The time of emergence of artificial selection. Methods of artificial selection: methodological and unconscious. Artificial selection and its forms

Selection called the process of differential (unequal) reproduction of genotypes. At the same time, we should not forget that, in fact, selection is carried out according to phenotypes at all stages of ontogenesis of organisms (individuals). Ambiguous relationships between genotype and phenotype require testing of selected plants by progeny.

There are many forms of artificial selection. Let us consider in more detail the most commonly used forms of selection.

Mass selection – the entire group is selected. For example, seeds from the best plants are pooled and sown together. Mass selection is considered a primitive form of selection, since it does not eliminate the influence of modification variability (including long-term modifications). Used in seed production. It is recommended for the selection of new plants introduced into the culture or crops that have received little research in terms of breeding. The advantage of this form of selection is the preservation of a high level of genetic diversity in the selected group of plants.

Individual selection – individual individuals are selected, and the seeds collected from them are sown separately. Individual selection is considered a progressive form of selection, since it eliminates the influence of modification variability.

One of the most progressive selection methods, taking into account modification variability, is considered pedigree method (English pedigree - pedigree), based on individual selection of the best individuals with an assessment of their offspring. When evaluating material, it is not individual individuals that are rejected, but entire lines containing alleles that are undesirable for the breeder. This method is especially effective when selecting self-pollinators with a short life cycle (annuals). However, the pedigree method is not applicable for species prone to inbreeding depression, and even more so for dioecious plant species. Therefore, in the selection of cross-pollinating plants, a special form of individual selection is used - family selection (a family is a collection of individuals grown from seeds collected from one plant, and the pollen donor is usually unknown).

If different families are isolated from each other, then such selection is called individual-family. During the reproduction of each family, individuals with undesirable traits are discarded, and the remaining best individuals are freely cross-pollinated. The family is then assessed based on its offspring. Those families in which a large proportion of plants with undesirable traits are rejected and excluded from the breeding process, and families with high average indicators are used for further seed propagation and selection. This selection method is a modification of the pedigree method as applied to cross-pollinating plants.

The severity of selection presupposes the merciless culling of the worst families, from the point of view of the breeder, and this contradicts the idea of ​​biodiversity as one of the most important natural resources. Therefore, family selection must be complemented by iterative selection methods based on the preservation of the original material. With repeated selection in each generation, material is selected from the best individuals for cloning and preliminary variety testing. In parallel, seed regeneration continues in each family. At the same time, work is intensifying to create collections of families using the genetic potential of organisms living in other ecological and geographical conditions, as well as the genetic potential of experimentally obtained mutant plants.

To prevent homozygotization and inbreeding depression it is used family-group selection . This method is based on combining into one group families that are phenotypically similar in selectable traits, but differ in origin. Each such group is isolated from other similar groups. Then, within the group, cross-pollination occurs between members of different families.

A type of family selection is sib selection . Sib selection is based on selection for closest relatives (siblings - brothers and sisters). A special case of sib-selection is the selection of sunflower for oil content using the half-and-half method. When using this method, the sunflower inflorescence (basket) is divided in half. The seeds of one half are checked for oil content: if the oil content is high, then the second half of the seeds is used in further selection.

Let us briefly consider some other forms of artificial selection.

Negative, positive and modal. With negative selection, the worst individuals (from the breeder's point of view) are rejected; with positive selection, the best individuals are retained for further reproduction (again from the breeder’s point of view). With modal selection, individuals that are typical for a given variety or breed are retained for breeding; used to preserve stable gene combinations; modal selection is an analogue of the stabilizing form of natural selection and is used to preserve stable combinations of genes.

Conscious and unconscious selection. With conscious (methodological) selection, the final result is planned in advance (see above). With unconscious selection, the breeder controls only some of the traits that interest him. However, not all traits can be controlled by the breeder, then unexpected, often undesirable effects arise, for example, an increase in winter hardiness is accompanied by a decrease in productivity. In the 19th century In Russia, to obtain the largest seeds of cereals, double threshing was used: the sheaf was lightly struck on the ground, and at the same time, the largest grains fell off first: genotypes were selected that provided the largest mass of grains. But at the same time, there was an unconscious selection of genotypes that increased the shedding of cereals. In the course of artificial selection, aimed at enhancing traits beneficial to humans, natural selection always occurs, aimed at preserving traits beneficial to organisms. This contradiction can inhibit selection.

Multiple and single selection. Repeated selection takes place over many generations. It is usually used when high level genetic diversity of the source material. With repeated selection in each generation, part of the plants is used for variety testing, and part is preserved as source material. Repeated selection as a modern form of multiple selection will be discussed below. Single selection is used if the selected plants do not split in subsequent generations. This selection is effective for seed propagation of self-pollinating plants in the presence of homozygotes in the source material that are phenotypically different from heterozygotes. Then, as a result of a single selection, pure lines are created in which further selection is ineffective. Single selection in cross-pollinating plants is possible if the selected plants can be propagated vegetatively, then it is supplemented by clonal selection.

Clonal selection. Produced by vegetative propagation over 2...3 generations. In this case, the emergence of new genotypes due to recombination turns out to be impossible, and then each seedling can potentially be considered the ancestor of a new variety. Thus, clonal selection is a special form of selection aimed at identifying and eliminating somatic (bud) mutations and long-term modifications.

The creative role of artificial selection.

During artificial selection, undesirable traits are weakened and economically useful traits are greatly enhanced. The creative role of artificial selection is that forms are created that did not previously exist.

Selection for diversity. For a long time, the end result of selection was considered to be the creation of genetically homogeneous, homogeneous groups in which selection becomes impossible (or ineffective). The effectiveness of selection in populations (heterogeneous genetic systems) and the ineffectiveness of selection in pure lines (homogeneous genetic systems) were already at the beginning of the 20th century. proved by the outstanding Danish geneticist V.L. Johannsen. As a result of selection for homogeneity, the genetic potential of the source material is depleted. Then you need to look new material, new genotypes. Thus, artificial selection in its classical form inevitably leads to a decrease in the level of biological diversity. To maintain the required level of biodiversity, it is necessary to constantly implement a set of measures to preserve the gene pool (see above).

Selection for diversity is much less expensive. For example, out of several similar varieties (breeds), not the best variety (the most productive, the most resistant to diseases, the most competitive, etc.) is preserved, but the entire group of varieties (breeds). If several varieties (breeds) are obtained that are phenotypically indistinguishable but genetically different, then the entire group must be preserved. Thus, diversity itself is considered to be the most important biological resource (recall that a biological resource is considered to be genetic material, organisms or parts thereof, or ecosystems used or potentially useful to humanity, including the natural balance within and between ecosystems).

A special case of selection for diversity is selection for diversity of developmental rhythms.

Artificial selection- the basis of any selection process. Allowing only individuals endowed with any properties and characteristics needed by humans to be crossed is artificial selection. Back in ancient times, when man began to domesticate animals and cultivate plants, he used unconscious selection , leaving for reproduction the most suitable individuals in his opinion. At the same time, he did not have a clear goal of what particular breed or variety should be developed, much less outlined specific ways to solve the problem. However, our ancestors, who lived 5-10 thousand years ago, managed to achieve brilliant results - domesticate almost all types of modern domestic animals (horse, donkey, ram, goat, cat, dog, cattle) and cultivate the main grain species ( wheat, oats, barley, rye).

Selection carried out in accordance with the assigned tasks and based on knowledge genetic characteristics object, usually called methodical selection . This is what is now used in breeding.

There are two selection methods - mass And individual . Mass selection is used for numerous organisms, usually cereal plants, and individual selection is used for breeding breeds of domestic animals with low fertility and the number of offspring from crossing, as in cattle, no more than one. Material from the site

The mass selection process looks like this. First, the researcher sets the task, for example, to develop a variety of bread wheat (this type of wheat contains six sets of chromosomes, unlike durum wheat, which is a tetraploid), which would be not only highly productive, but also resistant to fungal diseases. First, the source material is selected for selection. They can serve, for example, various highly productive varieties, which, however, are not resistant to pathogenic fungi (Fig. 74). Each of these varieties is planted in a separate area and exposed to pathogens (from Greek pathos- suffering and genesis). As a result, single unaffected plants remain in each area. The grain of these plants is collected and sown again the following year. Much more resistant plants grow from it; fungi affect no more than half of all individuals. The grain obtained from the second generation is sown in the third year. This generation turns out to be quite stable: about 90% of plants resistant (from English resistance- resistance, opposition) to fungal diseases. It remains to find out which of the selected varieties, during the selection process for resistance to pathogenic fungi, retained high productivity, and to continue working with it in order to obtain a new plant variety. Obviously, this problem cannot be solved in three to five years. This usually takes decades.

Charles Darwin drew attention to the diversity of breeds and varieties. Having noted special changes, properties, and signs in animals and plants, a person selects for himself only individuals that have the characteristics he needs. For example, when selecting melon and watermelon, a person plans to obtain high-quality fruits. In carrots and beets, only sweet root vegetables are valued, in potatoes - large tubers, in poppy - indehiscent boxes. Charles Darwin called selection for traits useful to humans in plants and animals artificial selection. Charles Darwin was the first to notice that variability alone cannot explain all the differences between breeds and varieties. If variability in nature changes all organs of plants and animals, then with artificial selection a person selects organisms with the characteristics necessary for him, so Charles Darwin set himself the goal of finding out the mechanism of the evolutionary process. To do this, he began to study the agricultural practices of England.
Charles Darwin studied the production of new forms of organisms (animal breeds and plant varieties) in agricultural practice. He then began to study the reasons for the appearance of new species in nature and noticed that the main principle of the process of formation of new forms in nature and agricultural practice is selection. He was able to show the huge differences between artificial and natural selection. As a result, Charles Darwin laid the foundation for the doctrine of artificial and natural selection. In his work “Changes in Domestic Animals and Cultivated Plants,” published in 1868, Charles Darwin gave Full description origin of breeds and their diversity. He studied about 400 breeds of cattle. Animals differed from each other in skull size, body shape, coloring, and skeletal and muscle development. Despite many differences, he came to the conclusion that the ancestor of all breeds and varieties obtained through artificial selection is one wild species. The many examples given by Charles Darwin completely refuted the prevailing belief at that time that each breed or variety has its own wild ancestor. Refuting this opinion, Charles Darwin gave many examples. For example, all breeds of domestic chickens originated from the wild banker chicken, domestic ducks from the mallard duck, and breeds of rabbits from the wild European rabbit (Fig. 15). Cattle breeds originate from the wild European bull - the aurochs, dogs - from wolves, etc.

Rice. 15. Wild rabbit

Examples of artificial selection. In 1862, Charles Darwin, at the International Agricultural Exhibition in London, drew attention to a variety of multi-eared wheat bred by Major Gallet. After meeting Gallet, he learned how to breed this variety. Gallet began working on improving a wheat variety in 1856. First, he selected several specimens with the largest ears from a wheat field. Having counted 87 grains, he sowed them in a separate area. Of these, he received wheat with 10 large ears, the next year - with 17 ears, and in the fourth year - with 39 ears. So, every year he selected the most multi-spiked specimens for seeds. As a result, Gallet developed a variety of multi-eared wheat in 6 years. Selection at that time was used not only in crop production, but also in animal husbandry. Thus, it has been proven in practice how continuous selection of new animal breeds and plant varieties is carried out.
As a result of artificial selection, only the traits needed by humans change in animal breeds and plant varieties. However, at least one common feature remains. To prove this, let's look at a few examples.
Breeders have developed six varieties of cabbage that are not similar to each other. But the similarity between seeds and flowers remained unchanged. The numerous varieties of cultivated violets that currently exist with different flower colors are united by same shape leaves.
New varieties and breeds serve to meet human needs and are improved every year.
However, animals and plants bred by humans have highly developed individual traits that are useless and even harmful to the organism itself. A highly productive breed of short-legged pigs is very useful for humans (they produce a lot of fat). But obesity is very harmful to the pig's body. Obese, short-legged pigs are not adapted to living in natural conditions. Overweight, with short legs, she cannot defend herself from enemies. A pigeon with a large crop (pouter) receives food from the palm of a person, since the huge crop prevents it from pecking food from the ground (Fig. 16).
Thus, as a result of artificial selection, man creates only the breeds of animals and plant varieties he needs. At the same time, it is precisely the characteristic that is of particular interest to the person that develops. Uncertain (mutational) variability is very useful, since mutation is a rare phenomenon.
Artificial selection proceeds successfully among a larger number of individuals, while the possibilities of selection expand.

Artificial selection- the process of creating new breeds of animals and varieties of cultivated plants by preserving and propagating individuals with traits and properties valuable to humans.

Selection. Artificial selection.

1. In order to study artificial selection, Charles Darwin turned to the agricultural practice of England.

1. Lay the foundation for artificial selection.
2. He proved the unity of origin of various breeds and varieties from one wild species.
3. What examples did Charles Darwin use to prove the existence of wild species of breeds and varieties?
4. Give examples that prove that artificial selection is harmful to the body.
5. Explain the term artificial selection.
6. What are the results of artificial selection?
7. Explain how pigeon breeds differ from their wild ancestor (Fig. 16).

Rice. 16. Breeds of pigeons and their wild ancestor: 1 - wild rock pigeon; 2 - Jacobin; 3 - peacock pigeon; 4 - blower; 5 - shaggy pigeon; 6 - tumbler

1. Define artificial selection.

Which organs of these plants are changed by artificial selection? Enter your answers into the table, putting a “+” sign in the appropriate places. (Draw the table separately in your notebook.)

While creating evolutionary theory Darwin relied on the breeding material of his time. At that time, a large number of varieties of cultivated plants and breeds of domestic animals were known. Their ancestors were one or more wild species. For example, all known breeds of domestic dogs are descended from the wolf or jackal. Famous breeds of sheep are descendants of argali or mouflon, and a surprising variety of pigeons are descended from the rock pigeon. Modern cabbage varieties are descended from several forms of wild cabbage found in Europe today.

In the conditions of the dominance of ideas about the constancy and immutability of species, it was important for Charles Darwin to show how their diversity is formed. Therefore, he substantiated in detail the position on the variability of living organisms.

Darwin identified three forms of variability: definite (group), indefinite (individual) and correlative (correlative).

Specific (group) variability — the appearance of identical characteristics in all individuals and their offspring under the influence of a changed environmental factor.

A certain variability is widespread. With a lack of food, animals lose weight; in cold climates, mammals have thicker fur. The leaves of plants exposed to different lighting conditions differ in shape, etc. A certain variability increases the organism's adaptability to specific environmental conditions, but is not inherited. That is, when environmental conditions change, offspring do not retain the characteristics acquired by their parents.

Uncertain (individual) variability — the appearance in an individual individual within one variety, breed, species of a new trait that was not found in the parents. Thus, within the same breed of rabbits, different coat colors can be observed. Within one variety of Uzambara violets there are different flower colors. Darwin noted that even under similar environmental conditions, the offspring of a pair of parents differ from each other. This form of variability is the result of the specific influence of living conditions on each individual organism. The real reasons uncertain variability was unknown to Darwin. However, the scientist considered its hereditary nature and, as a result, a significant diversity of individuals to be the leading material for the evolutionary process. Gradually Darwin came to the conclusion that only heritable individual changes in organisms, because only they can accumulate and be passed on from generation to generation.

Correlative variability — a change in one organ or part of the body following changes in other parts of the body.

For example, with constant exercise of the lower limbs, domestic duck breeds develop a ridge on the femur for muscle attachment. In wading birds, the lengthening of the neck is accompanied by a simultaneous lengthening of the limbs. This form of variability is very important in breeding practice. IN in this case the breeder can anticipate deviations from the original form and select traits in the desired direction.

In addition to variability, Darwin considered heredity to be an important factor in evolution.

Heredity is the ability of organisms to pass on their characteristics and properties to their descendants..

Subsequently, G. Mendel, in his laws (on the uniformity of hybrids of the first generation and the splitting of characters in the second generation), explained the mechanisms of inheritance of characters. Thus, according to Darwin, heredity and variability are common properties of all living organisms. They are the main prerequisites for the evolutionary process.

Artificial selection

Having studied the reason for the diversity of animal breeds and plant varieties, Darwin began to elucidate the mechanisms of their occurrence. The scientist noted that in ancient times people sought to preserve the most valuable specimens of animals and plants and obtain offspring from them. Over the course of generations, such valuable traits accumulated and became fixed. This ensured better reproduction of the herd in animals and increased yield in plants. Darwin called this force, which gradually leads to the emergence of new breeds of animals and varieties of plants, artificial selection.

Artificial selection is the process of human selection of the most economically valuable animals and plants and their use for further breeding.

Darwin identified two forms of artificial selection - unconscious and methodical. At unconscious selection a person does not set himself the goal of creating a new breed or variety. By reproducing some individuals and removing others, it slowly changes the characteristics of organisms that are useful to itself. This is the most ancient form of artificial selection.

For example, a person selected wheat plants with the largest, healthiest and longest-stored seeds for subsequent sowing. Cows were selected based on milk yield and meatiness, and sheep were selected based on wool density.

Thanks to this differentiated approach from generation to generation, certain characteristics of the breeding individuals intensified. As a result, an unconscious form of artificial selection slowly but surely led to the formation of new breeds and varieties.

Methodical selection- purposeful human breeding of animal breeds or plant varieties. In this case, the breeder pays attention to the traits that are most desirable for him in specific conditions. Next, he designs the breed or variety. Based on the hereditary variability of organisms, a person purposefully selects pairs for crossing. It also ensures maximum development and consolidation of desired characteristics from generation to generation.

For example, cow breeds are bred for meatiness or milk yield. Breeds of chickens - according to egg production, amount of meat and even fighting qualities, dogs - according to their ability to various types hunting, official use, decorative purposes, etc.

Artificial selection theory showed that this is the main mechanism that determined the emergence of diversity of cultivated plants and domestic animals. At the same time, Darwin emphasized the special importance of unconscious selection. Of course, its course is in many ways similar to those phenomena that occur in nature. Therefore, it is unconscious selection that is the connecting link between artificial and natural selection.

The theory of artificial selection created by Darwin today not only has not lost its significance, but has also expanded its scope. In Darwin's time, the starting material for developing new breeds of animals and plant varieties was selected from nature. Today, thanks to the methods of biotechnology and genetic engineering, source material for further selection with specified characteristics is obtained. Thanks to the successes of genetic engineering, new genetically modified varieties of grains, tomatoes, potatoes and other crops have been obtained that are more productive and resistant to diseases.

To date, the production of human insulin has been established using genetically modified bacteria, new breeds of mice have been bred for scientific research etc.

A genetically modified organism (GMO) is a living organism whose genotype is purposefully changed by humans for scientific or economic purposes using genetic engineering methods. For example, Belarusian and Russian scientists introduced into the genome of goats a human gene responsible for the synthesis of the protein lactoferrin. This protein will now be contained in goat milk in large quantities. Possessing, according to scientists, a unique antibacterial effect, lactoferrin will be indispensable in the formation of immunity in bottle-fed children.

Currently, the achievements of Belarusian breeders have made it possible to provide agriculture with its own animal breeds and plant varieties. The breeds of black-and-white and large white pigs meet all international standards. This fact is confirmed at international livestock exhibitions. The own varieties of most cultivated plants bred in Belarus fully comply with the natural and climatic conditions of the republic.

In the changing natural and climatic conditions of our country, the resistance of cultivated plants to spring frosts is very important. Another negative climatic fact may be low air temperature during the flowering period of plants. In this regard, domestic breeders have developed varieties of apple trees that are resistant to spring freezing: Verbnoye, Zaslavskoye, Belorusskoye Sladoe, etc.

Thus, modern advances selections based on the use of genetic achievements, various methods of crossing and mutagenesis confirm the creative role of artificial selection noted by Darwin in his evolutionary theory.

Darwin identified three forms of variability: definite (group), indefinite (individual) and correlative (correlative). Artificial selection is the main mechanism that determined the emergence and diversity of varieties of cultivated plants and breeds of domestic animals. Darwin described two forms of artificial selection: unconscious and methodical.

To develop new breeds, varieties or strains, people use artificial selection. The best individuals with the qualities required by humans are selected for further selection, unfavorable individuals are rejected.

Definition

Selection appeared long before Charles Darwin's theory of evolution. For many millennia, people have tamed and domesticated various animals, mastered agriculture and grown plants that gave rich harvests. People, without realizing it, selected the best representatives of flora and fauna, crossed them with each other and developed new breeds and varieties with traits useful to humans.

Rice. 1. Charles Darwin.

Darwin noticed that all domestic pets and cultivated plants have “analogues” in wildlife. A dog is very similar to a wolf, a cow is similar to a buffalo, and a rabbit is similar to a hare. Having studied different breeds and varieties, Darwin identified a number of similar features among them:

• a wide variety of breeds and varieties in each related group;
• domesticated animals and cultivated plants differ both from each other (each breed has its own characteristics) and from their supposed wild ancestors;
• all breeds and varieties are well adapted to human needs - a cow provides rich milk, a wheat field provides a winter supply of flour, dogs help in hunting, etc.

Darwin's doctrine of artificial selection prompted the development of evolutionary theory, which is based on the hereditary variability of a population. It was the ability of living nature to change and accumulate useful characteristics in genes that allowed man to select individuals with the necessary properties and create more “improved” offspring from them.

For natural selection to occur, changing environmental circumstances are necessary (the fittest individuals survive and produce offspring). With artificial selection driving force is a person's desire.

Characteristic

Basic characteristic features artificial selection are presented in the table.

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In the course of selection, a person leaves individuals with the best qualities. “Bad” individuals are rejected - sterilized or destroyed.

Forms

Charles Darwin identified two forms of artificial selection:

• unconscious - random selection, not aimed at a specific trait;
• methodical - meaningful breeding of offspring with certain characteristics and properties, developed in the 18th century.

Unconscious selection was carried out en masse, without a specific goal. Man left the seeds of sweeter or more fruitful plants, the healthiest individuals for reproduction. Thus, varieties of wheat were selected whose seeds do not fall, corn with large cobs was grown, and varieties of melons (melon, pumpkin, watermelon) were bred.

Unconscious selection has been going on for many centuries.

Rice. 2. Melons.

Examples of methodical selection are breeds of dogs, chickens, and grape varieties for different needs. There are hunting, service, guard, and herding dogs. They breed egg-laying, downy, and meat-laying chickens. Grapes are grown for making wine and for desserts. This is a faster method of selecting individuals.

Rice. 3. Different breeds of chickens.

Unlike natural selection, artificial selection is not the driving force of evolution. Artificial selection is the basis of selection. Natural selection often influences human-bred breeds and varieties.

What have we learned?

From the 11th grade biology lesson we learned about the features of artificial selection. Man, for his needs, using the method of selection (the best individuals are retained, defective ones are rejected) has created many new varieties, breeds and strains. Initially, humanity selected the best individuals unconsciously. Over time, targeted selection methods emerged. Unlike wild ancestors, domesticated individuals are highly productive.

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