Modifying mutation. Characteristics of modification variability

Variability is the occurrence of individual differences. Based on the variability of organisms, genetic diversity of forms appears, which, as a result of natural selection, are transformed into new subspecies and species. A distinction is made between modificational, or phenotypic, and mutational, or genotypic, variability.

TABLE Comparative characteristics forms of variability (T.L. Bogdanova. Biology. Assignments and exercises. A guide for applicants to universities. M., 1991)

Modification variability

Modifying variability does not cause changes in the genotype; it is associated with the reaction of a given, one and the same genotype to changes in the external environment: under optimal conditions, the maximum capabilities inherent in a given genotype are revealed. Thus, the productivity of outbred animals in conditions of improved housing and care increases (milk yield, meat fattening). In this case, all individuals with the same genotype respond to external conditions equally (C. Darwin called this type of variability definite variability). However, another trait - the fat content of milk - is weakly susceptible to changes in environmental conditions, and the color of the animal is even more stable sign. Modification variability usually fluctuates within certain limits. The degree of variation of a trait in an organism, i.e., the limits of modification variability, is called the reaction norm.

A wide reaction rate is characteristic of such characteristics as milk yield, leaf size, and color in some butterflies; narrow reaction norm - milk fat content, egg production in chickens, color intensity of flower corollas, etc.

The phenotype is formed as a result of interactions between the genotype and environmental factors. Phenotypic characteristics are not transmitted from parents to offspring; only the reaction norm is inherited, that is, the nature of the response to changes in environmental conditions. In heterozygous organisms, changing environmental conditions can cause different manifestations of this trait.

Properties of modifications: 1) non-heritability; 2) the group nature of the changes; 3) correlation of changes to the influence of a certain environmental factor; 4) the dependence of the limits of variability on the genotype.

Genotypic variability

Genotypic variability is divided into mutational and combinative. Mutations are abrupt and stable changes in units of heredity - genes, entailing changes in hereditary characteristics. The term "mutation" was first introduced by de Vries. Mutations necessarily cause changes in the genotype, which are inherited by the offspring and are not associated with crossing and recombination of genes.

Classification of mutations. Mutations can be combined into groups - classified according to the nature of their manifestation, by location, or by the level of their occurrence.

Mutations, according to the nature of their manifestation, can be dominant or recessive. Mutations often reduce viability or fertility. Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal, and those incompatible with life are called lethal. Mutations are divided according to the place of their occurrence. A mutation that occurs in germ cells does not affect the characteristics of a given organism, but appears only in the next generation. Such mutations are called generative. If genes change in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But with asexual reproduction, if an organism develops from a cell or group of cells that has a changed - mutated - gene, mutations can be transmitted to offspring. Such mutations are called somatic.

Mutations are classified according to the level of their occurrence. There are chromosomal and gene mutations. Mutations also include a change in the karyotype (change in the number of chromosomes). Polyploidy is an increase in the number of chromosomes, a multiple of the haploid set. In accordance with this, plants are distinguished into triploids (3p), tetraploids (4p), etc. More than 500 polyploids are known in plant growing (sugar beets, grapes, buckwheat, mint, radishes, onions, etc.). All of them are distinguished by a large vegetative mass and have great economic value.

A wide variety of polyploids is observed in floriculture: if one original form in the haploid set had 9 chromosomes, then cultivated plants of this species can have 18, 36, 54 and up to 198 chromosomes. Polyploids evolve as a result of exposure of plants to temperature, ionizing radiation, chemicals (colchicine) that destroy the cell division spindle. In such plants, the gametes are diploid, and when fused with the haploid germ cells of a partner, a triploid set of chromosomes appears in the zygote (2n + n = 3n). Such triploids do not form seeds, they are sterile, but highly productive. Even-numbered polyploids form seeds.

Heteroploidy is a change in the number of chromosomes that is not a multiple of the haploid set. In this case, the set of chromosomes in a cell can be increased by one, two, three chromosomes (2n + 1; 2n + 2; 2n + 3) or decreased by one chromosome (2l-1). For example, a person with Down syndrome has one extra chromosome on the 21st pair and the karyotype of such a person is 47 chromosomes. People with Shereshevsky-Turner syndrome (2p-1) are missing one X chromosome and 45 chromosomes remain in the karyotype. These and other similar deviations in numerical relationships in a person’s karyotype are accompanied by health disorders, mental and physical disorders, decreased vitality, etc.

Chromosomal mutations are associated with changes in the structure of chromosomes. There are the following types chromosome rearrangements: separation of various sections of a chromosome, doubling of individual fragments, rotation of a section of a chromosome by 180°, or attachment of a separate section of a chromosome to another chromosome. Such a change entails disruption of the function of genes in the chromosome and hereditary properties organism, and sometimes its death.

Gene mutations affect the structure of the gene itself and entail changes in the properties of the body (hemophilia, color blindness, albinism, color of flower corollas, etc.). Gene mutations occur in both somatic and germ cells. They can be dominant or recessive. The former appear in both homozygotes and. in heterozygotes, the second - only in homozygotes. In plants, somatic gene mutations that arise are preserved during vegetative propagation. Mutations in germ cells are inherited during seed reproduction of plants and during sexual reproduction of animals. Some mutations have a positive effect on the body, others are indifferent, and others are harmful, causing either the death of the body or a weakening of its viability (for example, sickle cell anemia, hemophilia in humans).

When breeding new varieties of plants and strains of microorganisms, induced mutations are used, artificially caused by certain mutagenic factors (X-ray or ultraviolet rays, chemicals). Then the resulting mutants are selected, preserving the most productive ones. In our country, many economically promising plant varieties have been obtained using these methods: non-lodging wheat with large ears, resistant to diseases; high-yielding tomatoes; cotton with large bolls, etc.

Properties of mutations:

1. Mutations occur suddenly, spasmodically.
2. Mutations are hereditary, that is, they are persistently transmitted from generation to generation.
3. Mutations are undirected - any locus can mutate, causing changes in both minor and vital signs.
4. The same mutations can occur repeatedly.
5. According to their manifestation, mutations can be beneficial and harmful, dominant and recessive.

The ability to mutate is one of the properties of a gene. Each individual mutation is caused by a cause, but in most cases these causes are unknown. Mutations are associated with changes in external environment. This is convincingly proven by the fact that through exposure to external factors it is possible to sharply increase their number.

Combinative variability

Combinative hereditary variability arises as a result of the exchange of homologous sections of homologous chromosomes during the process of meiosis, as well as as a consequence of independent divergence of chromosomes during meiosis and their random combination during crossing. Variation can be caused not only by mutations, but also by combinations of individual genes and chromosomes, a new combination of which, during reproduction, leads to changes in certain characteristics and properties of the organism. This type of variability is called combinative hereditary variability. New combinations of genes arise: 1) during crossing over, during the prophase of the first meiotic division; 2) during independent divergence of homologous chromosomes in anaphase of the first meiotic division; 3) during the independent divergence of daughter chromosomes in anaphase of the second meiotic division and 4) during the fusion of different germ cells. The combination of recombined genes in a zygote can lead to a combination of characteristics different breeds and varieties.

In breeding, the law of homologous series of hereditary variability, formulated by the Soviet scientist N. I. Vavilov, is of great importance. It reads: inside different types and genera that are genetically close (i.e., having the same origin), similar series of hereditary variability are observed. This type of variability has been identified in many cereals (rice, wheat, oats, millet, etc.), in which the color and consistency of the grain, cold resistance and other qualities vary similarly. Knowing the nature of hereditary changes in some varieties, one can predict similar changes in related species and, by influencing them with mutagens, induce similar beneficial changes in them, which greatly facilitates the production of economically valuable forms. Many examples of homological variability are known in humans; for example, albinism (a defect in the synthesis of dye by cells) was found in Europeans, blacks and Indians; among mammals - in rodents, carnivores, primates; short dark-skinned people - pygmies - are found in tropical forests equatorial Africa, in the Philippine Islands and in the jungles of the Malacca Peninsula; Some hereditary defects and deformities inherent in humans are also noted in animals. Such animals are used as a model to study similar defects in humans. For example, cataracts of the eye occur in mice, rats, dogs, and horses; hemophilia - in mice and cats, diabetes - in rats; congenital deafness - in guinea pig, mice, dogs; cleft lip - in a mouse, dog, pig, etc. These hereditary defects are convincing confirmation of the law homologous series hereditary variability N. I. Vavilova.

Table. Comparative characteristics of forms of variability (T.L. Bogdanova. Biology. Assignments and exercises. A manual for applicants to universities. M., 1991)

Variability is the ability of organisms to change their characteristics and properties, which is manifested in the diversity of individuals within a species.

There are 2 forms of variability:

non-hereditary (phenotypic) or modification

hereditary (genotypic)

Modification variability is the variability of the phenotype that

is the response of a specific genotype to changing environmental conditions. They are not inherited and arise as a reaction of the body, that is, they represent an adaptation.

Modification variability is characterized by the following features:

is of a group nature

is reversible

environmental influences can change the phenotypic manifestation of a trait. The reaction norm is the limit of modification variability of a trait determined by the genotype. For example, such quantitative characteristics as the body weight of an animal and the size of plant leaves vary quite widely, that is, they have a wide reaction rate. The sizes of the heart and brain vary within narrow limits, that is, they have a narrow reaction rate. The reaction norm is expressed as a variation series.

has transitional forms.

A variation curve is a graphical expression of modification variability, reflecting the scope of variation and the frequency of occurrence of individual variants.

Genotypic variability is divided into:

combinative

mutational

Combinative variability- a type of hereditary variability caused by various recombinations of existing genes and chromosomes. It is not accompanied by changes in the structure of genes and chromosomes.

Its source is: - recombination of genes as a result of crossing over;

Recombination of chromosomes during meiosis; - a combination of chromosomes as a result of the fusion of germ cells during fertilization.

Mutational variability is a type of hereditary variability caused by the manifestation of various changes in the structure of genes, chromosomes or genome.

COMPARATIVE CHARACTERISTICS OF FORMS OF VARIATION

Mutational variability

Mutations are the basis of mutational variability.

Mutations- These are sudden, natural or artificially induced changes in genetic material that lead to changes in the characteristics of an organism. The foundations of the doctrine of mutations were laid by Hugo de Vries in 1901.

Mutations are characterized by a number of properties:

They appear suddenly, without transitional forms;

These are qualitative changes, do not form continuous series and are not grouped around an average value;

They have a non-directional effect - under the influence of the same mutagenic factor, any part of the structure carrying genetic information;

Passed on from generation to generation.

Mutagens are factors that cause mutations. Divided into three categories:

physical (radiation, electromagnetic radiation, pressure, temperature, etc.).

chemical (salts of heavy metals, pesticides, phenols, alcohols, enzymes, narcotic substances, medicines, food preservatives, etc.)

CLASSIFICATION OF MUTATIONS:

By level of occurrence

1. chromosomal;

   genomic

By type of allelic interactions

dominant;

recessive;

Title page of On the Origin of Species, 1859

Modification variability- ability of organisms with the same genotype develop differently in different conditions environment. In this case, the phenotype changes, but the genotype does not change. In English-language literature until the 90s of the XX century. The concept of “adaptive modification” was often used in a similar meaning; at present, the concept of “phenotypic plasticity” is predominantly used. It is this class of phenomena that primarily underlies the “definite variability” that Charles Darwin described, as opposed to the “indeterminate variability” based mainly on mutations in the genetic apparatus.

Characteristics of modification variability

Reaction rate

Long-term modification variability

In most cases, modification variability is non-hereditary in nature and is only a reaction of the genotype of a given individual to environmental conditions with a subsequent change in phenotype. However, there are also examples of heritable environment-dependent changes described in some bacteria, protozoa and multicellular eukaryotes. Most often these cases are now defined as “transgenerational epigenetic inheritance,” but in Soviet-era textbooks the concept of “long-term modification” is applied to such cases.

To understand the possible mechanism of inheritance of modification variability, let us first consider the concept of a genetic trigger.

Modification variability in human life

The practical use of patterns of modification variability has great value in crop production and animal husbandry, as it allows one to foresee and plan in advance the maximum use of the capabilities of each plant variety and animal breed (for example, individual indicators of sufficient light for each plant). Creation of known ones optimal conditions for the implementation of the genotype ensures their high productivity.

This also makes it possible to expediently use the child’s innate abilities and develop them from childhood - this is the task of psychologists and teachers who are still school age trying to determine the inclinations of children and their abilities for one or another professional activities, increasing within the normal reaction level the level of realization of genetically determined abilities of children.

Modification variability - changes in the phenotype of an organism in most cases are adaptive in nature and are formed as a result of the interaction of the genotype with the environment. Changes in the body and modifications are NOT inherited. In general, the concept of “modification variability” corresponds to the concept of “definite variability”, which was introduced by Charles Robert Darwin

Conditional classification of modification variability

• According to the nature of changes in the body
  • Morphological changes
  • Physiological and biochemical adaptations- homeostasis
• According to the reaction norm spectrum
  • Narrow
  • Wide
• By value
  • Adaptive modifications
  • Morphoses
  • Phenocopies
• By duration
  • Observed only in individuals exposed to certain environmental factors (one-term)
  • Observed in the descendants of these individuals (long-term modifications) over a certain number of generations

Mechanism of modification variability

Gene → protein → change in the phenotype of an organism Environment

Modified variability is the result not of changes in the genotype, but of its response to environmental conditions. That is, the structure of genes does not change, but gene expression changes.

As a result, under the influence of environmental factors on the body, the intensity of enzymatic reactions changes, which is caused by a change in the intensity of their biosynthesis. Some enzymes, for example MAP kinase, regulate gene transcription, which depends on environmental factors. Thus, environmental factors are able to regulate the activity of genes and their production of a specific protein, the functions of which are most consistent with the environment.

As an example of adaptive modifications, consider the mechanism of formation of the melanin pigment. According to its production, there are four genes that are located in different chromosomes. Largest quantity alleles of these genes - 8 - are present in people with dark body color. If the integument is intensively affected by environmental factors, ultraviolet radiation, then when it penetrates into the lower layers of the epidermis, the cells of the latter are destroyed. Endothelin-1 and eicosanoids (fatty acid breakdown products) are released, which causes activation and enhanced biosynthesis of the tyrosinase enzyme. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further formation of melanin occurs without the participation of tyrosinase, but increased biosynthesis of tyrosinase and its activation causes the formation of a tan and corresponds to environmental factors.

Another example is the seasonal change in fur color in animals (molting). Molting and subsequent color changes are caused by the effect of temperature on the pituitary gland, which stimulates the production of thyroid-stimulating hormone. This causes an impact on the thyroid gland, under the influence of hormones which causes molting.

Reaction rate

The reaction norm is the spectrum of gene expression with a constant genotype, from which the level of activity of the genetic apparatus that is most suitable for environmental conditions is selected and forms a specific phenotype. For example, there is an allele of the gene X a, which causes the production more wheat ears, and the Y b gene allele, which produces a small number of wheat ears. The expression of alleles of these genes is interrelated. The entire spectrum of expression lies between the maximum expression of allele a and the maximum expression of allele b, and the intensity of expression of these alleles depends on environmental conditions. At favorable conditions(with sufficient moisture, nutrients) “dominance” of the allele occurs, and when unfavorable, the manifestation of allele b predominates. The reaction norm has a limit of manifestation for each species - for example, increased feeding of an animal will cause an increase in its mass, but it will be within the range of detection of this trait for a given species. The reaction rate is genetically determined and inherited. For various changes there are different faces manifestations of reaction norms. For example, the amount of milk yield, the productivity of cereals (quantitative changes) varies greatly, the color intensity of animals varies weakly, etc. (qualitative changes). In accordance with this, the reaction norm can be narrow (qualitative changes - the color of the pupae and adults of some butterflies) and wide (quantitative changes - the size of plant leaves, the body size of insects depending on the nutrition of their pupae). However, some quantitative changes are characterized by a narrow reaction rate (milk fat content, the number of toes in guinea pigs), and some qualitative changes are characterized by a wide reaction rate (seasonal color changes in animals northern latitudes). In general, the reaction norm and the intensity of gene expression based on it determine the dissimilarity of intraspecific units.

Characteristics of modification variability

• Turnover - changes disappear when the specific environmental conditions that led to the appearance of the modification disappear;
• Group character;
• Changes in the phenotype are NOT inherited - the norm of the genotype reaction is inherited;
• Statistical regularity of variation series;
• Modifications differentiate the phenotype without changing the genotype.

Analysis and patterns of modification variability

The displays of the manifestations of modification variability are ranked - a variation series - a series of modification variability of a property of an organism, which consists of individual interconnected properties of the organism's phenotype, arranged in ascending or descending order of the quantitative expression of the property (leaf size, changes in fur color intensity, etc.). A single indicator of the relationship between two factors in a variation series (for example, the length of fur and the intensity of its pigmentation) is called a variant. For example, wheat growing in one field can vary greatly in the number of heads and heads due to different soil conditions. By comparing the number of spikelets in one spikelet with the number of spikelets, you can get the following variation series:

Variation curve

A graphical display of the manifestation of modification variability - a variation curve - reflects both the range of variation in properties and the frequency of observation of individual variations.

After constructing the curve, it is clear that the most common are the average variants of manifestation of the property (Quetelet’s law). The reason for this is the effect of environmental factors on the course of ontogenesis. Some factors suppress gene expression, others enhance it. Almost always, these factors, equally affecting ontogenesis, neutralize each other, i.e. extreme manifestations of the trait are minimized in frequency of occurrence. This is the reason for the greater occurrence of individuals with an average manifestation of the trait. For example, the average height of a man - 175 cm - is the most common.

When constructing a variation curve, you can calculate the value of the standard deviation and, based on this, construct a graph of the standard deviation from the median - the manifestation of the characteristic that occurs most often.

Forms of modification variability

Phenocopies

Phenocopies are changes in phenotype under the influence of unfavorable environmental factors, similar to mutations. The genotype does not change. Their causes are teratogens - certain physical, chemical (drugs, etc.) and biological agents (viruses) with the occurrence of morphological abnormalities and developmental defects. Phenocopies often resemble hereditary diseases. Sometimes phenocopies originate from embryonic development. But most often examples of phenocopies are changes in ontogenesis - the spectrum of phenocopies depends on the stage of development of the organism.

Morphoses

Morphoses are changes in phenotype under the influence of extreme environmental factors. For the first time, morphoses appear precisely in the phenotype and can lead to adaptive mutations, the epigenetic theory of evolution is taken as the basis for the movement of natural selection based on modification variability. Morphoses are non-adaptive and irreversible in nature, that is, like mutations, they are labile. Examples of morphosis are scars, certain injuries, and burns.

Long-term modification variability

Most modifications are NOT inherited and are only a reaction of the genotype to environmental conditions. Of course, the descendants of an individual that has been exposed to certain factors that have shaped a broader reaction rate may also have the same broad changes, but these will only appear when exposed to certain factors that act on genes that cause intense enzymatic reactions. However, in some protozoa, bacteria and even eukaryotes there is so-called long-term modification variability due to cytoplasmic inheritance. To clarify the mechanism of long-term modification variability, let us first consider the regulation of the trigger by environmental factors.

Adjusting the trigger with modifications

As an example of long-term modification variability, consider the bacterial operon. An operon is a way of organizing genetic material in which genes encoding jointly or sequentially working proteins are combined under a single promoter. The bacterial operon contains, in addition to gene structures, two sections - a promoter and an operator. The operator is located between the promoter (the site from which transcription begins) and the structural genes. If the operator is associated with certain repressor proteins, then together they prevent RNA polymerase from moving along the DNA chain, starting from the promoter. If there are two operons and if they are interconnected (the structural gene of the first operon encodes a repressor protein for the second operon and vice versa), then they form a system called a trigger. When the first component of the trigger is active, the other component is passive. But, under the influence of certain environmental factors, a switch of the trigger to the second operon may occur due to interruption of the encoding of the repressor protein for it.

The effect of switching triggers can be observed in some noncellular life forms, such as bacteriophages, and in prokaryotes, such as coli.

Let's consider both cases.

Escherichia coli is a collection of bacterial species that interact with certain organisms to obtain a common benefit (mutualism). They have high enzymatic activity towards sugars (lactose, glucose), and they cannot simultaneously break down glucose and lactose. The ability to break down lactose is regulated by the lactose operon, which consists of a promoter, an operator and a terminator, as well as a gene encoding a repressor protein for the promoter. In the absence of lactose in the environment, the repressor protein combines with the operator and transcription stops. If lactose enters a bacterial cell, it combines with the repressor protein, changes its conformation, and dissociates the repressor protein from the operator.

Bacteriophages are viruses that infect bacteria. When bacteria enter a cell, unfavorable conditions environment, bacteriophages remain inactive, penetrating into genetic material and being transmitted to daughter cells during binary division of the mother cell. When favorable conditions appear in the bacterial cell, the trigger switches into the bacteriophage as a result of inducing nutrients, and the bacteriophages multiply and escape from the bacterium.

This phenomenon is often observed in viruses and prokaryotes, but it almost never occurs in multicellular organisms.

Cytoplasmic inheritance

Cytoplasmic heredity is heredity that consists of the entry of an inductor substance into the cytoplasm, which triggers gene expression (activates an operon) or in the autoreproduction of parts of the cytoplasm.

For example, when a bacterium budding, the inheritance of a bacteriophage occurs, which is located in the cytoplasm and plays the role of a plasmid. Under favorable conditions, DNA replication already occurs and the genetic apparatus of the cell is replaced by the genetic apparatus of the virus. A similar example of variability in E. coli is the operation of the lactase operon of E. Coli - in the absence of glucose and the presence of lactose, these bacteria produce an enzyme to break down lactose due to a switch in the lactase operon. This operon switch can be inherited by budding by introducing lactose into the daughter bacterium during its formation, and the daughter bacteria also produce an enzyme (lactase) to break down lactose even in the absence of this disaccharide in the environment.

Also, cytoplasmic inheritance associated with long-term modification variability can be found in such representatives of eukaryotes as the Colorado potato beetle and Habrobracon ichneumon wasps. When the pupae of the Colorado potato beetle were exposed to intense thermal indices, the color of the beetles changed. Under the obligatory condition that the female beetle also experienced the effects of intense thermal indicators, the descendants of such beetles maintained the present manifestation of the trait for several generations, and then the previous norm of the trait returned. This continued modification variability is also an example of cytoplasmic inheritance. The reason for inheritance is the autoreproduction of those parts of the cytoplasm that have undergone changes. Let us consider the mechanism of autoreproduction as the cause of cytoplasmic inheritance in detail. Organelles that have their own DNA and RNA, as well as other plasmogens, can autoreproduce in the cytoplasm. Organelles that are capable of self-reproduction are mitochondria and plastids, which are capable of self-duplication and protein biosynthesis through replication and the steps of transcription, processing and translation. This ensures the continuity of autoreproduction of these organelles. Plasmogens are also capable of self-reproduction. If, under the influence of the environment, plasmogens have undergone changes that determine the activity of this gene, for example, during the dissociation of a repressor protein or the association of a protein-coding protein, then it begins to produce a protein that forms a certain trait. Since plasmogens are able to be transported through the membrane of female eggs and thus inherited, their specific state is also inherited. At the same time, modifications caused by the gene by activating its own expression are also preserved. If the factor that caused the activation of gene expression and protein biosynthesis is preserved throughout ontogenesis to the offspring of an individual, then the trait will be transmitted to the next offspring.

Thus, a long-term modification persists as long as the factor causing this modification exists. When a factor disappears, the modification slowly fades away over several generations. This is what makes long-term modifications different from regular modifications.

Modification variability and theories of evolution

Natural selection and its influence on modification variability

Natural selection is the survival of the fittest individuals and the appearance of offspring with fixed successful changes. Four types of natural selection:

Stabilizing selection. This form of selection leads to: a) neutralization of mutations through selection, neutralizes their oppositely directed effect, b) improvement of the genotype and process individual development with a constant phenotype and c) the formation of a reserve of neutralized mutations. As a result of this selection, organisms with an average reaction rate dominate in low-minimal conditions of existence.

Driving selection. This form of selection leads to: a) the opening of mobilization reserves consisting of neutralized mutations, b) the selection of neutralized mutations and their compounds, and c) the formation of new phenotypes and genotypes. As a result of this selection, organisms with a new average reaction rate that is more consistent with the changing environmental conditions in which they live dominate.

Disruptive selection. This form of selection leads to the same processes as driving selection, but it is aimed not at the formation of a new average reaction norm, but at the survival of organisms with extreme reaction norms.

Sexual selection. This form of selection facilitates the meeting between the sexes, limiting the participation in the reproduction of the species of individuals with less developed sexual characteristics.

In general, most scientists consider the substrate of natural selection, together with other constant factors (genetic drift, struggle for existence), to be hereditary variability. These views were realized in conservative Darwinism and neo-Darwinism (synthetic theory of evolution). However, in lately Some scientists began to adhere to a different point of view, according to which the substrate before natural selection is morphosis - separate type modification variability. This view evolved into the epigenetic theory of evolution.

Darwinism and neo-Darwinism

From the point of view of Darwinism, one of the main factors of natural selection that determines the fitness of organisms is hereditary variability. This leads to the dominance of individuals with successful mutations, as a consequence of this - to natural selection, and, if the changes are very pronounced, to speciation. Modification variability depends on the genotype. The synthetic theory of evolution, which was created in the 20th century, adheres to the same view regarding modification variability. M. Vorontsov. As can be seen from the above text, these two theories consider the genotype to be the basis for natural selection, which changes under the influence of mutations, which are one of the forms of hereditary variability. Changes in the genotype cause a change in the reaction norm, since it is the genotype that determines it. The reaction norm causes a change in the phenotype, and thus mutations appear in the phenotype, which makes it more consistent with environmental conditions if mutations are appropriate. The stages of natural selection according to Darwinism and neo-Darwinism consist of the following stages:

1) First, an individual appears with new properties (due to mutations)

2) She then finds herself able or unable to leave descendants;

3) If an individual leaves descendants, then changes in its genotype are fixed in generations, and this ultimately leads to natural selection.

Epigenetic theory of evolution

The epigenetic theory of evolution considers phenotype as a substrate of natural selection, and selection not only fixes beneficial changes, but also participates in their creation. The main influence on heredity is not the genome, but the Epigenetic system - a set of factors acting on ontogeny. During morphosis, which is one of the types of modification variability, a stable development trajectory (creod) is formed in the individual - an epigenetic system that adapts to the morphosis. This development system is based on the genetic assimilation of organisms, which consists of conforming to the modification of a certain mutation - a modification gene copy, caused by an epigenetic change in the chromatin structure. This means that changes in gene activity can be the result of both mutations and environmental factors. That is, based on a certain modification under intense environmental influence, mutations are selected that adapt the body to new changes. In this way, a new genotype is formed, which forms a new phenotype. Natural selection, according to it, consists of the following stages:

1) Extreme environmental factors lead to morphosis;

2) Morphoses lead to destabilization of ontogenesis;

3) Destabilization of ontogenesis leads to the appearance of an abnormal phenotype that most closely matches the morphosis;

4) If the new phenotype is successfully matched, gene copying of modifications occurs, which leads to stabilization - the formation new normal reactions;

Comparative characteristics of hereditary and non-hereditary variability

Comparative characteristics of forms of variability

Property	Non-hereditary (modification)	Hereditary
Change object	Phenotype within the normal range of reaction	Genotype
Origin factor	Changes in environmental conditions	Gene recombination as a result of gamete fusion, crossing over and mutations
Inheritance of traits	Traits are not inherited (only the reaction norm is inherited)	Inherited
Value for an individual	Adaptation to environmental conditions, increasing vitality	Beneficial changes lead to survival, harmful changes lead to death
Meaning for view	Promotes survival	Leads to the emergence of new populations and species as a result of divergence
Role in evolution	Adaptation of organisms	Material for natural selection
Form of variability	Group	Individual, combined
Pattern	Statistical (variation series)	Law of homological series of hereditary variability

Modification variability in human life

Man, in general, has long used the knowledge of modification variability, for example, in farming. With knowledge of certain individual characteristics each plant (for example, the need for light, water, temperature conditions) can be planned maximum level using (within normal reaction limits) this plant is to achieve high fruitfulness. That's why various types people place plants for their formation in different conditions- in different seasons and the like. The situation is similar with animals - knowledge about the needs of, for example, cows leads to increased production of milk and, as a result, increased milk yield.

Since in humans the functional asymmetry of the cerebral hemispheres is formed upon reaching a certain age and it is less in illiterate, uneducated people, it can be assumed that the asymmetry is a consequence of modification variability. Therefore, at the stages of learning, it is very advisable to identify the child’s abilities in order to most fully realize her phenotype.

Examples of modification variability

• In insects and animals
  • Increase in the level of red blood cells when climbing mountains in animals (homeostasis)
  • Increased skin pigmentation with intense exposure to ultraviolet radiation
  • Development of the motor system as a result of training
• Scars (morphosis)
  • Changes in the color of Colorado potato beetles when their pupae are exposed to high or low temperatures for a long time
  • Changes in fur color in some animals when weather conditions change
  • The ability of butterflies from the genus Vanessa to change their color with changes in temperature
• In plants
  • Different structures of underwater and above-water leaves in aquatic Buttercup plants
  • Development of low-growing forms from seeds of lowland plants grown in the mountains
• In bacteria
  • operation of the lactase operon genes of Escherichia coli
• Tan

We know that modification variability is special case non-hereditary variability.

Modification variability – the ability of organisms with the same genotype develop differently under different environmental conditions. In a population of such organisms, a certain set of phenotypes. In this case, organisms must be same age.

Modifications - these are phenotypic non-hereditary differences that arise under the influence of environmental conditions in organisms of the same genotype (Karl Nägeli, 1884).

Examples of modifications widely known and numerous.

Leaf morphology water buttercup And arrowhead depends on the environment in which, air or underwater, they develop.

Arrowhead (Sagittaria sagittaefolia) has different leaves: arrow-shaped (above-water), heart-shaped (floating) and ribbon-shaped (underwater). Consequently, the arrowhead is hereditarily determined not by a specific leaf shape, but by the ability, within certain limits, to change this shape depending on the conditions of existence, which is adaptive feature body.

If the aerial part of the stem potatoes artificially deny access to light, tubers develop on it, hanging in the air.

U flounder , Leading a bottom lifestyle, the upper side of the body is dark, which makes it invisible to approaching prey, and the lower side is light. But if an aquarium with a glass bottom is illuminated not from above, but from below, then the lower surface of the body becomes dark.

Ermine rabbits have white fur on the body except the end of the muzzle, paws, tail and ears. If you shave an area, for example, on the back and keep the animal at a low temperature (0-1 °C), then black hair grows on the shaved area. If you pluck some of the black hair and place the rabbit in high temperatures, the white fur will grow back.

This is due to the fact that each part of the body is characterized by its own level of blood circulation and, accordingly, temperature, depending on which the black pigment is formed or degrades - melanin . The genotype remains the same.

Wherewarm , there the pigment degrades →white coat color, whereCold (distal areas), the pigment does not degrade there →black wool.

Modification properties

S. M. Gershenzon describes the following modification properties :

1. Degree of modification severity proportional to strength and duration action on the body of the factor causing the modification. This pattern fundamentally distinguishes modifications from mutations, especially gene mutations.

2. In the vast majority of cases, the modification is useful, adaptive reaction body for one way or another external factor. This can be seen in the above modifications in various organisms.

3. Only those modifications that are caused have adaptive significance normal changes in nature given conditions with whom this type I've encountered it many times before. If the body enters unusual , extreme circumstances , then modifications arise that are devoid of adaptive significance - morphoses .

If acting on larvae or pupae fruit flies X-ray or ultraviolet rays, as well as the maximum tolerated temperature, then developing flies exhibit a variety of morphoses ( flies with wings curled upward, with notches on the wings, with spread wings, with small wings, phenotypically indistinguishable from flies of several mutant lines of Drosophila).

4. Unlike mutations, modifications reversible , i.e., the resulting change gradually disappears if the effect that caused it is eliminated. So, a person’s tan goes away when the skin stops being exposed to insolation, muscle volume decreases after stopping training, etc.

5. Unlike mutations, modifications are not inherited . This position has been most hotly debated throughout human history. Lamarck believed that any changes in the body can be inherited, acquired during life (Lamarckism). Even Darwin recognized the possibility of inheritance of some modification changes.

The first serious blow to the idea of ​​inheritance of acquired characteristics came from A. Weisman . For 22 generations, he cut off the tails of white mice and crossed them with each other. A total of 1,592 individuals were examined, and tail shortening was never found in newborn mice. The results of the experiment were published in 1913, but there was no particular need for it, since intentional injury to humans, made for ritual or “aesthetic” reasons - circumcision, ear piercing, mutilation of the feet, skull, etc., as is known, are also not inherited.

In the USSR in the 30-50s. erroneous theories have become widespread Lysenko about the inheritance of “acquired characteristics,” i.e., actually modifications. Many experiments carried out on different organisms have shown the non-heritability of modifications, and studies of this kind now represent only historical interest. In 1956-1970 F. Creek formulated the so-called "the central dogma of molecular biology" , according to which information transfer is possible only from DNA to proteins, but not in the opposite direction.