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Every organism has the ability to adapt to conditions environment- this is modification variability. It is thanks to modifications that the life of living beings is possible.

Without the ability to adapt, the slightest changes in temperature, nutrition, and light would bring entire species to the brink of extinction.

What is modification (phenotypic) variability

Modification variability has developed as a result of evolution, as the body’s reaction to changes in living conditions.

A distinctive feature of modifications is that changes occur within the boundaries of the phenotype, i.e., a set of external and internal characteristics of the organism that appeared during its development.

Therefore, in the literature there is an equivalent name - phenotypic variability.

Impact on a living cell invariably leads to a response. In response to an external stimulus, cells send signals to genes, which leads to changes in the synthesis of proteins responsible for the physiology of the body. However, the changes that occur in the phenotype have a limit, which is called the reaction norm.

1. Depending on the degree to which one or another characteristic of the phenotype changes, the following reaction norms are distinguished in biology: Wide
2. – the trait is characterized by a high degree of variability. Most often it manifests itself in quantitative terms. Narrow

– under the influence of the environment, the characteristic changes slightly and is usually of a qualitative nature.

Options for the development of modification of the organism, arranged in ascending or descending order, form a variation series. The relationship between a phenotype trait and the frequency of its manifestation is clearly reflected in a graph in the form of a curve. These statistical methods are necessary in important areas of human activity: agriculture

, medicine, industry. The variation curve allows us to identify patterns of phenotypic variability, boundaries of reaction norms, and predict the values ​​of indicators.

Examples of modification variability

Modification changes in the body are a response to changes in living conditions. Diet, ambient temperature, humidity and light levels - these and many other factors depend on appearance

Examples of phenotypic differences are available at every step - a dandelion grown in a field differs from a dandelion grown in the mountains in stem height, leaf arrangement, and development of the root system.

Another example is that plants of the same species will vary in size depending on the level of light and the amount of nutrients in the soil. Depending on the temperature, the color of the fur of some animals changes.

Phenotypic changes can also be observed in humans. The most striking example is tanning, which occurs in the form of defensive reaction to exposure to ultraviolet radiation.

Residents northern countries dark skin color is a temporary phenomenon, which indicates the adaptive nature of this modification. Frequent physical exercise also lead to a change in phenotype - the muscles and bones of the body are strengthened.

Not all modification changes are manifested externally; sometimes they occur only at the cellular level. In rarefied air conditions, the human body, in an effort to maintain vital functions, increases the level of red blood cells, which deliver oxygen to organs and tissues.

This phenomenon is observed when climbing mountains. Therefore, climbers pay Special attention adaptation to dramatically changed environmental conditions.

Properties of modification variability

Modifying heredity is not inherited. Its manifestations are temporary. There is no change in the genotype - the genes that are passed on to descendants are not affected.

Individuals of the same species placed in the same conditions will have similar changes in the phenotype, which indicates the group nature of modification variability.

Usually modifications do not persist for long and disappear when returning to the original conditions. These signs determine the regularity and predictability of changes.

Is modification variability beneficial or harmful? Here the answer is simple - modifications help the body adapt to changing environmental conditions, and therefore survive.

The difference between mutational and modification variability

Mutation, like modification, leads to a change in the body, but this happens due to changes in the hereditary material, through the rearrangement of genes, chromosomes, and genome.

An individual subjected to mutations remains so until the end of its life, and subsequently passes on the gene with the mutation to its descendants.

Exposure to radiation chemical substances, temperature change – common reasons occurrence of mutations. Their appearance is spontaneous - under the influence of the same factor, the signs that appear are likely to be different.

At the same time, mutation is the most important engine of evolution, since during natural selection Only the carriers of useful changes that provide them with a competitive advantage continue their lineage.

Modifications and their characteristics

Changes in phenotype occur for various reasons, and the degree of their manifestation depends on the intensity of exposure to environmental factors.

The types of modification variability can be classified as follows:

1. Age- changes occur as a result life cycle body. They are especially pronounced in organisms that undergo metamorphoses during development - amphibians spend part of their lives in the form of tadpoles, insects - in the form of larvae, and only then do they take on the appearance of an adult.
2. Seasonal changes are closely related to changes in temperature. So, for example, in winter, some animals' coat color changes - these are hair pigments that react to the cold.
3. Environmental– arise in response to changing environmental conditions. Modifications of this type can persist throughout the life of the organism if exposure to the factors that caused the change in phenotype continues.

It is worth noting: such a division is quite arbitrary, since the phenotype is often formed as a combination of all changes.

Medical significance of phenotypic variability

Like all living things, humans are subject to modification. Knowledge of the laws of this process and the limits of reaction norms is important for medicine, whose activities are aimed at ensuring healthy development human body.

Analysis of variation series and curves allows us to characterize normal condition health, as well as identify the values ​​at which deviations from the norm occur.

In humans: an increase in the level of red blood cells when climbing mountains; increased skin pigmentation with intense exposure to ultraviolet rays; development of the musculoskeletal system as a result of training; scars (an example of morphosis)

In insects and other animals: color changes in the Colorado potato beetle due to prolonged exposure to high or low temperatures on their pupae; change in fur color in some mammals when weather conditions change (for example, a hare); different colors of nymphalid butterflies (for example, Araschnia levana) that developed at different temperatures

In plants: different structures of underwater and above-water leaves in water buttercup, arrowhead, etc.; development of low-growing forms from seeds of lowland plants grown in the mountains

In bacteria: the work of the genes of the lactose operon of Escherichia coli (in the absence of glucose and in the presence of lactose, they synthesize enzymes for processing this carbohydrate)

Mutational variability

Mutational called variability caused by the occurrence of mutation. Mutations- these are heritable changes in genetic material that lead to changes in certain characteristics of the organism.

The main provisions of the mutation theory were developed by G. De Vries in 1901-1903. and boil down to the following:

• · Mutations occur suddenly as discrete changes in characteristics;
• · New forms are stable;
• · Unlike non-hereditary changes, mutations do not form continuous series. They represent qualitative changes;
• · Mutations manifest themselves in different ways and can be both beneficial and harmful;
• · The probability of detecting mutations depends on the number of individuals studied;
• · Similar mutations may occur repeatedly;
• · Mutations are undirected (spontaneous), that is, any part of the chromosome can mutate, causing changes in both minor and vital signs.

By the nature of genome changes There are several types of mutations - genomic, chromosomal and gene.

Genomic mutations (aneuploidy and polyploidy) is a change in the number of chromosomes in the cell's genome.

Chromosomal mutations, or chromosomal rearrangements, are expressed in changes in the structure of chromosomes, which can be identified and studied under a light microscope. Known perestroikas different types(normal chromosome -- ABCDEFG):

• · deficiency, or deficiency, is the loss of the terminal sections of a chromosome;
• · deletions - loss of a section of a chromosome in its middle part (ABEFG);
• · duplications - two- or multiple repetitions of a set of genes localized in a certain region of the chromosome (ABCDECDEFG);
• · inversions - rotation of a chromosome section by 180° (ABEDCFG);
• · translocation - transfer of a section to the other end of the same chromosome or to another, non-homologous chromosome (ABFGCDE).

During deficiencies, divisions and duplications, the amount of genetic material on chromosomes changes. The degree of phenotypic change depends on how large the corresponding chromosome regions are and whether they contain important genes. Examples of chromosomal rearrangements are known in many organisms, including humans. The severe hereditary disease “cry of the cat” syndrome (named after the nature of the sounds made by sick babies) is caused by heterozygosity for deficiency in the 5th chromosome. This syndrome is accompanied by mental retardation. Children with this syndrome usually die early.

Duplications play significant role in the evolution of the genome, since they can serve as material for the emergence of new genes, since different mutation processes can occur in each of two previously identical sections.

During inversions and translocations, the total amount of genetic material remains the same, only its location changes. Such mutations also play a role significant role in evolution, since crossing mutants with the original forms is difficult, and their F 1 hybrids are most often sterile. Therefore, only crossing the original forms with each other is possible here. If such mutants have a favorable phenotype, they can become the initial forms for the emergence of new species. In humans, all of these mutations lead to pathological conditions.

Modification (phenotypic) variability- changes in the body associated with changes in phenotype due to environmental influences and, in most cases, of an adaptive nature. The genotype does not change. Generally modern concept“adaptive modifications” corresponds to the concept of “definite variability”, which was introduced into science by Charles Darwin.

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Conditional classification of modification variability

• According to changing signs of the body:
  • morphological changes
  • physiological and biochemical adaptations - homeostasis (increased levels of red blood cells in the mountains, etc.)
• According to the range of the reaction norm
  • narrow (more typical for qualitative traits)
  • broad (more typical for quantitative traits)
• By value:
  • modifications (useful for the body - manifested as an adaptive response to environmental conditions)
  • morphoses (non-hereditary changes in phenotype under the influence of extreme environmental factors or modifications that arise as an expression of newly emerged mutations that do not have an adaptive nature)
  • phenocopies (various non-hereditary changes that copy the manifestation of various mutations)
• By duration:
  • exists only in an individual or group of individuals that have been influenced by the environment (not inherited)
  • long-term modifications - persist for two or three generations

Mechanism of modification variability

Environment as a reason for modifications

Modifying variability is the result not of changes in the genotype, but of its response to environmental conditions. With modification variability, the hereditary material does not change, but the expression of genes changes.

Under the influence of certain environmental conditions on the body, the course of enzymatic reactions (enzyme activity) changes and the synthesis of specialized enzymes can occur, some of which (MAP kinase, etc.) are responsible for the regulation of gene transcription, depending on changes in the environment. Thus, environmental factors are able to regulate gene expression, that is, the intensity of their production of specific proteins, the functions of which respond to specific environmental factors.

Four genes, located on different chromosomes, are responsible for the production of melanin. Nai large quantity dominant alleles of these genes - 8 - are found in people of the Negroid race. When exposed to a specific environment, for example, intense exposure to ultraviolet rays, epidermal cells are destroyed, which leads to the release of endothelin-1 and eicosanoids. They cause activation of the enzyme tyrosinase and its biosynthesis. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further education melanin occurs without the participation of enzymes, but a larger amount of enzyme causes more intense pigmentation.

Reaction rate

The limit of manifestation of modification variability of an organism with an unchanged genotype is the norm of reaction. The reaction rate is determined by the genotype and varies among different individuals of a given species. In fact, the reaction norm is a spectrum possible levels gene expression, from which the level of expression most suitable for given environmental conditions is selected. The reaction norm has limits or boundaries for each biological species (lower and upper) - for example, increased feeding will lead to an increase in the weight of the animal, but it will be within the reaction norm characteristic of a given species or breed. The reaction rate is genetically determined and inherited. For different traits, the reaction norm limits vary greatly. For example, wide limits of the reaction norm are the value of milk yield, cereal productivity and many other quantitative characteristics, narrow limits are the color intensity of most animals and many other qualitative characteristics.

However, some quantitative traits are characterized by a narrow reaction norm (milk fat content, number of toes guinea pigs), and for some qualitative characteristics it is wide (for example, seasonal color changes in many animal species of northern latitudes). In addition, the boundary between quantitative and qualitative characteristics is sometimes very arbitrary.

Characteristics of modification variability

• reversibility - changes disappear when the specific environmental conditions that provoked them change
• group character
• changes in the phenotype are not inherited, the norm of the genotype reaction is inherited
• statistical regularity of variation series
• affects the phenotype without affecting the genotype itself.

Analysis and patterns of modification variability

Variation series

A ranked display of the manifestation of modification variability is a variation series - a series of modification variability of a property of an organism, which consists of individual modifications arranged in order of increasing or decreasing quantitative expression of the property (leaf size, change in the intensity of coat color, etc.). A single indicator of the relationship between two factors in a variation series (for example, the length of the coat and the intensity of its pigmentation) is called variant. For example, wheat growing in one field can differ greatly in the number of ears and spikelets due to different soil conditions and moisture content in the field. By compiling the number of spikelets in one ear and the number of ears of corn, we can obtain a variation series in statistical form:

Variation curve

A graphical display of the manifestation of modification variability - a variation curve - displays both the range of variation of a property and the frequency of individual variants. The curve shows that the most common are the average variants of manifestation of the trait (Quetelet’s law). The reason for this, apparently, is the effect of environmental factors on the course of ontogenesis. Some factors suppress gene expression, while others, on the contrary, enhance it. Almost always, these factors, while simultaneously acting on ontogeny, neutralize each other, that is, neither a decrease nor an increase in the value of the trait is observed. This is the reason why individuals with extreme expressions of the trait are found in significantly smaller numbers than individuals with average size. For example, average height men - 175 cm - most common in European populations.

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 most common value of the attribute.

Modification variability in the theory of evolution

Darwinism

In 1859, Charles Darwin published his work on the topic of evolution entitled “The Origin of Species by Natural Selection, or the Preservation of Favored Races in the Struggle for Life.” In it, Darwin showed the gradual development of organisms as a result of natural selection. Natural selection consists of the following mechanism:

• first, an individual appears with new, completely random properties (formed as a result of mutations)
• then she is or is not able to leave offspring, depending on these properties
• finally, if the outcome of the previous stage is positive, then she leaves offspring and her descendants inherit the newly acquired properties

New properties of an individual are formed as a result of hereditary and modification variability. And if hereditary variability is characterized by changes in the genotype and these changes are inherited, then with modification variability the ability of the genotype of organisms to change the phenotype when exposed to the environment is inherited. With constant exposure to the same environmental conditions, mutations can be selected on the genotype, whose effect is similar to the manifestation of modifications, and, thus, modification variability turns into hereditary variability (genetic assimilation of modifications). An example is the constant large percentage of melanin pigment in the skin of black and Mongoloid race compared to Caucasoid.

Darwin called modification variability definite (group).

A certain variability is manifested in all normal individuals of a species exposed to a certain influence. A certain variability expands the limits of existence and reproduction of an organism.

Natural selection and modification variability

Modification variability is closely related to natural selection. Natural selection has four directions, three of which are directly aimed at the survival of organisms with in different forms non-hereditary variability. This is stabilizing, driving and disruptive selection.

Stabilizing selection is characterized by the neutralization of mutations and the formation of a reserve of these mutations, which determines the development of the genotype with a constant phenotype. As a result, organisms with an average reaction rate dominate in constant conditions of existence. For example, generative plants maintain a flower shape and size that matches the shape and size of the insect that pollinates the plant.

Disruptive selection is characterized by the opening of reserves with neutralized mutations and the subsequent selection of these mutations for the formation of new genotypes and phenotypes that are suitable for the environment. As a result, organisms with extreme reaction rates survive. For example, insects with strong wings have greater resistance to gusts of wind, while insects of the same species with weak wings are blown away.

Driving selection is characterized by the same mechanism as disruptive selection, but it is aimed at the formation of a new average reaction norm. For example, insects become resistant to chemicals.

Epigenetic theory of evolution

According to the main provisions of the epigenetic theory of evolution, published in 1987, the substrate for evolution is a holistic phenotype - that is, morphoses in the development of an organism are determined by the impact of environmental conditions on its ontogenesis (epigenetic system). At the same time, a stable development trajectory is formed, based on morphoses (creod) - a stable epigenetic system is formed, adaptive to morphoses. This development system is based on the genetic assimilation of organisms (modification gene copying), which consists of conforming to any modification of a specific mutation. That is, this means that a change in the activity of a particular gene can be caused by both a change in the environment and a certain mutation. When a new environment acts on an organism, mutations are selected that adapt the organism to new conditions, therefore the organism, first adapting to the environment through modifications, will then become adapted to it genetically (motor selection) - a new genotype arises, on the basis of which a new one arises phenotype. For example, with congenital underdevelopment musculoskeletal system In animals, a restructuring of the supporting and motor organs occurs in such a way that the underdevelopment turns out to be adaptive. This trait is further fixed by hereditary stabilizing selection. Subsequently, a new mechanism of behavior arises aimed at adapting to adaptation. Thus, the epigenetic theory of evolution considers postembryonic morphosis based on special conditions environment as a driving lever of evolution. Thus, natural selection in the epigenetic theory of evolution consists of the following stages:

Thus, synthetic and epigenetic theories of evolution are quite different. However, there may be cases that are a synthesis of these theories - for example, the appearance of morphoses due to the accumulation of neutral mutations in reserves is part of the mechanism of both synthetic (mutations appear in the phenotype) and epigenetic (morphoses can lead to genecopying modifications if the initial mutations did not determine this ) theories.

Forms of modification variability

In most cases, modification variability contributes to the positive adaptation of organisms to environmental conditions - the response of the genotype to the environment improves and a restructuring of the phenotype occurs (for example, the number of red blood cells increases in a person who has climbed the mountains). However, sometimes, under the influence of unfavorable environmental factors, for example, the influence of teratogenic factors in pregnant women, changes in the phenotype occur that are similar to mutations (non-hereditary changes, similar to hereditary ones) - phenocopies. Also, under the influence of extreme environmental factors, organisms may develop morphoses (for example, a disorder of the musculoskeletal system due to injury). Morphoses are irreversible and non-adaptive in nature, and in their labile nature the manifestations are similar to spontaneous mutations. Morphoses are accepted by the epigenetic theory of evolution as the main factor in evolution.

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, long-term modifications are also known, described in some bacteria, protozoa and multicellular eukaryotes. To understand the possible mechanism of long-term modification variability, let us first consider the concept of a genetic trigger.

For example, bacterial operons contain, in addition to structural genes, two sections - a promoter and an operator. The operator of some operons is located between the promoter and structural genes (in others it is part of the promoter). If the operator is associated with a protein called a repressor, then together they prevent RNA polymerase from moving along the DNA chain. In bacteria E. coli a similar mechanism can be observed. When there is a lack of lactose and an excess of glucose, a repressor protein (Lacl) is produced, which attaches to the operator, preventing RNA polymerase from synthesizing mRNA for translation of the enzyme that breaks down lactose. However, when lactose enters the cytoplasm of the bacterium, lactose (an inducer substance) attaches to the repressor protein, changing its conformation, which leads to the dissociation of the repressor from the operator. This causes the beginning of the synthesis of an enzyme to break down lactose.

In bacteria, when dividing, the inductor substance (in the case of E. coli- lactose) is transferred to the cytoplasm of the daughter cell and triggers the dissociation of the repressor protein from the operator, which entails the manifestation of enzyme activity (lactase) to break down lactose in rods even in the absence of this disaccharide in the medium.

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), they form a system called a trigger. When the first operon is active, the second one is disabled. However, under the influence of the environment, the synthesis of the repressor protein by the first operon can be blocked, and then the trigger switches: the second operon becomes active. This trigger condition can be inherited by subsequent generations of bacteria. Molecular triggers can provide long-lasting modifications in eukaryotes as well. This can occur, for example, through the cytoplasmic inheritance of cytoplasmic inclusions in bacteria during their reproduction.

The trigger switching effect can be observed in non-cellular life forms, such as bacteriophages. When a bacteria enters a cell due to a lack of nutrients, it remains inactive, incorporating itself into the genetic material. When favorable conditions in the cell, phages multiply and break out of the bacterium - a trigger switch occurs due to a change in the environment.

Cytoplasmic inheritance

Comparative characteristics of forms of variability

Together, hereditary and modificational variability provide the basis for natural selection. In this case, qualitative or quantitative changes in the manifestations of the genotype in the characteristics of the phenotype (hereditary variability) determine the result of natural selection - the survival or death of the individual.

Modification variability in human life

Practical use patterns of modification variability has great importance in crop production and animal husbandry, as it allows one to anticipate 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). The creation of known optimal conditions for the implementation of the genotype ensures their high productivity.

This also makes it possible to efficiently use innate abilities child 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 activity, increasing within the normal reaction level the level of realization of genetically determined abilities of children.

We know that modification variability is a special case of 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 the 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 to one 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 , which this species has encountered 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 impact 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. Crick 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.

Non-hereditary (phenotypic) variability is not associated with changes in genetic material. It is the body's response to specific environmental changes. The study of the influence of new conditions on humans showed that such characteristics as the type of metabolism, predisposition to certain diseases, blood type, skin patterns on the fingers and others are determined by the genotype and their expression depends little on environmental factors. Other characteristics, such as intelligence level, weight, height, etc., have a wide range of changes, and their manifestation is largely determined by the environment. Those external differences that are caused by the environment are called modifications. Modifications are not associated with changes in the genetic structures of an individual, but are only a partial reaction of the genotype to specific changes in the environment (temperature, oxygen content in the inhaled air, nature of nutrition, upbringing, training, etc.). However, the extent of these changes in a trait in response to environmental influences is determined by the genotype. Specific changes are not inherited; they are formed during the life of an individual. The genotype with its specific norm of reaction to environmental changes is inherited. Thus, the set of characteristics of an individual (its phenotype) is the result of the implementation of genetic information in specific environmental conditions. The phenotype is formed in the process individual development starting from the moment of fertilization. A person’s physical, mental and mental health is the result of the interaction of a person’s inherited characteristics with environmental factors that affect him throughout his life. Neither heredity nor surrounding a person the environment is not immutable. This important principle underlies the modern understanding of the processes of Variation and Heredity. It is impossible to find two people in the world, with the exception of identical twins (developed from the same fertilized egg), who have the same set of genes. It is also impossible to find two people who lived their lives in the same conditions. Heredity and environment are not opposed to each other: they are one and unthinkable without the other.

Modification variability

Among various types variability discussed above, non-hereditary variability was identified, which is also called modification. The general patterns of variability are much less known than the laws of inheritance.

Modifying variability is phenotypic differences in genetically identical individuals.

External influences can cause changes in an individual or group of individuals that are harmful, indifferent or beneficial for them, i.e. adapted.

As is known, the evolutionary theory developed by J.B. Lamarck (1744-1829), was based on the erroneous postulate about the inheritance of changes acquired during life, i.e. about inheritance of modification. The very performance of J.B. Lamarck's account of the evolution of organic forms was undoubtedly progressive for its time, but his explanation of the mechanism of evolutionary progress was incorrect and reflected a common misconception characteristic of 18th-century biologists.

Charles Darwin (1809-1882) in his “Origin of Species...” divided variability into certain And uncertain. This classification generally corresponds to the current division of variability into non-hereditary and hereditary.

One of the first researchers to study modification variability was K. Naegeli (1865), who reported that if alpine forms of plants, for example hawkweeds, are transferred to the rich soil of the Munich Botanical Garden, then they exhibit an increase in power, abundant flowering, and some plants change beyond recognition. If the forms are transferred again to poor rocky soils, they return to their original form. Despite the results obtained, K. Naegeli remained a supporter of the inheritance of acquired properties.

For the first time, a strict quantitative approach to the study of modification variability from the standpoint of genetics was used by V. Johansen. He studied the inheritance of the mass and size of bean seeds - traits that vary significantly under the influence of both genetic factors and plant growing conditions.

A. Weisman (1833-1914) was a staunch opponent of the inheritance of properties acquired in ontogenesis. Consistently defending the Darwinian principle of natural selection as the driving force of evolution, he proposed to separate the concepts somatogenic And blastogenic changes, i.e. changes in the properties of somatic cells and organs, on the one hand, and changes in the properties of generative cells, on the other. A. Weisman pointed out the impossibility of the existence of a mechanism that would transmit changes in somatic cells to reproductive cells in such a way that in the next generation the organisms change adequately to the modifications that the parents underwent during their ontogenesis.

Illustrating this point, A. Weisman conducted the following experiment, which proved the non-inheritance of acquired characteristics. For 22 generations, he cut off the tails of white mice and crossed them with each other. In total, he examined 1,592 individuals and never found tail shortening in newborn mice.

Types of modification variability

Distinguish age, seasonal And environmental modifications. They come down to changing only the degree of expression of the trait; There is no disruption of the genotype structure. It should be noted that it is impossible to draw a clear line between age-related, seasonal and environmental modifications.

Age , or ontogenetic, modifications are expressed in the form of a constant change in characteristics during the development of an individual. This is clearly demonstrated by the example of the ontogeny of amphibians (tadpoles, young of the year, adults), insects (larva, pupa, adult) and other animals, as well as plants. In humans, modifications of morphophysiological and mental characteristics are observed during development. For example, a child will not be able to develop correctly both physically and intellectually if normal external, including social, factors do not influence him in early childhood. For example, long stay a child in a socially disadvantaged environment can cause an irreversible defect in his intelligence.

Ontogenetic variability, like ontogenesis itself, is determined by the genotype, where the development program of the individual is encoded. However, the peculiarities of the formation of the phenotype in ontogenesis are determined by the interaction of the genotype and the environment. Under the influence of unusual external factors, deviations in the formation of a normal phenotype may occur.

Seasonal modifications , individuals or entire populations manifest themselves in the form of a genetically determined change in characteristics (for example, a change in coat color, the appearance of down in animals), occurring as a result of seasonal changes in climatic conditions [Kaminskaya E.A.].

A striking example of such variability is the experience with the ermine rabbit. A certain area of ​​the ermine rabbit's back is shaved bald (the back of the ermine rabbit is normally covered with white hair) and then the rabbit is placed in the cold. It turns out that in this case, darkly pigmented hair appears on a bare area exposed to low temperature and, as a result, on the back - dark spot. It is obvious that the development of one or another characteristic of a rabbit is its phenotype, V in this case ermine coloring depends not only on its genotype, but also on the entire set of conditions in which this development occurs.

Soviet biologist Ilyin showed that environmental temperature is more important in the development of pigment in the ermine rabbit, and each area of ​​the body has its own temperature threshold, above which white wool grows, and below which black wool grows (Fig. 1).

Fig 1. Map of temperature thresholds for fur pigmentation in the ermine rabbit (from Ilyin according to S.M. Gershenzon, 1983)

Seasonal modifications can be classified as environmental modifications. The latter represent adaptive changes in phenotype in response to changes in environmental conditions. Ecological modifications are phenotypically manifested in changes in the degree of expression of a trait. They can occur in the early stages of development and persist throughout life. An example would be various shapes leaves of the arrowhead, due to the influence of the environment: arrow-shaped surface, wide floating, ribbon-shaped underwater.

An arrowhead plant that produces three types of leaves: submerged, floating and emergent. Photo: Udo Schmidt

Environmental modifications affect quantitative (number of petals in a flower, offspring in animals, weight of animals, plant height, leaf size, etc.) and qualitative (color of flowers in lungwort, woodland, primrose; skin color in humans under the influence of ultraviolet rays, etc. ) signs. For example, Levakovsky, when growing a blackberry branch in water until it blossomed, discovered significant changes in anatomical structure her fabrics. In a similar experiment, Constantin revealed phenotypic differences in the structure of the above-water and underwater parts of the buttercup leaf.

Rice. Water buttercup leaves and a frog:) Photo: Radio Tonreg

In 1895, the French botanist G. Bonnier conducted an experiment that became a classic example of ecological modification. He divided one dandelion plant into two parts and grew them in different conditions: on the plain and high in the mountains. The first plant reached normal height, but the second one turned out to be dwarf. These kinds of changes also occur in animals. For example, R. Wolterk in 1909 observed changes in the height of the helmet in daphnia depending on feeding conditions.

Ecological modifications, as a rule, are reversible with a change of generations, subject to changes external environment may appear. For example, the offspring of low-growing plants on well-fertilized soils will be of normal height; a certain number of petals in the flower of a plant may not be repeated in the offspring; A person with bow legs due to rickets has completely normal offspring. If conditions do not change over a number of generations, the degree of expression of the trait in the offspring is preserved, and it is often mistaken for a persistent hereditary trait (long-term modifications).

With the intensive action of many agents, non-inherited changes are observed, random (in their manifestation) in relation to the effect. Such changes are called morphoses. Very often they resemble the phenotypic manifestation of known mutations. Then they are called phenocopies these mutations. In the late 30s - early 40s I.A. Rapoport studied the effects on Drosophila of many chemical compounds, showing that, for example, antimony compounds are brown (brown eyes); arsenous acid and some other compounds - changes in wings, body pigmentation; boron compounds – eyeless (eyeless), aristopredia (transformation of aristas into legs), silver compounds – yellow (yellow body), etc. Moreover, some morphoses, when exposed to a certain stage of development, were induced with a high frequency (up to 100%).

Characteristics of modification variability:

1. Adaptive changes (example, arrow leaf).

2. Adaptive nature. This means that in response to changing environmental conditions, individuals exhibit phenotypic changes that contribute to their survival. An example is the change in moisture content in the leaves of plants in arid and humid areas, the color of a chameleon, and the shape of a leaf in an arrowhead, depending on environmental conditions.

3. Reversibility within one generation, i.e. with changes in external conditions in adult individuals, the degree of expression of certain signs changes. For example, in cattle, depending on the conditions of detention, the milk yield and fat content of milk may fluctuate, in chickens - egg production).

4. Modifications are adequate, i.e. The degree of severity of a symptom is directly dependent on the type and duration of action of a particular factor. Thus, improving livestock management helps to increase live weight of animals, fertility, milk yield and fat content of milk; on fertilized soils at optimal climatic conditions the productivity of grain crops increases, etc.

5. Mass character. Mass distribution is determined by the fact that the same factor causes approximately the same change in individuals that are genotypically similar.

6. Long modifications. They were first described in 1913 by our compatriot V. Yollos. By irritating the ciliates of the shoes, he caused them to develop a series of morphological features, which persisted for large number generations, as long as reproduction was asexual. When development conditions change, long-term modifications are not inherited. Therefore, the opinion that education and external influence can fix a new trait in the offspring is erroneous. For example, it was assumed that well-trained animals produce offspring with better “acting” characteristics than those from untrained animals. The offspring of trained animals are indeed easier to educate, but this is explained by the fact that they do not inherit the skills acquired by the parents, but the ability to train, due to the inherited type of nervous activity.

7. Norm of reactions (limit of modification). It is the reaction norm, and not the modifications themselves, that are inherited, i.e. the ability to develop a particular trait is inherited, and the form of its manifestation depends on environmental conditions. The reaction norm is a specific quantitative and qualitative characteristics of the genotype, i.e. a certain combination of genes in the genotype and the nature of their interaction.

Table. Comparative characteristics of hereditary and non-hereditary variability

Examples of modification variability

In humans:

Increase in the level of red blood cells when climbing mountains

Increased skin pigmentation with intense exposure to ultraviolet rays.

Development of the musculoskeletal system as a result of training

Scars (an example of morphosis).

In insects and other animals:

Changes in coloration in the Colorado potato beetle due to prolonged exposure to high or low temperatures on their pupae.

Changes in fur color in some mammals when weather conditions change (for example, a hare).

Different colors of nymphalid butterflies (for example, Araschnia levana) that developed at different temperatures.

In plants:

Different structures of underwater and above-water leaves of water buttercup, arrowhead, etc.

Development of low-growing forms from seeds of lowland plants grown in the mountains.

In bacteria:

The work of the genes of the lactose operon of Escherichia coli (in the absence of glucose and in the presence of lactose, they synthesize enzymes for processing this carbohydrate).