Our mother nature is very wise. A weak and ill-adapted body simply does not have a chance of survival. Is it possible according to natural laws to allow a sick individual to produce the same unhealthy offspring? Of course not, so all organisms are forced to fight for their survival. The winner in this fight is the strong, tough, most fit and healthy. This is how natural selection is made. The material for selection and its principles will be considered in more detail in the article.
The concept of natural selection
If you 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 the competition. The synthetic theory of evolution considers natural selection, material for selection, as the main reason for the development of all adaptations and the formation of supraspecific categories.
Although natural selection is considered to be the reason for the adaptation of organisms to the environment, it is not alone that is the culprit of evolution in nature. The term itself was introduced by Charles Darwin, who devoted many of his works to the study of this issue.
What follows from natural selection
Genes in any body are capable of mutations that can occur for a variety of reasons. In the process of natural selection, they are fixed, but only those that contribute to increasing the adaptability of organisms to the environment. Quite often, natural selection is called a self-evident mechanism, since it follows from several factors:
- Each organism is able to produce offspring much more than it can survive.
- In any population there is hereditary variability, this is the source material for natural selection.
- Genetically heterogeneous organisms differ among themselves not only in survival, but also in their ability to reproduce.
These factors contribute to the creation of competition between organisms in survival and reproduction, and they are simultaneously a prerequisite for the evolution of wildlife through natural selection. In nature, it is so arranged that organisms with predominant hereditary traits pass them on to their offspring, while individuals without such superiority have the least chance of transmission.
Selection mechanism
Charles Darwin and Alfred Wallace were the first to express the fact that a certain mechanism, which is similar to artificial selection, was laid in nature itself. They were sure that nature does not have to delve into all situations - it is enough to create a wide variety of individuals, among which the fittest will survive. The selection mechanism can be represented as follows:
- The appearance of an individual with new traits.
- If the signs are useful, then the body survives and leaves offspring.
- Descendants inherit useful qualities and begin to pass them on to future generations.
Despite the fact that modern genetic discoveries make adjustments, the essence of Darwin's theory remains unchanged. Perhaps only changes occur much faster, and not smoothly, as he claimed, due to mutations that are spasmodic.
Source material for natural selection
Hereditary variation serves as the material that leads to natural selection. All hereditary changes result from mutations. But for evolutionary transformations, only those that affect germ cells are of interest, because it is through them that information is transmitted to the next generation.
Most mutations are recessive in nature, that is, they cannot immediately appear, as they are suppressed by dominant genes. But they can accumulate, populations do not disappear anywhere from the gene pool, although they do not affect fitness and do not manifest phenotypically.
The mutation process is ongoing, the number of such mutations is constantly accumulating, and at one fine moment there are two recessive genes and the trait necessarily appears. The material for selection is hereditary variability, but such changes do not always lead to increased vitality and fitness. Quite a few mutations, on the contrary, reduce these qualities, as they provoke various disturbances in metabolic processes.
But you can give examples where, it would seem, a harmful mutation when changing the conditions of existence is useful. Indoor flies have a mutation, which leads to a decrease in the speed of nerve impulses. If an organism turns out to be homozygous for this characteristic, then the mutation is fatal, but heterozygotes retain viability, although they are inferior in fitness to healthy individuals. But when the nerve agent has an effect on the population of flies, heterozygotes outperform normal individuals, since the slowed down speed of the pulse 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 traits that can vary in different ranges. Depending on this, the types of selection distinguish the following:
- Directional . The average value of the characteristic changes over time. This may include an increase in body size.
- Disruptive selection is aimed at selecting extreme indicators (for example, too large or, conversely, small).
- Stabilizing is directed against the manifestation of extreme values of the sign.
Sexual is also a natural selection. Material for selection at such a level is any sign that increases the likelihood of mating by increasing the attractiveness of an individual for the opposite sex. This is well manifested in males of some species (for example, huge deer horns, bright colors of plumage in birds).
Forms of Natural Selection
The forms of selection can be classified in different ways, but the criteria for selecting material are almost always the same:
- The usefulness of this trait for the individual.
- Necessity and importance of the trait for survival in certain conditions.
- The positive influence of the trait 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 are given signs that are necessary for a person, and not for the body, for which they can be quite harmful. You can give an example with a breed of depth, which is called datysh. They have a large goiter, which makes them unusual and attractive to humans, but in nature such individuals will be completely helpless and will not withstand competition with their counterparts. They simply cannot find food for themselves. So it turns out that the selection of material, the basic principles are completely different for natural and artificial selection.
Depending on the influence of selection on the variability of the trait in the population, the following forms are distinguished:
- Moving.
- Stabilizing.
- Tearing, or disruptive.
There is a need to consider each selection separately in more detail.
Driving Selection Features
The reason for this selection is always a change in the conditions of existence of the species. Individuals who, as a result, show signs that deviate from the average due to the fact that the material for natural selection supplies hereditary variability, are in a better position. From generation to generation there is a shift of the trait in a certain direction, as a result it is formed exactly that which helps organisms to survive in new conditions.
A striking example is the evolution of color in the birch moth. She from the moment of her appearance lived on the trunks of birches, which are white in color. Accordingly, the wings of this butterfly are also white.
But with the development of industry, the atmosphere began to become polluted, 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 offspring of butterflies, the advantage was that which, due to mutations, had a darker color, since light ones were quite noticeable for birds and they were often eaten. So gradually 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 action is already directed against the appearance of deviations from the norm. You can give an example: for all organisms, it would seem that increased fertility is only good, as it leads to an increase in population. But actually it is not. Individuals with average fertility rates benefit, since it is quite difficult to feed large offspring.
The selection in favor of average indicators can be seen on the example of many signs. For example, coastal birds prefer medium-sized wings. If there are too short, it is problematic to take off, and if 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 the cessation of natural selection and evolution as a whole. This type of selection ensures the stable functioning of organisms in familiar external conditions.
Disruptive selection
With this form of selection, the conditions of existence are suitable for extreme manifestations of the trait. As a result of this, several forms of existence appear.
Disruptive selection leads to the formation of polymorphism, and may even cause 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 rattle plant has two forms - one begins to bloom and bear fruit in the middle of summer, and the second - after haying, that is, in August.
The positive role of selection and the negative
Rather, this is not even a role, but forms of selection that have different effects.
- Positive selection leads to an increase in the number of organisms that have signs useful for survival under the given conditions, which increase the survival of the species as a whole.
- Negative, or it is also called cut-off selection, leads to the destruction of individuals with signs that sharply reduce survival and fitness. This selection helps remove harmful alleles from the population.
Selection effects
We have already found out what is the material for selection, examined its forms. But it is necessary to clarify what effects this or that selection gives. The driving force leads to the emergence of new devices, the results of its action are manifested in this:
- Accumulating . This effect shows the accumulation of useful traits from generation to generation. This applies not only to the body, but also to individual organs. For example, enlargement of the forebrain, development of the cerebral cortex - all this is an example of the cumulative effect of driving selection.
- The transforming effect is manifested in the fact that useful signs are enhanced, and those that have lost adaptive value are weakened.
If we talk about selection as a whole (variability is the material for natural selection), then we can name the distributing effect and the supporting one .
The first is that, under the most favorable conditions, organisms often survive and produce offspring. Where these conditions do not meet all the requirements, with survivability and fertility problems.
The supporting effect is that adaptive traits cannot decrease, they can intensify 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 natural selection in evolution. Modern synthetic theory also considers it the main regulator of the development and appearance of adaptations in organisms.
In the 19-20 centuries, the discovery in genetics of the discrete nature of the inheritance of traits led some scientists to deny the important role of natural selection. The synthetic theory of evolution, also called neo-Darwinism, is based on a quantitative analysis of the frequency of occurrence of alleles in a population, which changes under the influence of the same natural selection.
But science does not stand still and the discoveries of recent decades in various fields confirm the failure of the classical synthetic theory to describe 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 today. Perhaps this is a question that is almost impossible to give an exact answer. But one thing can be said: the moment is ripe when the whole evolutionary theory requires a revision.