The reason for modification variability is. Modification variability. Mechanism, meaning, examples

Variability, its types. Characteristics of modification variability, examples

The variability of organisms is manifested in the diversity of individuals (of the same species, breed or variety), differing from each other in a set of characteristics, properties and qualities. The reasons for this may be different. In some cases, these differences (with identical genotypes in organisms) are determined by the environmental conditions in which the development of individuals occurs. In others, the differences are due to different genotypes of organisms. Based on this, two types of variability are distinguished: non-hereditary(modification, phenotypic) And hereditary(genotypic).

Modification (phenotypic) variability lies in the fact that under the influence of different conditions external environment In organisms of the same species, genotypically identical, changes in characteristics (phenotype) are observed. These changes are individual and are not inherited, that is, they are not transmitted to individuals of subsequent generations. Let us consider the manifestation of such a pattern using several examples.

In one of the experiments, a dandelion rhizome was cut lengthwise with a sharp razor and the halves were planted in different conditions- in the lowlands and in the mountains. By the end of the season, these seedlings had completely grown similar friend on each other's plants. The first of them (in the lowland) was tall, with large leaves and a large flower. The second, grown in the mountains, in harsh conditions, turned out to be short, with small leaves and flowers (Fig. 1).

The genotype of these two plants is absolutely identical (after all, they grew from halves of the same rhizome), but their phenotypes differed significantly as a result of different growing conditions. The descendants of these two plants, grown under the same conditions, were no different from each other. Therefore, phenotypic changes are not inherited.

Rice. 1.Changes in dandelion under the influence of external environmental conditions (according to Bonnier): à - plant grown in lowlands; á - in the mountains; both plants are cuttings of the same individual

The biological significance of modification variability is to ensure the individual adaptability of the organism to different conditions external environment.

Let's look at another example. Let's imagine that two brothers, identical twins (that is, with identical genotypes), chose different hobbies in childhood: one devoted himself to weightlifting, and the other to playing the violin. Obviously, in ten years there will be a significant physical difference between them. And it is also clear that the athlete’s newborn son will not be born with “athletic” characteristics.

Changes in the phenotype under the influence of environmental conditions can not occur indefinitely, but only within a limited range (wide or narrow), which is determined by the genotype. The range within which a characteristic can vary is called reaction norms. So, for example, the characteristics of cows that are taken into account in animal husbandry - milk yield (i.e., the amount of milk produced) and milk fat content - can change, but within different limits. Depending on the conditions of keeping and feeding the animals, milk yield varies significantly (from glasses to several buckets per day). IN in this case talk about wide normal reaction. But the fat content of milk fluctuates very slightly depending on the conditions of detention (only by hundredths of a percent), i.e. this characteristic is characterized narrow reaction norm.

So, environmental conditions determine changes in the trait within the reaction norm. The boundaries of the latter are dictated by the genotype. Consequently, changes in the reaction norm itself can only occur as a result of a change in the genotype (i.e., as a result of genotypic variability).

2.49. Combinative variability and its mechanism

Combinative variability has two main components; 1) random, equally probable divergence of chromosomes in meiosis (it ensures the recombination of parental chromosomes and serves as a cytological substantiation of the law of free combination formulated by G. Mendel) and 2) recombination of linked genes localized in homologous chromosomes. In a narrower sense, recombination means the recombination of genes, and therefore the prerequisite for it, in particular, and for combinative variability in general, is the heterozygosity of the organism for one or more genes. This heterozygosity, and therefore recombination, occurs in eu- and prokaryotes in different ways: for their implementation in prokaryotes there is conjugation, transformation and transduction, as well as joint infection (in viruses). In eukaryotes, heterozygosity is ensured by the diploidity of the genome, and recombination itself can occur in both germ and somatic cells. The result of recombination is ultimately the transfer of DNA sections from one molecule to another. In the case of reciprocal recombination, this transfer is mutual, and in non-reciprocal recombination, it is one-sided.

There are two approaches to studying the process recombination. The first of them, the classic one, analyzes the inheritance of traits and, if traits tend to be inherited together, estimates the degree of their linkage, or the frequency of recombination between the corresponding loci. This approach arose in the “home-lecular” time and represents statistical analysis observed divergence of traits during their transmission to subsequent generations. The second approach to studying genetic recombination, molecular, is aimed at analyzing the subtle mechanisms of this process. Although both approaches focus on the same process, the concept of genetic recombination itself is ambiguous.

There are three types recombination:
general(occurs between homologous DNA sequences; this is recombination between homologous chromatids in meiosis, less often in mitosis);
site-specific(affects DNA molecules characterized by limited structural similarity, and is observed during the integration of the phage genome and the bacterial chromosome);
illegal(occurs during transposition not based on homology of DNA sequences).

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 has shown 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 level of intelligence, weight, height, etc., have a wide range of changes, and their manifestation is largely determined 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 modern understanding processes of variability 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 was not highlighted hereditary variability, which is also called modification. General patterns variability is known much less well 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, 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 1592 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 early childhood it will not be influenced by normal external, including social, factors. For example, a child's long stay 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 specific area on the back of an ermine rabbit is shaved bald (the back of an 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, a dark spot on the back. It is obvious that the development of one or another characteristic of a rabbit is its phenotype, in this case the ermine coloration, 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 is the different leaf shapes of the arrowhead, determined by the influence of the environment: arrow-shaped above-water, 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 the anatomical structure of its tissue. 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 classic example 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, and the second 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.

Environmental modifications, as a rule, are reversible with a change of generations, subject to changes in the external environment that may occur. 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, at the large cattle depending on the conditions of detention, the milk yield and fat content of milk may fluctuate, and 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 under optimal climatic conditions, the yield 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, it is a mistaken opinion that education and external influence a new trait can be fixed in the offspring. 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 train, 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 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:

Change in color 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).

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

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Subtitles

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.

Subject to certain conditions environment on organism the course of enzymatic processes changes reactions(activity enzymes) and can happen synthesis specialized enzymes, some of which ( MAP kinase etc.) are responsible for the regulation transcriptions genes depending on changes environment. Thus, factors environment capable of regulating expression genes, that is, the intensity of their production of specific proteins , functions which meet specific factors environment.

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 humans 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, however, 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 reaction norm. The reaction norm is determined genotype and varies among different individuals of a given species. In fact, the reaction norm is a spectrum of possible gene expression levels, from which the expression level 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 normal reaction range 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 rate (milk fat content, the number of toes in guinea pigs), while some qualitative traits are characterized by a wide reaction rate (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 wool and the intensity of its pigmentation) is called option. 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 evolutionary topics entitled “The Origin of Species by Natural Selection, or the Preservation of Favorable 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. When a genotype is continually exposed to the same environmental conditions, mutations whose effect is similar to that of the modification may be selected for, 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 the Negroid and Mongoloid races compared to the Caucasoid race.

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 different forms of 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 basic principles 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 influence 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 teratogenic factors in pregnant women, phenotypic changes similar to mutations occur (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 They 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, in operons bacteria contain, in addition to structural genes, two sections - promoter And 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 bound to a protein called a repressor, then together they prevent movement RNA polymerase 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, e.g. bacteriophages. When bacteria are introduced into a cell due to a lack of nutrients, they remain inactive, incorporating themselves into the genetic material. When favorable conditions appear in the cell, phages multiply and break out of the bacterium - the trigger switches 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

The practical use of patterns of modification variability has great importance 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). The creation of known 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 activity, increasing within the normal reaction level the level of realization of genetically determined abilities of children.

There are two main types variability living organisms: hereditary and non-hereditary. The first can be mutational and combinative. The second one is called modification variability. It includes changes in characteristics that are not preserved during sexual reproduction, since these changes do not affect the genotype. She is also called phenotypic variability.

Modification variability arises as a result of the interaction of organisms with the environment, i.e. in the process of realizing genetic information. Different organisms react differently to environmental factors. There is such a thing as a reaction norm. These are the limits of modification variability, which are determined by the capabilities of a given genotype.

Characteristic feature modification is that the same impact causes the same change in all individuals that were exposed to it. For this reason, Charles Darwin called modification variability definite. Modifications are especially good to observe in individuals that are identical in genotype, but placed in different environmental conditions. Thus, significant differences in many traits appear in plants of the same species growing in mountainous and valley conditions. In the mountains, plants are usually squat, with short stems, basal leaves, and deep roots; in the valley the plants are higher, their root system located closer to the soil surface. When plants are moved to another habitat, the modifications disappear. Modifications of plants that occur under the influence of different lighting, sowing density, and changes in nutrition are well known.

Modifications in animals are no less varied. Changes in the physique of fish are known depending on the nature of the reservoir. For example, in lakes and slow rivers (i.e. in large bodies of water), crucian carp are larger and rounder. In ponds and small marshy lakes, the fish are much smaller and have an elongated body.

In chickens, egg production changes under the influence of daylight hours; in cattle and horses with large physical activity muscle volume increases, lung volume increases, blood circulation increases.

Of particular interest is modification variability in humans. To evaluate it, a very effective twin method. Studies performed on twins have demonstrated huge role heredity in the development of the organism. Identical twins brought up in different environments have striking physical and psychological similarities, although differences in upbringing, of course, leave an imprint on their intellectual abilities and behavior.

In most cases, the modification represents a beneficial adaptive response of the organism, i.e. is adaptive in nature. Plants growing in the shade have large leaf blades to maximize the capture of solar energy. In arid areas, plants, on the contrary, decrease leaf blade, the number of stomata decreases, the epidermis thickens, i.e. signs appear that protect plants from moisture loss.

The change in color in many insects, fish, and amphibians, depending on their habitat, either has a protective function or, conversely, helps to lie in wait for prey. A person has a tan - defensive reaction against insolation.

The adaptive nature is usually inherent in modifications that are caused by the influence of ordinary environmental factors. If the body comes under the influence of an unusual factor or the intensity of the usual one sharply increases, then non-adaptive modifications may occur, often having the nature of deformities. Such changes are called morphoses. They are often caused by chemicals and radiation. For example, when seeds are irradiated, they grow into seedlings with wrinkled leaves, cotyledons of different shapes, and uneven green color. Drosophila sometimes develop real monsters when irradiated.

In plants, morphoses often occur as a result of an excess or deficiency of a substance in the soil, most often a microelement. Thus, a lack of copper causes severe tillering in cereals. In this case, the inflorescences do not come out of the leaf wrappers and dry out. In fish fry developing in water mixed with lithium chloride, only one eye located in the middle is formed.

Some modifications that occur under the influence of radiation, extreme temperatures and other potent factors mimic specific mutations. Thus, under the influence of the temperature shock to which Drosophila pupae were exposed, flies with curved wings, notched wings, and short wings appeared, indistinguishable from flies of some mutant lines. Such modifications are called phenocopies.

The adaptive nature of modifications is due to the norm of the genotype reaction, which allows a trait to change without disturbing the structure of the corresponding gene (i.e., without mutation). The wider the reaction norm, the higher the adaptive potential of an individual, population or species.

Unlike mutations, modifications have varying degrees of persistence. Many modifications disappear soon after the factor that caused them (for example, tanning) ceases to act. Others may persist throughout the life of the individual. For example, people who suffered from rickets in childhood due to a lack of vitamin D may remain bow-legged for the rest of their lives.

Sometimes there is an aftereffect of modifications. Thus, in mammals, offspring carried by an exhausted mother are smaller and weaker than normal. However, this influence quickly disappears if the factor that caused the modification in the mother is eliminated.

It is very rare that modifications persist for several generations. This is observed only during vegetative or parthenogenetic propagation. Long-term modifications have been described in unicellular algae and protozoa. For example, resistance to increased concentrations of arsenic in the slipper ciliate persisted for 10.5 months, after which it decreased to the initial level. The mechanism of long-term modifications is not entirely clear.