the following three types of selection, which are described in more detail later in this article:
- Transformational or directional selection
- Stabilizing selection
- Divisive or disruptive selection
selection types
There are basically three different types of selection:
forms of selection
In contrast to mutation and recombination, selection acts in a certain direction. This selection pressure leads to different effects on the gene pool of a population.
Transformational or directional selection
As environmental conditions change, some variants may gain a selective advantage. This increases the reproductive success. Therefore, the proportion of variants in the population increases. Thus, the gene pool is also better adapted.
If the changed environmental conditions exist for a long time, then the selective pressure acts in such a way that the characteristics of the individuals change from one side only. Species change may occur and the «differently» developed species no longer reproduce with each other. Thus, the variant that occurs most frequently is disadvantaged.
Figure 1: Transformational selection.
Stabilizing selection
When environmental conditions are long-lasting but stable, some individuals evolve to be optimally adapted. Most variants that emerge are worse than the existing fit. Therefore, these are not chosen by the selection. As a result, the species remains the same because it does not change. When selection pressure is constant and applied from both sides, individuals evade the pressure.
Figure 2: Stabilizing selection.
Divisive or disruptive selection
The resulting variants make up the majority of this population. The selection pressure acts on them. The original population is disadvantaged for this. The population may split up. Then there are two new populations. A split can e.g. B. caused by parasites or predators.
Figure 3: Disruptive selection.
selection factors
The strength and direction of selection are environmentally dependent. To put it simply, the demands of the environment on living beings can be abiotic and biotic selection factors split up.
abiotic factors
the abiotic factors include all influences of inanimate nature (Climate, soil conditions, geographic location, etc.).
For example, in very arid areas, only plants that have effective protection against evaporation (e.g., a thick cuticle) and develop thickened epidermis or hair coats and water-storing tissues (stem, leaf succulence as convergent developments in different plant families) can grow.
biotic factors
Biotic factors are those location factors the living environment of an organism, e.g. E.g. predators and prey, pathogens and parasites, competitors or humans. Camouflage protects prey from predators. Conversely, camouflage the predator also prevents it from being quickly discovered by the prey.
Many animals are camouflaged by coloring and patterning that matches their environment, e.g. B. hare, frog, lion, fawn, peppered moth.
Unintentional human influences can also be part of the biotic factors of a distribution area. The phenomenon that dark colored mutants of many insect and spider species are common in industrial areas is called industrial melanism.
Mimesis is camouflage where an item is replicated/copied.
Striking red-black or yellow-black color patterns are intended to signal inedibility or danger to predators. as mimicry or pseudo-warning costume is the imitation of defensive or poisonous animals by completely harmless animals. A prerequisite for the effect of mimicry is the ability of the predator to learn and a small number of imitators, so that the predator knows how dangerous the originals are.
Examples of biotic selection factors
Here are two more examples of biotic selection factors: transformative and sexual selection.
transformative selection
Through intraspecific (Intraspecific) competition for food, living space and sexual partners increases the variability within a population, since e.g. B. the mutants, which can use other food sources because of their more extreme characteristics, have a greater reproductive success. One ecological niche formation can be the result.
Take a look at the relevant article on ecological niche development!
interspecific (Interspecific) competition causes a shift in the variability of the species concerned in opposite directions. Each species is adapted to environmental conditions in its own specific way. It comes to niche.
sexual selection
Sexual selection (sexual selection) as a special case of intraspecific competition occurs mainly in animals where several males are courting for the favor of a female. The success of a male depends primarily on the effectiveness of his sexual triggers, which are intended to induce the female to mate.
When males compete for females, physical traits or behaviors are often developed to impress the rival and cause him to retreat. Males with alleles that lead to the optimal development of such triggering signals are then present in a larger proportion in the next generation. In this way, sometimes very pronounced differences between the sexes, the so-called sexual dimorphism, develop.
Selection – The most important thing
- Selection is reflected in carriers of certain genotypes having greater reproductive success than others. It causes a targeted shift in gene or allele frequencies in the gene pool of a population.
- Selection is one of four evolutionary factors that affect a population’s gene pool.
- One can distinguish between natural, sexual and artificial selection.
- Selection pressure leads to different effects on the gene pool of a population. A distinction is made between:
- Transformational or directional selection
- Stabilizing selection
- Divisive or disruptive selection
- In addition, a distinction is made within selection between abiotic and biotic selection factors.