Comparative characteristics of artificial and natural selection. Comparative characteristics of natural and artificial selection

Dwelling in natural conditions, there is individual variability, which can manifest itself in three forms - useful, neutral and harmful. Usually, organisms with harmful variability die at various stages of individual development. The neutral variability of organisms does not affect their viability. Individuals with beneficial variability survive by virtue of an advantage in intraspecific, interspecific, or against adverse conditions environment.

driving selection

When environmental conditions change, those individuals of the species survive in which hereditary variability has manifested itself and, in connection with this, signs and properties have developed that correspond to new conditions, and those individuals that did not have such variability die. During his voyage, Darwin discovered that on oceanic islands where strong winds prevail, there are few long-winged insects and many insects with rudimentary wings and wingless insects. As Darwin explains, insects with normal wings could not withstand the strong winds on these islands and died. And insects with rudimentary wings and wingless did not rise at all into the air and hid in the cracks, finding shelter there. This process, which was accompanied by hereditary variability and natural selection and continued for many thousands of years, led to a decrease in the number of long-winged insects on these islands and the appearance of individuals with rudimentary wings and wingless insects. Natural selection, which ensures the emergence and development of new features and properties of organisms, is called motive selection.

Disruptive selection

Disruptive selection- this is a form of natural selection, leading to the formation of a number of polymorphic forms that differ from each other within the same population.

Indicators	Natural selection	artificial selection
Initial material for selection		Individual signs of the body
Selecting factor		Environmental conditions
The Path of Favorable Change	Selected, become productive	Remain, accumulate, be inherited
The Path of Unfavorable Changes	Selected, discarded, destroyed	Destroyed in the struggle for existence
Nature of action	Creative - directed accumulation of signs for the benefit of a person	Creative - selection of adaptive traits for the benefit of an individual, population, species, leading to the emergence of new forms
Selection result	New plant varieties, breeds of animals, strains of microorganisms	New species
Selection Forms	Mass, individual, unconscious, methodical	Motive, stabilizing, destabilizing, disruptive, sexual

Lesson 5–6. plant breeding

Equipment: tables on general biology, illustrating the diversity of breeds and varieties, the main methods and achievements of plant breeding.

DURING THE CLASSES

I. Knowledge Test

A. Oral knowledge test

1. Ch.Darwin about the reasons for the diversity of breeds and varieties.
2. Forms of artificial selection and their characteristics.
3. The creative role of artificial selection.

B. Card work

№1. Why can a breed or variety be considered a man-made population, i.e. a population created by the will and efforts of people?

№2. Show examples of the influence of selection on the direction of breed and variety formation.

№3. Why is mass selection used for cross-pollinated plants? Does mass selection produce genetically homogeneous material? Why is re-selection necessary for mass selection?

II. Learning new material

1. Features of plant biology taken into account in breeding

Selection must take into account the following features plant biology:

– high fecundity and numerous offspring;
– the presence of self-pollinating species;
- the ability to reproduce by vegetative organs;
– the possibility of artificial production of mutant forms.

These features of plants determine the choice of breeding methods.

2. Crossing as a method of increasing the diversity of material for artificial selection

The main methods of plant breeding are hybridization and selection. Usually these methods are used together. Hybridization increases the diversity of the material with which the breeder works. But in itself, most often, it cannot lead to a purposeful change in the characteristics of organisms, i.e. crosses without artificial selection are ineffective. Crossbreeding is preceded by a careful selection of parental pairs. For successful search, selection and use of source material great importance have the teachings of N.I. Vavilov about the centers of origin cultivated plants, his law of homological series in hereditary variability, ecological and geographical principles of plant taxonomy, and also created by N.I. Vavilov, his followers and students collection of agricultural plants.

Hybridization can be carried out different schemes. There are simple crosses (paired) and complex crosses (step, return, or backcrosses).

Simple , or doubles , is called crossing between two parental forms, produced once. A variety of them are the so-called mutual(reciprocal) crossovers. Recall that their essence lies in the fact that two crossings are carried out, and the paternal form of the first crossing is used in the second crossing as the maternal one, and the maternal form, respectively, as the paternal one. Such crosses are used in two cases: when the development of the most valuable trait is due to cytoplasmic heredity (for example, frost resistance in some varieties of winter wheat) or when seed setting in hybrids depends on whether one or another variety is taken as the maternal or paternal form. Reciprocal crossings show that sometimes the influence of the cytoplasm of the maternal variety is very significant.
So, in the Research Institute of Oilseeds. V.S. Pustovoita (Krasnodar), as a result of reciprocal crossings of sunflower varieties 3519 and 6540, intervarietal hybrids were obtained, which significantly (by 2.5 times) differed in the degree of broomrape damage, depending on which variety was taken as the mother variety, and which - as a paternal form. Naturally, hybrids with greater resistance to broomrape were included in the breeding process.

complex called crosses in which more than two parental forms are used or the hybrid offspring are re-crossed with one of the parents. A distinction is made between stepped and back compound crosses.
Complex step hybridization- This is a system of successive crossings of the resulting hybrids with new forms, as well as hybrids with each other. In this way, you can collect in one variety best qualities many original forms. This method was first developed and successfully applied by the famous Soviet breeder A.P. Shekhurdin when creating varieties of soft spring wheat Lutescens 53/12, Albidum 43, Albidum 24, Steklovidnaya, Saratovskaya 210, Saratovskaya 29, etc., as well as a number of varieties of durum spring wheat.
At backcrosses the resulting hybrids are crossed with the parent form, the trait of which they want to enhance. If such crossings are repeated many times, they are called saturating, or absorption(backcrosses). In this case, the hybrid is saturated with the genetic material of one of the parents, and the genetic material of the other parent is displaced (absorbed), and one or more genes responsible for some valuable trait remain in the hybrid genome, for example, drought resistance or resistance to one of the diseases. As a rule, local wild-growing forms, which are most often low-productive, are used as donors of such traits, which is why breeders have to resort to backcrosses.

The following types of crosses are used in plant breeding.

Inbreeding, or inbreeding, are used as one of the stages of increasing productivity. For this, self-pollination of cross-pollinated plants is carried out, which leads to an increase in homozygosity. After 3-4 generations, the so-called pure lines arise - genetically homogeneous offspring obtained by individual selection from one individual or a pair of individuals in a series of generations. Many abnormal traits are recessive. In pure lines they appear phenotypically. This leads to an adverse effect, a decrease in the viability of organisms, called inbreeding depression. But, despite the adverse effect of self-pollination in cross-pollinated plants, it is often and successfully used in breeding to obtain pure lines. They are necessary for the hereditary fixation of desirable, valuable traits, as well as for interline crossing. In self-pollinating plants, there is no accumulation of unfavorable recessive mutations, since they quickly become homozygous and are eliminated by natural selection.

Interline crossing– cross pollination between different self-pollinating lines, as a result of which, in some cases, high-yielding interline hybrids appear. For example, to obtain interline hybrids of corn, panicles are plucked from selected plants and, when stigmas of pistils appear, they are pollinated with pollen from the same plant. To prevent pollination by pollen from other plants, the inflorescences are covered with paper insulators. This is how several pure lines are obtained over a number of years, and then the pure lines are crossed with each other and select those whose offspring gives the maximum increase in yield.

Crossbreeding- plant crossing different varieties among themselves for the purpose of manifestation in hybrids of combinative variability. This type of crossing is the most common in breeding and underlies the production of many high-yielding varieties. It is also used for self-pollinated species, such as wheat. Anthers are removed from the flowers of a plant of one variety of wheat, a plant of another variety is placed next to it in a jar of water, and both plants are covered with a common insulator. As a result, get hybrid seeds, combining the traits of different varieties necessary for the breeder.

distant hybridization- plant crossing different types, and sometimes childbirth, contributing to the receipt of new forms. Usually, interbreeding occurs within a species. But sometimes it is possible to obtain hybrids from crossing plants of different species of the same genus and even different kinds. So, there are hybrids of rye and wheat, wheat and wild cereal Aegilops. However, distant hybrids are usually sterile. The main causes of infertility:

- in distant hybrids, the normal course of maturation of germ cells is usually impossible;
- the chromosomes of both parental plant species are so dissimilar that they are unable to conjugate, as a result of which there is no normal reduction in their number, the process of meiosis is disrupted.

These disturbances are even more significant when the crossing species differ in the number of chromosomes (for example, the diploid number of chromosomes in rye is 14, in common wheat - 42). There are many cultivated plants created as a result of distant hybridization. For example, as a result of many years of work of Academician N.V. Tsitsina and his collaborators obtained valuable varieties of cereals based on the hybridization of wheat with the perennial weed wheatgrass. As a result of the hybridization of wheat with rye (these hybrids are usually sterile), a new cultivated plant was obtained, called triticale (lat. triticum- wheat, secale- rye). This plant is very promising as a fodder and grain crop, which gives high yields and is resistant to adverse environmental influences.

3. The phenomenon of hybrid power and its genetic basis

Even in the middle of the XVIII century. Russian academician I.Kelreuter drew attention to the fact that in some cases, when crossing plants, hybrids of the first generation are much more powerful than parental forms. Then Charles Darwin concluded that hybridization in many cases is accompanied by a more powerful development of hybrid organisms. Higher viability, productivity of hybrids of the first generation in comparison with crossed parental forms is called heterosis. Heterosis can occur when crossing breeds in animals, varieties and pure lines in plants. Thus, an intervarietal hybrid of Grushevskaya and Dnepropetrovsk corn gives 8–9% yield increase, and an interline hybrid of two self-pollinated lines of the same varieties gives 25–30% yield increase. Cases of heterosis are also known with distant crossings of species and genera of plants and animals.

Thus, the phenomenon of heterosis as a hereditary expression of the effects of hybridization has been known for a long time. However, its use in the breeding process began relatively recently, in the 1930s. The discovery and understanding of the phenomenon of heterosis made it possible to determine a new direction in the selection process - the creation of highly productive hybrids of plants and animals.

A new period in the study of the phenomenon of heterosis begins in the 1920s. 20th century from the works of American geneticists J. Shell, E. East, R. Hell, D. Jones. As a result of their work, inbred lines were obtained in corn by self-pollination, which differ from the original plants in reduced productivity and viability, i.e. severe inbreeding depression. But when Shell crossed pure lines, he unexpectedly obtained very powerful hybrids of the first generation, significantly exceeding in all parameters of productivity both the original lines and the varieties from which these lines were obtained by self-pollination. With these works, the widespread use of heterosis in the selection process began.

What explains the phenomenon of heterosis, i.e. power of hybrids, from a genetic point of view? Geneticists have proposed several hypotheses to explain it. The following two are the most common.

Dominance hypothesis developed by the American geneticist D. Jones. It is based on the idea of ​​favorably acting dominant genes in a homozygous or heterozygous state. If the crossed forms have only two dominant favorably acting genes ( AAbbCCdd x aaBBccDD), then the hybrid has four of them ( AaBbCcDd), regardless of whether they are in the homozygous or heterozygous state. This, according to the supporters of this hypothesis, determines the heterosis of the hybrid, i.e. its advantages over the original forms.

Overdominance hypothesis proposed by American geneticists J. Shell and E. East. It is based on the recognition that a heterozygous state for one or more genes gives an advantage over homozygous states for one or more genes. The scheme illustrating the one-gene overdominance hypothesis is quite simple. It indicates that the heterozygous state for the gene Ah has advantages in the synthesis of a gene-controlled product over homozygotes for the alleles of this gene. Starting from the second generation of hybrids, the effect of heterosis fades, because. some of the genes go into the homozygous state:

P- Ah X Ah;
F2- AA; 2Ah; aa.

There are a number of other hypotheses of heterosis. The most interesting of them the hypothesis of the compensation complex of genes, proposed by the domestic geneticist V.A. Strunnikov. Its essence is as follows. Let there be mutations that greatly reduce viability and productivity. As a result of selection, a compensatory complex of genes is formed in homozygotes, which largely neutralizes the harmful effects of mutations. If then such a mutant form is crossed with a normal one (without mutations) and thereby the mutations are transferred to a heterozygous state, i.e. neutralize their action with a normal allele, then the compensation complex that has developed in relation to mutations will provide heterosis.

Thus, despite the fact that the genetic basis of heterosis has not yet been fully elucidated, one thing is certain: a positive role in hybrids is played by high heterozygosity, leading to the manifestation of increased physiological activity.

4. Overcoming infertility of interspecific plant hybrids

Remote hybridization is not widely used in breeding due to the sterility of the resulting hybrids. One of the outstanding achievements of modern genetics and breeding has been the development of a method for overcoming the infertility of interspecific hybrids, leading in some cases to obtaining normally breeding hybrids. This was first achieved in 1922–1924. Russian geneticist, student N.I. Vavilov, Georgy Dmitrievich Karpechenko (1899–1942) when crossing radish and cabbage. Both of these species have (in the diploid set) 18 chromosomes each. Accordingly, their gametes carry 9 chromosomes each (haploid set). The hybrid has 18 chromosomes, but it is completely sterile, because. "rare" and "cabbage" chromosomes in meiosis do not conjugate with each other.

Cabbage-rare hybrid (rafanobrassika)

G.D. Karpechenko by the action of colchicine doubled the number of chromosomes of the hybrid. As a result, there were 36 chromosomes in the hybrid organism, consisting of two complete diploid sets of radish and cabbage. This created normal opportunities for meiosis as each chromosome had a pair. "Cabbage" chromosomes were conjugated with "cabbage", and "rare" - with "rare". Each gamete carried one haploid set of radish and cabbage (9 + 9 = 18). Species that have combined different genomes in one organism, and then their multiple increase, are called allopolyploids. The zygote again had 36 chromosomes.

Thus, the resulting cabbage-rare hybrid, called rafanobrassica, became prolific. The hybrid did not split into parental forms, because radish and cabbage chromosomes always ended up together. This man-made plant looked neither like a radish nor like a cabbage. The pods consisted of two halves, one of which resembled a cabbage pod, the other a radish. Distant hybridization combined with a doubling of the number of chromosomes (polyploidy) led to the restoration of fertility.

G.D. Karpechenko was the first to clearly demonstrate the relationship between distant hybridization and polyploidy in obtaining fertile forms. This is of great importance for both evolution and selection.

5. Use of somatic mutations in plant breeding

The use of somatic mutations is applicable to the selection of vegetatively propagating plants. Via vegetative propagation a beneficial somatic mutation can be preserved, or any heterozygous form possessing economically useful traits can be preserved and propagated. For example, only with the help of vegetative propagation, the properties of many varieties of fruit and berry crops are preserved. During sexual reproduction, the properties of varieties consisting of heterozygous individuals are not preserved, and their splitting occurs.

6. Artificial selection in plant breeding

As we have already said, hybridization is effective in selection only in combination with selection. In plant breeding, both mass and individual selection are used.

During the mass selection a large number individuals choose a group of plants with the best phenotypes, whose genotypes are unknown. Mass selection is carried out among cross-pollinated plants. Joint cultivation of selected plants promotes their free crossing, which leads to heterozygosity of individuals. Mass selection is carried out repeatedly in a number of subsequent generations. It is resorted to in the case when it is required to improve one or another variety relatively quickly. But the presence of modification variability reduces the value of varieties bred by mass selection.

Individual selection in plant breeding is used as a way of conservation for reproduction. best plants. They are grown in isolation from each other in order to identify valuable traits in the offspring through comparison with the original forms and among themselves. As we already know, most often self-pollinating plants are the object of individual selection, and its result is pure lines.

7. The role of natural selection in plant breeding

Natural selection plays a decisive role in breeding. A whole range of environmental factors acts on any plant throughout its life, and it must be resistant to pests and diseases, adapted to a certain temperature and water regime. Therefore, due to natural selection, individuals form adaptations to the environment. There cannot be cultivated plants that are equally productive in any locality. Varieties are classified under the influence of natural selection.

8. Induced mutagenesis, polyploidy and their use in plant breeding

Induced mutagenesis is based on the impact of various radiations and chemical mutagens on the body to obtain mutations. Mutagens allow you to get a wide range of different mutations. Out of 1,000 artificially obtained mutations, 1-2,000 turn out to be beneficial. But in this case, strict individual selection of mutant forms is necessary and further work with them.

Mutagenesis methods are successfully used in plant breeding. Now more than 1 thousand varieties have been created in the world, leading a pedigree from individual mutant plants obtained as a result of artificial mutagenesis. Known variety spring wheat Novosibirskaya 67 was obtained at the Institute of Cytology and Genetics of the Siberian Branch of the Russian Academy of Sciences after processing the seeds of the initial material of the variety Novosibirskaya 7 x-rays. This variety has a short and strong straw, which prevents plants from lodging during the harvest period.

In plant breeding, the method of obtaining polyploid forms is also widely used. Polyploidy is a type of genomic mutation and consists in a multiple increase in the number of chromosomes compared to a haploid one. Polyploid forms can be obtained by treating seeds with colchicine during their germination.

A multiple increase in the number of chromosomes is accompanied by an increase in the mass of seeds and fruits, which leads to an increase in the yield of agricultural plants. Academician P.M. Zhukovsky: "Humanity feeds and clothes mainly on the products of polyploidy." In Russia, experimentally obtained polyploid potato varieties, wheat, sugar beet, buckwheat and other cultivated plants.

III. Consolidation of knowledge

Generalizing conversation in the course of learning new material.

IV. Homework

Study the paragraph of the textbook (features of plant biology taken into account in breeding, the main methods of plant breeding and their characteristics).

To be continued

artificial selection. To substantiate the historical principle of the development of wildlife, Darwin deeply studied the centuries-old practice of agriculture and animal husbandry and came to the conclusion that the diversity of breeds of domestic animals and cultivated plant varieties is the result of variability, heredity and artificial selection.

Artificial selection is carried out by man and can be twofold: conscious (methodical) - in accordance with the goal that the breeder sets for himself, and unconscious, when a person does not set himself the Goal of breeding a breed or variety with predetermined properties, but simply eliminates less valuable individuals and leaves the best for the tribe. Unconscious selection has been carried out by man for many millennia: even savages, during a famine, left more useful animals for the tribe, and killed less valuable ones. In unfavorable periods, primitive man primarily used unsalted fruits or smaller seeds, and in this case also made selection, but unconsciously. In all cases of such selection, the most productive forms of animals and more productive varieties plants, although man here acted as a blind selection factor, which can be any other environmental factor. .one

Many valuable forms have been bred by the centuries-old practice of artificial selection. In particular, by the middle of the XIX century. more than 300 varieties of wheat are registered in agricultural practice, in the deserts North Africa 38 varieties of date palm were cultivated, in Polynesia - 24 forms of breadfruit and the same number of varieties of banana, in China - 63 varieties of bamboo. There were about 1000 varieties of grapes, more than 300 gooseberries, about 400 breeds of cattle, 250 breeds of sheep, 350 breeds of dogs, 150 breeds of pigeons, many valuable breeds rabbits, chickens, ducks, etc. Supporters of species constancy believed that each such variety or breed originates from its direct ancestor. However, Darwin proved that the source of the diversity of animal breeds and varieties of cultivated plants is one or a small number of wild ancestors, whose descendants were transformed by man in different directions in accordance with his economic goals, tastes and interests. In this case, the breeder used the hereditary variability inherent in the selected forms.

Darwin distinguished between definite (now called modificational) and indefinite variability. With a certain, or group, variability, all or almost all the offspring of individuals exposed to the same conditions change in one direction; for example, when there is a shortage of food, animals lose weight; in cold climates, mammals have thicker hair 1 t. one orta, one breed, one species. Currently, this form of variability is called genotypic. Variability is transmitted to offspring not only during sexual reproduction, but also during vegetative reproduction: often a plant grows shoots with new properties or develops buds, from which fruits with new qualities (grapes, gooseberries) are formed - the result of a mutation in the somatic cell of the kidney.

In the phenomena of variability, Darwin discovered a number of important regularities, namely: when one organ or feature changes, others can change. For example, a crest develops at the site of attachment of the exercised muscle to the bone, in wading birds the neck lengthens simultaneously with the lengthening of the limbs, the thickness of the hair in sheep changes accordingly with an increase in the thickness of the skin. Such variability is called correlative or correlative. On the basis of correlative variability, the breeder can predict certain deviations from the original form and select in the desired direction.

Natural selection unlike artificial, it is carried out in nature itself and consists in the selection within the species of the most adapted individuals to the conditions of a particular environment. Darwin discovered a certain commonality in the mechanism of artificial and natural selection: in the first form of selection, the conscious or unconscious will of a person is embodied in the results, in the second, the laws of nature dominate. In both cases, new forms are created, however, with artificial selection, despite the fact that variability affects all organs and properties of animals and plants, the resulting animal breeds and plant varieties retain features that are useful for humans, but not for the organisms themselves. On the contrary, natural selection preserves individuals in which the changes are beneficial to their own existence under given conditions.

In nature, definite and indefinite variability is constantly observed. Its intensity is less pronounced here than in domestic forms, since the change natural environment occurs insidiously and extremely slowly. The emerging qualitative heterogeneity of individuals within species, as it were, brings many "candidates" to the evolutionary arena, leaving natural selection to reject those less adapted to survival. The process of natural "culling", according to Darwin, is carried out on the basis of variability, the struggle for existence and natural selection. The material for natural selection is supplied by the indeterminate (genotypic) variability of organisms. It is for this reason that the offspring of any pair of wild (as well as domestic) organisms turns out to be heterogeneous. If the changes are beneficial, it increases the chances of survival and procreation. Any change harmful to the organism will inevitably lead to its destruction or the inability to leave offspring. The survival or death of an individual is the final result of the "struggle for existence", which Darwin understood not in a direct, but in a figurative sense. He distinguished three forms of struggle for existence:

A) intraspecific - the most fierce, since individuals of the same species need similar food sources, which are also limited, in similar conditions for reproduction, the same shelters;

C) the struggle of living organisms with factors of inanimate nature - environmental conditions during drought, floods, early frosts, hail fall, many small animals, birds, worms, insects, grasses die.

As a result of all these complex relationships, many organisms die or, being weakened, do not leave offspring. Individuals with at least minimal beneficial changes survive. Adaptive traits and properties do not appear immediately, they are accumulated by natural selection from generation to generation, which leads to the fact that the descendants differ from their ancestors at the species and higher systematic level.

The struggle for existence is inevitable in connection with the intensive reproduction that exists in nature. This pattern knows no exceptions. There are always more organisms born than those capable of surviving to adulthood and leaving descendants. Calculations show: if all born mice survived, then within seven years the offspring of one pair would occupy the entire land the globe. A female cod fish lays up to 10 million eggs at a time, one shepherd's bag plant produces 73 thousand seeds, henbane - 446,500, etc. However, " geometric progression reproduction" is never carried out, since between organisms there is a struggle for space, food, shelter from enemies, competition in choosing a sexual partner, a struggle for survival with fluctuations in temperature, humidity, lighting, etc. In this "fight" most of those born die , leaving no offspring, and therefore in nature the number of individuals of each species, on average, remains constant.

Table Forms of selection (T.L. Bogdanova. Biology. Tasks and exercises. A guide for applicants to universities. M., 1991)

Indicators	artificial selection	Natural selection
Initial material for selection		Individual signs of the body
Selecting factor		Environmental conditions (living and inanimate nature)
Change path:
favorable	Selected, become productive	Remain, accumulate, be inherited
unfavorable	Selected, discarded, destroyed	Destroyed in the struggle for existence
Nature of action	Creative - directed accumulation of signs for the benefit of a person	Creative - selection of adaptive traits for the benefit of an individual, population, species, leading to the emergence of new forms
Selection result	New plant varieties, animal breeds, strains of microorganisms	New species
Selection Forms	Mass; individual; unconscious (spontaneous); methodical (conscious)	Driving, supporting deviations in changing environmental conditions; stabilizing, maintaining the constancy of the average reaction rate under constant environmental conditions

The doctrine of artificial selection is considered. We will analyze the main characteristics, types and features of this concept in our article.

Driving Forces of Evolution

According to evolutionary theory, modern views arose as a result of a series of adaptive changes in wild animals. Under the influence of what processes did this happen? These include hereditary variability and the struggle for existence, the consequence of which is natural selection. The essence of the latter lies in the prevailing survival of the fittest species. It happens in nature even now.

Characteristics of artificial selection

Man has long learned to use selection to obtain species with useful properties. To do this, he saves the descendants of the most productive individuals. This type of selection is called artificial. Its purpose is to bring valuable economic relations plants and strains of microorganisms.

Their formation began with the domestication and cultivation of wild species. For example, all modern dog breeds have a single ancestor, which is a wolf. Initially, the main characteristic of artificial selection was its unconscious nature. This means that a person carried it out without a specific goal. He left the largest individuals of animals for reproduction, and the best seeds for sowing on next year. Less valuable specimens were used for food. The results of such a process will be visible only after a long time.

How to achieve the emergence of new traits in self-pollinating plants and animals that are capable of self-fertilization? In this case, breeders use mutations - sudden abrupt changes in the genotype that occur as a result of the action of certain factors. They are called mutagens. This has been proven experimentally. If self-pollination of plants with the largest seeds is carried out, then useful signs do not appear even after six generations.

Conscious selection is more effective. It is also called methodical. At the same time, a person consciously deduces artificial look with specific properties. Such selection is carried out in a number of generations until the desired result is achieved.

Comparative characteristics of artificial and natural selection

Both types of selection have a number of similar features. Their basis is hereditary variability - the property of organisms to transmit certain signs and developmental characteristics of offspring. In both cases, properties that increase the viability of individuals are valuable. In natural selection, species that do not have favorable changes die as a result of the struggle for existence. And with artificial they are rejected or destroyed.

The main characteristic of artificial selection is the direct participation of man and the high rate of obtaining results. The necessary changes can be achieved over a period of 10 to 20 years. In nature, these processes take hundreds and even millions of years.

Mass selection

There are two forms of artificial selection. One of them is massive. In this case beneficial features source material are determined only on the basis of phenotypic traits. Thus, a person visually determines which species to use for further reproduction and cultivation.

Such artificial selection is an example of use simple methods in the selection. It is used quite often, but has a number of disadvantages. Despite external similarity, individuals can be genetically heterogeneous: heterozygous or homozygous for the dominant allele. In this case, the selection efficiency is significantly reduced. The expected result will appear only in the case of crossing heterozygotes. But in the next generations, the manifestation of beneficial traits will decrease, as the number of homozygous organisms will increase.

Individual selection

This form has a number of advantages. Individual artificial selection, examples of which we are considering, is carried out taking into account the genotype of the source material. For this, the method of analyzing crosses is used, as well as the study of pedigrees.

After choosing parental pairs, a crossing system - hybridization is used. It can be carried out within the same or different types. In any case, breeders encounter a number of difficulties. So, after a series of related crosses, the homozygosity of the offspring increases. The consequence of this is the degeneration, weakening and death of the line. But this method is ideal for getting clean lines.

With unrelated crossing, initially heterozygosity increases. This leads to the appearance of hybrid vigor in the descendants of the first generation. This phenomenon is called heterosis. Hybrids at the same time have a greater viability compared to their parents. But in subsequent generations, this effect weakens.

So, the main characteristics of artificial selection include directed human activity, the rapid pace of obtaining results, and taking into account the characteristics of the genotype of the selection material.

PRACTICAL WORK № 4

Topic:Comparison of natural and artificial selection.

Target:Give a comparative description of natural and artificial selection, find similarities and differences, find out the role of natural and artificial selection.

Equipment:tab. natural selection, artificial selection.

Working process

1. Natural selection is the survival and reproduction of organisms of a certain species that are most adapted to environmental conditions. Artificial selection is the breeding of new varieties of organisms of a certain species by man.

№ p/n	Properties	Selection type
		Natural	Artificial
	Source of evolutionary change	Hereditary variability, struggle for existence	hereditary variability
	Cause	Effect of environmental factors and population size	Human factor
	Driving force	Evolution	Selections
	What forms are saved	Forms with vital signs adapted to the environment	Forms with useful features for humans. These signs can be harmful to the body
	What forms are eliminated	Forms that are not viable or unadapted to environmental conditions	Forms with features necessary for a person
	Consequences of selection	Formation of new species: a) stabilizing b) driving c) bursting	Breeding of new breeds and varieties: a) conscious b) unconscious
	Selection types

Output:Similarities: the basis or source of evolutionary changes in artificial and natural selection is hereditary variability. As a result of natural and artificial selection, new organic forms are formed.

Features of difference: The basis of natural selection is hereditary variability and the struggle for existence. This is the main driving force of evolution. It always acts for the benefit of the organism, the population and the species as a whole, because it contributes to the survival of the fittest organisms.

Of the various hereditary changes, only those that meet the conditions of existence remain. These changes eventually lead to the emergence of new types of organisms.

This is the creative role of natural selection.

There are types of natural selection: stabilizing, driving and rozryvayuchy (disruptive): a) Stabilizing selection - is reduced to the elimination of individuals with a large deviation of the Feature from the stable (average). It maintains the constancy of the phenotype under stable conditions; b) Driving - acts in the event of a change in the conditions of existence and is reduced to the elimination of individuals with stable traits. There is a shift in the norm of the reaction in a certain direction; c) Disruptive - operates in unstable conditions and is reduced to the elimination of individuals with medium, intermediate characteristics and the preservation of extreme types. Leads to polymorphism in the population.

Artificial selection is carried out by a person who selects and stores in living organisms only traits that are useful for himself. The creative role of artificial selection is the breeding of new ones. plant varieties, animal breeds and strains of microorganisms. Artificial selection can be conscious and unconscious: a). Unconscious - when a person unconsciously selects to