Hereditary variability, its types. Types of mutations, their causes. The role of mutations in the evolution of the organic world and selection. Hereditary variability

Study the patterns and significance of variability for evolution.

Laboratory work (for 15-17 minutes).

"Variability of Organisms".

Target: characterize heredity and variability - the most important factors evolution.

Prepare a herbarium (separate assessment!):

5 leaves with one tree iron (dry) and secure on one page;

on another page – pin 5 leaves from different trees one type .

Progress:

Exercise 1. Compare leaves plucked from one plant. Explain the reasons for the similarities and differences between the leaves. Expand the meaning of non-hereditary (modification) variability for organisms.

Task 2. Compare leaves of the same species, but taken from different plants. Explain the reasons for the similarities and differences between the leaves. Expand the significance of hereditary variability (mutational and combinational) for evolution.

Variability

Variability call the ability of living organisms to acquire new characteristics and properties. Thanks to variability, organisms can adapt to changing environmental conditions.

There are two types of variability:

Non-hereditary, or phenotypic, - variability in which no changes in the genotype occur. She is also called group , certain .

Hereditary, or genotypic , individual, uncertain- changes in the characteristics of the organism due to changes in the genotype;

it happens:

mutational

combinative

mutational- arising as a result of a sudden change in the state of genes or chromosomes; combinative- resulting from the formation and fusion of germ cells;

Darwin distinguishes two main forms of variation: group, or certain(modified according to modern terminology) and individual, or uncertain .

Group variability depends on the conditions in which the organisms are located, while there is no change in the genotypes of individuals, and inheritance of characteristics does not occur. For example, the mass of a large cattle depends on feeding; cows at good feeding give more milk.

Non-hereditary variability

Arrowhead leaves are ribbon-shaped under water, heart-shaped on water, arrow-shaped in the air;

The underwater leaves of the aquatic buttercup are thread-like, while the above-water leaves have a wide leaf blade.

People tan in the sun, this is also a certain variability.

Meaning in nature?

Hereditary variability

For evolution and selection, only hereditary variability, variability associated with changes not only in phenotype, but also in genotype.

Hereditary variability supplies material for natural or artificial selection.

Hereditary variability can be:

mutational- resulting from a sudden change in the state of genetic material.

combinative- resulting from sexual reproduction.

Hereditary variability

Mutations are the material for evolution. Mutations are random and undirected. Can change genes, chromosomes and chromosome number.

For example, polyploidy is a type of mutation in which the number of chromosomes increases, a multiple of the haploid one. Polyploids in plants are more viable than diploid organisms.

Hereditary variability

Mutations can be:

dominant (manifest necessarily in the presence of a dominant gene);

recessive (they do not appear in the presence of a dominant gene).

Dominant mutations immediately come under selection control.

Hereditary variability

But most mutations are harmful and recessive; they do not appear and do not come under the control of selection until germ cells with recessive mutations combine.

Hereditary variability

Combinative variability.

When germ cells are formed, a recombination of the already existing genetic material of the organism occurs; there are no two identical germ cells in one organism.

When unique gametes fuse, a unique genotype is formed, which comes under the control of selection.

Repetition:

What types of variability did Charles Darwin distinguish?
Leaves of the same age from the same tree are different. What kind of variability is this? Explain your answer.
What is the significance of certain variability for organisms?
What type of variability do brother and sister have? Explain your answer.
What is the significance of combinational variability?
What kind of variability is called mutational?
What is the significance of mutational variability?
What is the elementary evolutionary material?

Repetition:

Combinative variability:

When does recombination of the genetic material of the parents occur?
Effect on genotype?
Effect on phenotype?
What does it mean for the body?

Mutational variability:

Can it be considered a certain variability?
Can it be considered group variability?
Effect on genotype?
Effect on phenotype?
Inheriting received changes?
What does it mean for the body?

Repetition:

Modification variability

Can it be considered a certain variability?
Can it be considered group variability?
Effect on genotype?
Effect on phenotype?
Inheriting received changes?
The significance of the organism?
Importance to the species?

The textbook complies with the Federal State educational standard secondary (full) general education, recommended by the Ministry of Education and Science of the Russian Federation and included in the Federal List of Textbooks.

The textbook is addressed to 10th grade students and is designed to teach the subject 1 or 2 hours a week.

Modern design, multi-level questions and tasks, Additional Information and the possibility of parallel work with an electronic application contribute to the effective assimilation of educational material.

Book:

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Remember!

Give examples of signs that change under the influence external environment.

What are mutations?

Variability– one of the most important properties of living things, the ability of living organisms to acquire differences from individuals of both other species and their own species.

There are two types of variability: non-hereditary(phenotypic, or modification) and hereditary(genotypic).

Non-hereditary (modification) variability. This type of variability is the process of the emergence of new characteristics under the influence of environmental factors that do not affect the genotype. Consequently, the resulting modifications of characteristics - modifications - are not inherited (Fig. 93). Two identical (monozygotic) twins who have exactly the same genotypes, but by the will of fate grew up in different conditions, can be very different from each other. Classic example arrowhead, which proves the influence of the external environment on the development of traits. This plant develops three types of leaves depending on the growing conditions - in the air, in the water column or on its surface.

Rice. 93. Oak leaves grown in bright light (A) and in a shaded place (B)

Rice. 94. Change in color of the fur of the Himalayan rabbit under the influence of different temperatures

Influenced by temperature environment The coat color of the Himalayan rabbit changes. The embryo, developing in the mother's womb, is exposed to elevated temperatures, which destroys the enzyme necessary for pigment synthesis, so rabbits are born completely white. Soon after birth, certain protruding parts of the body (nose, tips of the ears and tail) begin to darken because the temperature there is lower than elsewhere and the enzyme is not destroyed. If you pluck an area of white fur and cool the skin, black wool will grow in this place (Fig. 94).

Under similar environmental conditions in genetically similar organisms, modification variability has group character, for example, in summer period In most people, under the influence of UV rays, a protective pigment - melanin - is deposited in the skin, people tan.

In the same species of organisms, under the influence of environmental conditions, variability various signs can be completely different. For example, in cattle, milk yield, weight, and fertility very much depend on feeding and housing conditions, and, for example, the fat content of milk changes very little under the influence of external conditions. Manifestations of modification variability for each trait are limited by their reaction norm. Norm of reaction- these are the limits within which a change in a trait is possible in a given genotype. In contrast to the modification variability itself, the reaction norm is inherited, and its boundaries are different for different traits and in individual individuals. The narrowest reaction norm is characteristic of traits that provide vital qualities of the organism.

Due to the fact that most modifications have adaptive significance, they contribute to adaptation - the adaptation of the organism, within the limits of the normal reaction, to existence in changing conditions.

Hereditary (genotypic) variability. This type of variability is associated with changes in the genotype, and the traits acquired as a result of this are inherited by subsequent generations. There are two forms of genotypic variability: combinative and mutational.

Combinative variability consists in the appearance of new characteristics as a result of the formation of other combinations of parents’ genes in the genotypes of offspring. This type of variability is based on the independent divergence of homologous chromosomes in the first meiotic division, the random meeting of gametes in the same parental pair during fertilization and random selection parent couples. The exchange of sections of homologous chromosomes that occurs in the first prophase of meiosis also leads to recombination of genetic material and increases variability. Thus, in the process of combinative variability, the structure of genes and chromosomes does not change, but new combinations of alleles lead to the formation of new genotypes and, as a consequence, to the appearance of descendants with new phenotypes.

Mutational variability is expressed in the emergence of new qualities of the body as a result of the formation of mutations. The term “mutation” was first introduced in 1901 by the Dutch botanist Hugo de Vries. According to modern ideas mutations– these are sudden natural or artificially caused heritable changes in genetic material, leading to changes in certain phenotypic characteristics and properties of the organism. Mutations are non-directional, i.e. random, in nature and are the most important source of hereditary changes, without which the evolution of organisms is impossible. At the end of the 18th century. in America, a sheep with shortened limbs was born, giving rise to the new Ancona breed (Fig. 95). In Sweden at the beginning of the 20th century. A mink with platinum-colored fur was born on a fur farm. The huge variety of traits in dogs and cats is the result of mutational variability. Mutations arise spasmodically, as new qualitative changes: awnless wheat was formed from awned wheat, short wings and strip-shaped eyes appeared in Drosophila, and white, brown, and black colors appeared in rabbits from the natural agouti color as a result of mutations.

According to the place of occurrence, somatic and generative mutations are distinguished. Somatic mutations arise in the cells of the body and are not transmitted through sexual reproduction to subsequent generations. Examples of such mutations are age spots and skin warts. Generative mutations appear in germ cells and are inherited.

Rice. 95. Ancona sheep

Based on the level of change in genetic material, gene, chromosomal and genomic mutations are distinguished. Gene mutations cause changes in individual genes, disrupting the order of nucleotides in the DNA chain, which leads to the synthesis of an altered protein.

Chromosomal mutations affect a significant portion of the chromosome, disrupting the functioning of many genes at once. A separate fragment of a chromosome can be doubled or lost, which causes serious disruptions in the functioning of the body, including the death of the embryo in the early stages of development.

Genomic mutations lead to a change in the number of chromosomes as a result of violations of chromosome segregation during meiotic divisions. The absence of a chromosome or the presence of an extra one leads to adverse consequences. Most famous example The genomic mutation is Down syndrome, a developmental disorder that occurs when an extra 21st chromosome appears. Such people total number There are 47 chromosomes.

In protozoa and plants, an increase in the number of chromosomes that is a multiple of the haploid number is often observed. This change in chromosome set is called polyploidy(Fig. 96). The emergence of polyploids is associated, in particular, with the nondisjunction of homologous chromosomes in meiosis, as a result of which in diploid organisms diploid rather than haploid gametes can be formed.

Mutagenic factors. The ability to mutate is one of the properties of genes, so mutations can occur in all organisms. Some mutations are incompatible with life, and the embryo that receives them dies in the womb, while others cause persistent changes in characteristics that are significant to varying degrees for the life of the individual. Under normal conditions, the frequency of mutation of an individual gene is extremely low (10–5), but there are environmental factors that significantly increase this value, causing irreversible damage to the structure of genes and chromosomes. Factors whose impact on living organisms leads to an increase in the frequency of mutations are called mutagenic factors or mutagens.

Rice. 96. Polyploidy. Chrysanthemum flowers: A – diploid form (2 n); B – polyploid form

All mutagenic factors can be divided into three groups.

Physical mutagens are all types of ionizing radiation (?-rays, X-rays), ultraviolet radiation, high and low temperatures.

Chemical mutagens– these are analogues of nucleic acids, peroxides, salts of heavy metals (lead, mercury), nitrous acid and some other substances. Many of these compounds cause problems with DNA replication. Substances used in agriculture for pest and weed control (pesticides and herbicides), industrial waste, certain food colors and preservatives, some medications, components of tobacco smoke.

In Russia and other countries of the world, special laboratories and institutes have been created that test for mutagenicity all new synthesized chemical compounds.

To the group biological mutagens include foreign DNA and viruses, which, when integrated into the host DNA, disrupt the functioning of genes.

Review questions and assignments

1. What types of variability do you know?

2. What is a reaction norm?

3. Explain why phenotypic variability is not inherited.

4. What are mutations? Describe the main properties of mutations.

5. Give a classification of mutations according to the level of changes in hereditary material.

6. Name the main groups of mutagenic factors. Give examples of mutagens belonging to each group. Assess whether there are mutagenic factors in your environment. What group of mutagens do they belong to?

Think! Do it!

1. Do you think environmental factors can influence the development of an organism carrying a lethal mutation?

2. Can combinative variability appear in the absence of the sexual process?

3. Discuss in class what ways there are to reduce the effect of mutagenic factors on humans in the modern world.

4. Can you give examples of modifications that are not adaptive in nature?

5. Explain to someone unfamiliar with biology how mutations differ from modifications.

6. Complete the study: “Study of modification variability in students (using the example of body temperature and pulse rate, periodically measured over 3 days).”

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

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Think!

Questions

1. Which chromosomes are called sex chromosomes?

2. What are autosomes?

3. What is homogametic and heterogametic sex?

4. When does genetic determination of sex occur in humans and what causes this?

5. What mechanisms of sex determination do you know? Give examples.

6. Explain what sex-linked inheritance is.

7. How is color blindness inherited? What color perception will children have whose mother is colorblind and whose father has normal vision?

Explain from the perspective of genetics why there are many more colorblind people among men than among women.

Variability- one of the most important properties of living things, the ability of living organisms to exist in various forms, acquire new characteristics and properties. There are two types of variability: non-hereditary(phenotypic, or modification) and hereditary(genotypic).

■ Non-hereditary (modification) variability. This type of variability is the process of the emergence of new characteristics under the influence of environmental factors that do not affect the genotype. Consequently, the resulting modifications of characteristics - modifications - are not inherited. Two identical (monozygotic) twins who have exactly the same genotypes, but by the will of fate grew up in different conditions, can be very different from each other. A classic example demonstrating the influence of the external environment on the development of traits is the arrowhead. This plant develops three types of leaves depending on the growing conditions - in the air, in the water column or on the surface.

Under the influence of ambient temperature, the color of the Himalayan rabbit's fur changes. The embryo, developing in the mother's womb, is exposed to elevated temperatures, which destroys the enzyme necessary for fur coloring, so rabbits are born completely white. Soon after birth, certain protruding parts of the body (nose, tips of the ears and tail) begin to darken because the temperature there is lower than elsewhere and the enzyme is not destroyed. If you pluck an area of white fur and cool the skin, black fur will grow in that area.

Under similar environmental conditions in genetically similar organisms, modification variability has a group character, for example, in the summer, most people, under the influence of UV rays, deposit a protective pigment in the skin - melanin, people sunbathe.

In the same species of organisms, under the influence of environmental conditions, the variability of various characteristics can be completely different. For example, in cattle, milk yield, weight, and fertility very much depend on feeding and housing conditions, and, for example, the fat content of milk changes very little under the influence of external conditions. Manifestations of modification variability for each trait are limited by their reaction norm. Norm of reaction- these are the limits within which a change in a trait is possible in a given genotype. In contrast to the modification variability itself, the reaction norm is inherited, and its boundaries are different for different traits and in individual individuals. The narrowest reaction norm is characteristic of traits that provide vital qualities of the organism.

Due to the fact that most modifications have adaptive significance, they contribute to adaptation - the adaptation of the organism, within the limits of the reaction norm, to existence in changing conditions.

■ Hereditary (genotypic) variability. This type of variability is associated with changes in the genotype, and the traits acquired as a result of this are inherited by subsequent generations. There are two forms of genotypic variability: combinative and mutational.

Combinative variability consists in the appearance of new characteristics as a result of the formation of other combinations of parents’ genes in the genotypes of offspring. This type of variability is based on the independent divergence of homologous chromosomes in the first meiotic division, the random encounter of gametes in the same parental pair during fertilization, and the random selection of parental pairs. The exchange of sections of homologous chromosomes that occurs in the first prophase of meiosis also leads to recombination of genetic material and increases variability. Thus, in the process of combinative variability, the structure of genes and chromosomes does not change, but new combinations of alleles lead to the formation of new genotypes and, as a consequence, to the appearance of descendants with new phenotypes.

Mutational variability is expressed in the emergence of new qualities of the organism as a result of the formation of mutations. The term “mutation” was first introduced in 1901 by the Dutch botanist Hugo de Vries. According to modern concepts, mutations are sudden natural or artificially caused inherited changes in genetic material, leading to changes in certain phenotypic characteristics and properties of the organism. Mutations are non-directional, i.e. random, in nature and are the most important source of hereditary changes, without which the evolution of organisms is impossible. At the end of the 18th century. In America, a sheep with shortened limbs was born, giving rise to the new Ancona breed. In Sweden at the beginning of the 20th century. A mink with platinum-colored fur was born on a fur farm. The huge variety of traits in dogs and cats is the result of mutational variability. Mutations arise spasmodically, as new qualitative changes: awnless wheat was formed from awned wheat, short wings and strip-shaped eyes appeared in Drosophila, and white, brown, and black colors appeared in rabbits from the natural agouti color as a result of mutations.

Chromosomal mutations affect a significant portion of the chromosome, leading to disruption of the functioning of many genes at once. A separate fragment of a chromosome can be doubled or lost, which causes serious disruptions in the functioning of the body, including the death of the embryo in the early stages of development.

Genomic mutations lead to a change in the number of chromosomes as a result of violations of chromosome segregation during meiotic divisions. The absence of a chromosome or the presence of an extra one leads to adverse consequences. The most well-known example of a genomic mutation is Down syndrome, a developmental disorder that occurs when an extra 21st chromosome appears. Such people have a total number of chromosomes of 47.

In protozoa and plants, an increase in the number of chromosomes that is a multiple of the haploid number is often observed. This change in chromosome set is called polyploidy. The emergence of polyploids is associated, in particular, with the nondisjunction of homologous chromosomes in meiosis, as a result of which in diploid organisms diploid rather than haploid gametes can be formed.

Mutagenic factors. The ability to mutate is one of the properties of genes, so mutations can occur in all organisms. Some mutations are incompatible with life, and the embryo that receives them dies in the womb, while others cause persistent changes in characteristics that are significant to varying degrees for the life of the individual. Under normal conditions, the frequency of mutation of an individual gene is extremely low (10 -5), but there are environmental factors that significantly increase this value, causing irreversible damage to the structure of genes and chromosomes. Factors whose impact on living organisms leads to an increase in the number of mutations are called mutagenic factors or mutagens.

All mutagenic factors can be divided into three groups.

Physical mutagens are all types of ionizing radiation (y-rays, x-rays), ultraviolet radiation, high and low temperatures.

Chemical mutagens- these are analogues of nucleic acids, peroxides, salts of heavy metals (lead, mercury), nitrous acid and some other substances. Many of these compounds cause problems with DNA replication. Substances used in agriculture to control pests and weeds (pesticides and herbicides), industrial waste, certain food colorings and preservatives, some medications, and components of tobacco smoke have a mutagenic effect.

In Russia and other countries of the world, special laboratories and institutes have been created that test all new synthesized chemical compounds for mutagenicity.

Heredity- This most important feature living organisms, which consists in the ability to transmit the properties and functions of parents to offspring. This transmission is carried out using genes.

A gene is a unit of storage, transmission and implementation of hereditary information. A gene is a specific section of a DNA molecule, the structure of which encodes the structure of a specific polypeptide (protein). It is likely that many sections of DNA do not code for proteins, but perform regulatory functions. In any case, in the structure of the human genome, only about 2% of DNA are sequences on the basis of which messenger RNA is synthesized (transcription process), which then determines the sequence of amino acids during protein synthesis (translation process). It is currently believed that there are about 30 thousand genes in the human genome.

Genes are located on chromosomes, which are located in the nucleus of cells and are giant DNA molecules.

Chromosome theory heredity was formulated in 1902 by Setton and Boveri. According to this theory, chromosomes are carriers of genetic information that determines the hereditary properties of the organism. In humans, each cell has 46 chromosomes, divided into 23 pairs. Chromosomes that form a pair are called homologous.

Sex cells (gametes) are formed using a special type of division - meiosis. As a result of meiosis, only one homologous chromosome from each pair remains in each sex cell, i.e. 23 chromosomes. Such a single set of chromosomes is called haploid. During fertilization, when the male and female reproductive cells fuse and form a zygote, the double set, which is called diploid, is restored. In a zygote, in the organism that develops from it, one chromosome from each chromosome is received from the paternal organism, the other from the maternal.

A genotype is a set of genes received by an organism from its parents.

Another phenomenon that genetics studies is variability. Variability is understood as the ability of organisms to acquire new characteristics - differences within a species. There are two forms of variability:
- hereditary;
- modification (non-hereditary).

Hereditary variability- this is a form of variability caused by changes in the genotype, which can be associated with mutational or combinational variability.

Mutational variability.
Genes undergo changes from time to time, which are called mutations. These changes are random and appear spontaneously. The causes of mutations can be very diverse. Available whole line factors whose influence increases the likelihood of mutation occurrence. This may be exposure to certain chemicals, radiation, temperature, etc. Using these means, mutations can be caused, but the random nature of their occurrence remains, and it is impossible to predict the appearance of a particular mutation.

The resulting mutations are passed on to descendants, i.e. they determine hereditary variability, which is associated with where the mutation occurred. If a mutation occurs in a reproductive cell, then it has the opportunity to be transmitted to descendants, i.e. be inherited. If the mutation occurs in a somatic cell, then it is transmitted only to those that arise from this somatic cell. Such mutations are called somatic; they are not inherited.

There are several main types of mutations.
- Gene mutations, in which changes occur at the level of individual genes, i.e. sections of the DNA molecule. This may be the waste of nucleotides, the replacement of one base with another, the rearrangement of nucleotides, or the addition of new ones.
- Chromosomal mutations associated with disruption of chromosome structure lead to serious changes that can be detected using a microscope. Such mutations include losses of chromosome sections (deletions), addition of sections, rotation of a chromosome section by 180°, and the appearance of repeats.
- Genomic mutations are caused by changes in the number of chromosomes. Extra homologous chromosomes may appear: in the chromosome set, trisomy appears in place of two homologous chromosomes. In the case of monosomy, there is a loss of one chromosome from a pair. With polyploidy, there is a multiple increase in the genome. Another variant of genomic mutation is haploidy, in which only one chromosome from each pair remains.

The frequency of mutations is influenced, as already mentioned, by a variety of factors. When a number of genomic mutations occur great importance has, in particular, the age of the mother.

Combinative variability.
This type of variability is determined by the nature of the sexual process. With combinative variation, new genotypes arise due to new combinations of genes. This type of variability manifests itself already at the stage of formation of germ cells. As already mentioned, in each sex cell (gamete) there is only one homologous chromosome from each pair. Chromosomes enter the gamete randomly, so the sex cells of one person can differ quite greatly in the set of genes on the chromosomes. An even more important stage for the emergence of combinative variability is fertilization, after which the newly emerged organism has 50% of its genes inherited from one parent and 50% from the other.

Modifying variability is not associated with changes in the genotype, but is caused by the influence of the environment on the developing organism.

The presence of modification variability is very important for understanding the essence of inheritance. Traits are not inherited. You can take organisms with absolutely the same genotype, for example, grow cuttings from the same plant, but place them in different conditions(light, humidity, mineral nutrition) and get quite different plants with different characteristics (growth, yield, leaf shape, etc.). To describe the actually formed characteristics of an organism, the concept of “phenotype” is used.

A phenotype is the entire complex of actually occurring characteristics of an organism, which is formed as a result of the interaction of the genotype and environmental influences during the development of the organism. Thus, the essence of inheritance lies not in the inheritance of a trait, but in the ability of a genotype to produce a certain phenotype as a result of interaction with developmental conditions.

Since modification variability is not associated with changes in the genotype, modifications are not inherited. Usually this position is difficult to accept for some reason. It seems that if, say, parents have trained in lifting weights for several generations and have developed muscles, then these properties must necessarily be passed on to their children. Meanwhile, this is a typical modification, and training is the environmental influence that influenced the development of the trait. No changes in the genotype occur during modification and the characteristics acquired as a result of modification are not inherited. Darwin called this type of variability non-hereditary.

To characterize the limits of modification variability, the concept of reaction norm is used. Some characteristics in humans cannot be changed due to environmental influences, for example, blood type, gender, eye color. Others, on the contrary, are very sensitive to environmental influences. For example, as a result of prolonged exposure to the sun, skin color becomes darker and hair becomes lighter. A person’s weight is greatly influenced by diet, illness, and the presence of bad habits, stress, lifestyle.

Environmental influences can lead not only to quantitative, but also to qualitative changes in the phenotype. In some species of primrose, red flowers appear at low air temperatures (15-20 C), but if the plants are placed in a humid environment with a temperature of 30 ° C, white flowers are formed.

Moreover, although the reaction norm characterizes a non-hereditary form of variability ( modification variability), it is also determined by the genotype. This point is very important: the reaction rate depends on the genotype. The same environmental impact on a genotype can lead to a strong change in one of its traits and not affect another.

There are 2 types of hereditary variability: mutational and combinative.

The basis of combinative variability is the formation of recombinations, i.e. such gene connections that the parents did not have. Phenotypically, this can manifest itself not only in the fact that parental characteristics are found in some offspring in other combinations, but also in the formation of new characteristics in the offspring that are absent in the parents. This happens when two or more non-allelic genes that differ between the parents influence the formation of the same trait.

The main sources of combinative variability are:

Independent segregation of homologous chromosomes in the first meiotic division;

Gene recombination, based on the phenomenon of chromosome crossing (recombination chromosomes, once in the zygote, cause the appearance of characteristics that are not typical for the parents);

Chance meeting gametes during fertilization.

Mutation variability is based on mutations - persistent changes in the genotype that affect entire chromosomes, their parts or individual genes.

1) Types of mutations, according to the consequences of their influence on the body, are divided into beneficial, harmful and neutral.

2) According to the place of occurrence, mutations can be generative if they arise in germ cells: they can manifest themselves in the generation that develops from germ cells. Somatic mutations occur in somatic (non-reproductive) cells. Such mutations can be transmitted to descendants only through asexual or vegetative reproduction.

3) Depending on what part of the genotype they affect, mutations can be:

Genomic, leading to a multiple change in the number of chromosomes, for example, polyploidy;

Chromosomal, associated with a change in the structure of chromosomes, the addition of an extra section due to a crossover, a rotation of a certain section of chromosomes by 180°, or a change in the number of individual chromosomes. Thanks to chromosomal rearrangements, the evolution of the karyotype occurs, and individual mutants that arose as a result of such rearrangements may turn out to be more adapted to the conditions of existence, multiply and give rise to a new species;

Gene mutations are associated with changes in the sequence of nucleotides in a DNA molecule. This is the most common type of mutation.

4) According to the method of occurrence, mutations are divided into spontaneous and induced.

Spontaneous mutations occur naturally under the influence of mutagenic environmental factors without human intervention.

Induced mutations occur when mutagenic factors are directed to the body. Physical mutagens include various types of radiation, low and high temperatures; chemical - various chemical compounds; to biological ones - viruses.

So, mutations are the main source of hereditary variability - a factor in the evolution of organisms. Thanks to mutations, new alleles appear (they are called mutant). However, most mutations are harmful to living beings, since they reduce their fitness and ability to produce offspring. Nature makes many mistakes, creating, thanks to mutations, many modified genotypes, but at the same time it always accurately and automatically selects those genotypes that give the phenotype most adapted to certain environmental conditions.

Thus, the mutation process is the main source of evolutionary change.