Natural selection. Material for selection. Basic principles of material selection

Our mother nature is very wise. A weak and unadapted organism simply does not stand a chance of survival. Is it possible by natural laws to allow a sick individual to give the same unhealthy offspring? Of course not, so all organisms are forced to fight for their survival. The winner in this struggle is strong, hardy, most fit and healthy. So natural selection is carried out. The material for selection and its principles will be discussed in more detail in the article.

The concept of natural selection

If we give a definition, then we can say that this is a process that leads to an increase in the number of the most viable and adapted individuals. Weak and poorly adapted simply can not stand competition. Synthetic theory of evolution considers natural selection, material for selection, as the main reason for the development of all adaptations and the formation of superspecies categories.

Natural selection, although considered the cause of adaptation of organisms to the environment, but he alone is the culprit of evolution in nature. The term was introduced by Charles Darwin, who devoted many of his works to the study of this issue.

From what follows natural selection

Genes in any organism are capable of mutations that can occur for a variety of reasons. In the process of natural selection, their fixing takes place, but only those that contribute to improving the adaptability of organisms to their habitat. Quite often, natural selection is called a self-evident mechanism, as it follows from several factors:

1. Each organism is capable of producing offspring much more than it can survive.
2. In any population there is a hereditary variability, this is the starting material for natural selection.
3. Genetically heterogeneous organisms differ among themselves not only by survival, but also by the ability to reproduce.

These factors contribute to the creation of competition between organisms in survival and reproduction, and they are simultaneously a necessary condition for the evolution of living nature through natural selection. In nature, it is so arranged that organisms with predominant hereditary traits transmit them to their offspring, while individuals that do not have such superiority have the least probability of transmission.

Selection mechanism

Charles Darwin and Alfred Wales spoke for the first time about the fact that nature itself contains some mechanism that is similar to artificial selection. They were sure that nature does not need to delve into all situations - it is enough to create a large variety of individuals, among which the fittest will survive. The selection mechanism can be presented as follows:

1. Appearance of an individual with new signs.
2. If the signs are useful, then the body survives and leaves offspring.
3. Descendants inherit useful qualities and begin to transfer them to the next generation.

Despite the fact that modern genetic discoveries are making their own adjustments, the essence of Darwin's theory remains unchanged. Perhaps, only changes occur much faster, rather than smoothly, as he argued, due to mutations that are spasmodic in nature.

Source material for natural selection

Hereditary variability serves as the material that leads to natural selection. All hereditary changes appear as a result of mutations. But for evolutionary transformations, only those that affect the sex cells are of interest, because it is through them that information is transmitted to the next generation.

Most mutations have a recessive character, that is, they can not immediately manifest, as they are suppressed by dominant genes. But they are able to accumulate, they do not disappear from the gene pool of the population, although they do not affect the fitness and do not manifest themselves phenotypically.

The mutation process is continuous, the number of such mutations is constantly accumulating, and at one fine moment there are two recessive genes and the symptom is necessarily manifested. The material for selection is hereditary variability, but such changes do not always lead to increased viability and fitness. Quite a lot of mutations, on the contrary, reduce these qualities, as they provoke various violations in metabolic processes.

But it is possible to give examples when a seemingly harmful mutation, when the conditions of existence change, is useful. In house flies there is a mutation, which leads to a decrease in the speed of conduction of nerve impulses. If the organism turns out to be homozygous for the given trait, then the mutation turns out to be fatal, but heterozygotes remain viable, although they are inferior in fitness to healthy individuals. But when the flies of the nerve agent are exposed to the flies, the heterozygotes win in normal individuals, since the slowed down rate of the impulse significantly reduces the effect of the poison on the body.

Types of natural selection

The source material for selection is hereditary variability, but it can lead to the appearance of features that can vary in a different range. Depending on this, the types of selection are as follows:

• Directed . The mean value of the characteristic changes over time. This includes increasing the size of the body.
• Diversive selection is aimed at selecting extreme indicators (for example, too large or, conversely, small).
• The stabilizer is directed against the manifestation of the extreme values of the trait.

Sexual is also a natural selection. The material for selecting this level is any sign that increases the likelihood of pairing by increasing the attractiveness of the individual for the opposite sex. This is well manifested in males of some species (for example, huge horns in deer, bright coloring of plumage in birds).

Forms of natural selection

The forms of selection can be classified in different ways, but the criteria for selecting the material are almost always the same:

• Usefulness of this characteristic for the individual.
• Necessity and importance of a sign for survival under certain conditions.
• The positive impact of the sign on the prosperity of the species as a whole.

Material for artificial selection is also hereditary variability, but the criteria are completely different. Here the palm of primacy is obtained by the signs that are necessary for a person, and not for an organism for which they can be quite harmful at all. You can give an example with a breed of depths, which are called pouters. They have a large goiter that makes them unusual and attractive to humans, but in nature such individuals will be completely helpless and will not withstand competition with their fellow humans. They simply can not find their sustenance. So it turns out that the selection of material basic principles is quite different with natural and artificial selection.

Depending on the effect of selection on the variability of the trait in the population, the following forms are distinguished:

1. Moving.
2. Stabilizing.
3. Tearing, or disruptive.

There is a need to consider each selection separately in more detail.

Features of driving selection

The reason for this selection is always changes in the conditions of existence of the species. Individuals that result in signs that deviate from the mean due to the fact that the material for natural selection delivers hereditary variability are in a more advantageous position. From generation to generation there is a shift in the trait in a certain direction, the result is exactly the one that helps organisms survive in new conditions.

A vivid example is the evolution of color in a birch moth. It has since its appearance inhabited the trunks of birches, which have a white color. Accordingly, the wings of this butterfly are also white.

But with the development of industry, the atmosphere began to get dirty, a lot of soot and soot appeared in the air, which settled on tree trunks. As a result, their color became far from white. Of all the progeny of butterflies, the one that benefited was the one that due to the mutations had a darker color, since the light ones were quite noticeable for the birds and they were often eaten up. So gradually the evolution went in the direction of changing the color of butterflies.

The manifestation of stabilizing selection

Consider stabilizing natural selection. The material for selection here is also hereditary variability, but its effect is already directed against the appearance of abnormalities. It is possible to give an example: for all organisms, it seems that increased fertility is only good, as it leads to an increase in the population size. But actually it is not. Benefits are given to individuals with average fertility rates, since it is quite difficult to feed a large number of offspring.

Selection in favor of average indicators can be seen in the case of many features. For example, it is preferable for middle-sized birds to have wings of medium size. If there are too short, it is problematic to take off, and if the wind is too long, the wind will interfere with the flight.

Stabilizing selection contributes to the accumulation of variability in populations. Even stable conditions for the existence of a species do not lead to an end to natural selection and evolution as a whole. This type of selection ensures the stable functioning of organisms in the usual external conditions.

Disruptive selection

With this form of selection, the conditions of existence are suitable for the extreme manifestations of the feature. As a result, several forms of existence appear.

Diversive selection leads to the formation of polymorphism, and may even become the cause of the formation of new species in nature.

This selection often takes effect when the population occupies a heterogeneous habitat. Different forms are forced to adapt to different niches and conditions. For example, a plant has two forms of a cellar - one starts to bloom and bear fruit in the middle of summer, and the second - after haymaking, that is, in August.

The positive role of selection and negative

Rather, it's not even a role, but forms of selection that have different effects.

1. Positive selection leads to an increase in the number of organisms that have useful characteristics for survival in these conditions, increasing the survival of the species as a whole.
2. Negative, or it is also called cutting off selection, leads to the destruction of individuals with signs that sharply reduce survival and fitness. This selection helps to remove harmful alleles from the population.

Selection Effects

We have already found out what is the material for selection, considered its forms. But it is necessary to clarify what effects this or that selection gives. The motivator leads to the appearance of new adaptations, in this the results of his action appear:

1. Accumulating . This effect shows the accumulation of useful traits from generation to generation. This is applicable not only to the body, but also to individual organs. For example, an increase in the forebrain, development of the cerebral cortex - all this is an example of the accumulating effect of motive selection.
2. The transforming effect is manifested in the fact that useful features are amplified, and those that lose their adaptive significance are weakened.

If we talk in general about the selection (the material for natural selection is variability), then we can name another distributing effect and supportive .

The first is that under the most favorable conditions organisms survive more often and give offspring. Where these conditions do not meet all the requirements, with the survival and fertility problems.

The supporting effect is that adaptive symptoms can not decrease, they can increase or remain at the same level.

The material for natural selection is hereditary variability, but this is not the only factor that contributes to the evolution of living organisms.

The role of natural selection in evolution

Charles Darwin also gave the palm to the natural selection in evolution. Modern synthetic theory also considers it to be the main regulator of the development and emergence of adaptations in organisms.

In the 19-20 centuries, the discovery in genetics of the discrete nature of the inheritance of characters led to the fact that some scientists began to deny the important role of natural selection. The synthetic theory of evolution, which is also called neo-Darwinism, is based on a quantitative analysis of the frequency of occurrence of alleles in a population that varies under the influence of the same natural selection.

But science does not stand still and the discoveries of the last decades in the most diverse fields confirm the inconsistency of the classical synthetic theory for describing all the nuances of the evolution of living organisms.

Disputes and discussions about the role of various factors in the historical development of the living world continue to this day. Perhaps this is such a question, on which it is almost impossible to give an exact answer. But we can say one thing: the moment has come when the entire evolutionary theory needs revision.