Selection

How does selection act as an evolutionary factor?

Selection (lat. Selectio = selection) consists in a broad sense of three forms:

 Natural Selection: Living organisms better adapted to their environment increase the likelihood of their genes being transmitted as less well-adapted creatures (see below for explanation).

 Sexual Selection: Intimate selection of sexual partners resulting from the competition for reproductive partners. Sexual selection also explains numerous phenotypic characteristics that would actually be detrimental to natural selection (for example, the peacock's plumage, which is a hindrance to escape from predators, but is an important "courtship").

 Artificial selection: Man-directed selection to promote certain traits in animal and plant species (e.g., higher milk yield from cows, disease-resistant crops such as wheat or small animal farming)

The basic assumption of natural selection is the fact that individuals who live longer can pass on their genes more often. The better an organism is adapted to its natural environment, the more frequently it will be able to pass its genes on to the next generation. One speaks also of the concept of the fitness.

Already Charles Darwin recognized this mechanism and spoke in his work "On the Origin of Species" of the "survival of the fittest". This phrase is often misunderstood and translated as "survival of the fittest". Darwin means the survival of the best adjusted individual. It is not the strongest who bring their genes into the next generation, but those who are best adapted to the external environment. This inevitably leads to a longer lifetime and thus to more offspring (in the optimal case). It happens quite well that less well adapted individuals reproduce and thus pass on their genes in the daughter generation. Selection is also a statistical probability process for this reason. For optimal adaptation to a habitat does not guarantee successful reproduction for a long time. Innumerable processes of an extra-species and intraspecific nature play a role. That's why you have to look at the populations in the selection more in the whole. Although less well-adjusted individuals also bring their genes into the next-generation gene pool, they are statistically less common than the better-adapted ones. In this way, favorable alleles in the gene pool are more common, adverse alleles rare.

At this point, the term selection pressure is briefly explained: Selection factors (see abiotic selection factors, biotic selection factors) act on all living beings, which "press" all populations and thus determine the direction of evolution. This does not happen in an active process, but passively. Populations are adapted to environments by selection and do not adapt themselves.

In the following the three selection types are presented:

Types of Selection - Transforming Selection

In transforming selection, the selection pressure acts from one side. The population moves accordingly away from the selection disadvantage in the other direction.

The X-axis corresponds to the intensity of the feature expression and the Y-axis to the number of individuals.

An example is the population of a deep-sea fish being hunted by a larger predatory fish. Escape speed proves to be an advantage for the prey fish, because it is eaten less often. In the long run, the population of the prey fish will thus change in the direction that the speed of each individual increases. Because: Faster fish can bring their genes (with the important alleles for increased swimming speed) more often into the next generation than slower ones.

Types of Selection - Stabilizing Selection

Stabilizing selection occurs when the selection pressure originates from both extreme sides of the characteristic expression. In this way, it comes in the long run to approximate the mean, because extreme forms are disadvantaged.

The X-axis corresponds to the intensity of the feature expression and the Y-axis to the number of individuals.

Particularly in the expression of the size of wings, the stabilizing selection is observed. Birds with extremely large and extremely small wings lose their ability to fly, so that in the long term stabilizes the average. As a result, extreme phenotypes are becoming rarer or no longer appear.

Types of Selection - Disruptive Selection

The central feature of disruptive selection is the formation of two extreme phenotypes. Ergo proves the expression of a feature in the average range as a disadvantage.

The X-axis corresponds to the intensity of the feature expression and the Y-axis to the number of individuals.

The splitting of a Darwinfinkenart (keyword: adaptive radiation) in insectivores and grains / nut eater is an example of disruptive selection. To catch and eat insects a fine, thin beak is necessary. For the cracking of nuts, on the other hand, a strong beak.

The mean value of a beak of these two forms, however, brings no benefits, or is inferior to the particular specialized forms. In this way, the population eventually experiences two extremes: a thin beak and a thick beak.

Summary

Selection (Latin selectio = selection) can be differentiated into natural selection, sexual selection and artificial selection

Transforming, stabilizing and disruptive selection are types of selection that may occur in the above forms