modification variability. Mechanism, meaning, examples. Modification Variability - Properties, Examples, and Meaning

Modification variability- this is a rather important property of organisms to adapt to the external environment. This is a complex of reactions that are an organism or an entire population to a change in environmental conditions. For example, under the sun, the skin more or less darkens in every person.

Modification variability and its properties

This property of organisms has some characteristics:

Modification variability affects only the phenotype (external features), but does not affect the genotype (individual set of genetic information).
It has a group character - if some environmental conditions affect a group of organisms, then all of its representatives experience the appearance of the same signs.
Reversibility - changes occur with the constant influence of certain factors. If the organism is transferred to other conditions or the influence of the factor is eliminated, then the phenotypic changes disappear.
Changes that have occurred under the influence of external factors are not inherited.

It should be noted that modification variability is of great importance for the process. The fact is that in nature those organisms survive that are most adapted to the conditions, especially with a sharp change in external factors. Combinatorial and far from fully provides the body with the ability to adapt.

Modification variability: examples

In nature, one can find countless examples of such changes in the body. Below are the most common ones.

When climbing mountains, where environmental conditions change, an increase in the number of red blood cells is observed in the blood of a person or animal, which ensures normal oxygen supply.
When exposed ultraviolet rays in the skin tissues, an increased release of pigments begins.
As a result of constant intense training muscle mass increases significantly. After the cessation of exercise, the body gradually loses its elasticity, the muscles decrease in size.
If a white Himalayan hare is moved to a temperate climate and the body area is shaved, the new coat will be gray in color.
If the trees already have fully blossomed leaves, and at night they will be affected subzero temperature, then in the morning you will notice a characteristic reddish tint.

In order to understand the nature of modification devices, it is necessary to consider other forms of variability.

Combinatorial variability

Such variability appears as a result during the fusion of gametes. Now consider an example: if the father of the child has dark hair, and the mother has blond hair and the Child can be born with green eyes and blond hair, or dark hair and blue eyes. It is these phenotypic changes in offspring that are provided by combinatorial variability.

Mutational variability

Changes occur when the body is exposed to mutagens of a chemical, physical or biological nature. Mutational variability in contrast to modification:

occurs spontaneously, and it is almost impossible to predict it;
causes changes in the genetic material;
mutational changes are persistent and are inherited;
mutations can be both benign and cause pathologies up to a lethal outcome;
they do not depend on environmental conditions;
occur in individual individuals;

As you can see, variability is a very complex process that affects both the genotype and phenotypic characteristics. It was thanks to modifications, combinations and mutations that organisms gradually changed, improving and adapting to changes.

Variation is the occurrence of individual differences. Based on the variability of organisms, a genetic diversity of forms appears, which, as a result of the action of natural selection, are transformed into new subspecies and species. There are modification variability, or phenotypic, and mutational, or genotypic.

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

Variability forms	Reasons for the appearance	Meaning	Examples
Non-hereditary modification (phenotypic)	A change in environmental conditions, as a result of which the organism changes within the norm of the reaction specified by the genotype	Adaptation - adaptation to given environmental conditions, survival, preservation of offspring	White cabbage in a hot climate does not form a head. Breeds of horses and cows brought to the mountains become stunted
Mutational	The influence of external and internal mutagenic factors, resulting in a change in genes and chromosomes	Material for natural and artificial selection, since mutations can be beneficial, harmful and indifferent, dominant and recessive	The appearance of polyploid forms in a plant population or in some animals (insects, fish) leads to their reproductive isolation and the formation of new species, genera - microevolution
Hereditary (genotypic) Combined	Occurs spontaneously within a population when crossing, when offspring have new combinations of genes	Distribution in a population of new hereditary changes that serve as material for selection	The appearance of pink flowers when crossing white-flowered and red-flowered primroses. When crossing white and gray rabbits, black offspring may appear
Hereditary (genotypic) Correlative (correlative)	Arises as a result of the properties of genes to influence the formation of not one, but two or more traits	The constancy of interrelated features, the integrity of the body as a system	Long-legged animals have a long neck. In table varieties of beets, the color of the root crop, petioles and leaf veins consistently changes.

Modification variability

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

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

The phenotype is formed as a result of interactions between the genotype and environmental factors. Phenotypic traits are not transmitted from parents to offspring, only the norm of reaction is inherited, that is, the nature of the response to changes in environmental conditions. At heterozygous organisms when environmental conditions change, various manifestations of this trait can be caused.

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

Genotypic variability

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

Mutation classification. Mutations can be combined, into groups - classified according to the nature of manifestation, in place or, according to the level of their occurrence.

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

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

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

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

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

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

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

Mutation properties:

1. Mutations occur suddenly, abruptly.
2. Mutations are hereditary, that is, they are persistently transmitted from generation to generation.
3. Mutations are not directed - any locus can mutate, causing changes in both minor and vital signs.
4. The same mutations can occur repeatedly.
5. According to their manifestation, mutations can be beneficial and harmful, dominant and recessive.

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

Combination variability

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

In breeding, the law of homologous series of hereditary variability, formulated by the Soviet scientist N. I. Vavilov, is of great importance. It says: inside different types and genera that are genetically close (i.e., having a common origin), similar series of hereditary variability are observed. Such a character of variability was found in many cereals (rice, wheat, oats, millet, etc.), in which the color and consistency of grain, cold resistance, and other qualities vary in a similar way. Knowing the nature of hereditary changes in some varieties, one can foresee similar changes in related species and, by acting on them with mutagens, cause similar beneficial changes in them, which greatly facilitates the production of economically valuable forms. Many examples of homological variability are also known in humans; for example, albinism (a defect in the synthesis of a dye by cells) was found in Europeans, blacks and Indians; among mammals - in rodents, carnivores, primates; short dark-skinned people - pygmies - are found in the tropical forests of equatorial Africa, the Philippine Islands and the jungles of the Malay Peninsula; some hereditary defects and deformities inherent in man are also noted in animals. Such animals are used as a model for studying similar defects in humans. For example, a cataract of the eye occurs in mice, rats, dogs, horses; hemophilia - in a mouse and a cat, diabetes - in a rat; congenital deafness - in guinea pigs, mice, dogs; cleft lip - in mice, dogs, pigs, etc. These hereditary defects are convincing confirmation of the law of homologous series of hereditary variability by N. I. Vavilov.

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

Characteristic	Modification variability	Mutational variability
Object of change	Phenotype within normal limits	Genotype
Selecting factor	Changing environmental conditions environments	Change of conditions environment
Inheritance signs	Not inherited	Inherited
Susceptibility to chromosome changes	Not exposed	undergo chromosomal mutation
Susceptibility to changes in DNA molecules	Not exposed	Exposed in case gene mutation
Significance for an individual	Raises or lowers viability. productivity, adaptation	Helpful Changes lead to victory in the struggle for existence, harmful - to death
View value	Promotes survival	Leads to the formation of new populations, species, etc. as a result of divergence
Role in evolution	fixture organisms to environmental conditions	Material for natural selection
Shape of variability	Certain (group)	Indefinite (individual), combinative
Subordination of regularity	Statistical regularity variation series	Homological law series of hereditary variability

Each organism has the ability to adapt to environmental conditions - this is modification variability. It is thanks to modifications that the vital activity of living beings is possible.

Without the ability to adapt, the slightest change in temperature, nutrition, light, would put entire species on the brink of extinction.

What is modification (phenotypic) variability

Modification variability has developed as a result of evolution, as a reaction of the organism to changes in the conditions of existence.

A distinctive feature of modifications is that changes occur within the boundaries of the phenotype, i.e., a set of external and internal features organisms that appeared in the course of its development. Therefore, in the literature there is an equivalent name - phenotypic variability.

Impact on a living cell invariably leads to a response. Responding 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. Nevertheless, the changes that occur in the phenotype have a limit, which is called the reaction rate.

Depending on the extent to which one or another sign of the phenotype changes, the following reaction norms are distinguished in biology:

Wide- the trait is characterized by a high degree of variability. Most often it manifests itself in a quantitative value.
narrow- under the influence of the environment, the sign changes slightly and usually has a qualitative character.

Options for the development of modification of the organism, arranged in ascending or descending order, form a variational series. The relationship between the trait of the phenotype and the frequency of its manifestation clearly reflects the graph in the form of a curve.

These statistical methods are needed in important areas of human activity: agriculture, medicine, industry. The variation curve allows you to identify patterns of phenotypic variability, the limits of reaction norms, and predict the values of indicators.

Examples of modification variability

Modification changes in the body are a response to changes in the conditions of existence.

Diet, ambient temperature, humidity and light levels - these and many other factors depend on appearance organism, the behavior of its cells.

Examples of differences in phenotype are available at every turn - a dandelion grown in the 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 coat of some animals changes.

Phenotypic changes can also be observed in humans. Most a prime example serves as a tan that occurs in the form of a protective reaction to exposure to ultraviolet radiation.

In northern countries, dark skin color is a temporary phenomenon, which indicates the adaptive nature of this modification. Frequent physical activity also leads to a change in the 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 conditions of rarefied air, the human body, trying to maintain vital activity, increases the level of red blood cells in the blood, which deliver oxygen to organs and tissues.

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

Properties of modification variability

Modificational 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 the modification variability.

Usually the modifications do not last long and disappear when you return 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 modificational variability

Mutation, like modification, leads to a change in the organism, but this happens due to changes in the hereditary material, by rearranging genes, chromosomes, and the genome.

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

Exposure to radiation, chemicals, change temperature regime – common causes the 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 in the course of natural selection only carriers of useful changes continue their genus, which provide them with a competitive advantage.

Modifications and their characteristics

Changes in the 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:

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

Its useful to note: such a division is rather arbitrary, since the phenotype is often formed as a set of all changes.

Medical significance of phenotypic variability

Like all living things, man is subject to modification changes. Knowledge of the laws of this process and the limits of the reaction norm is important for medicine, whose activity is aimed at ensuring the healthy development of the human body.

The analysis of variation series and curves makes it possible to characterize the normal state of health, as well as to identify the values at which there is a deviation from the norm.

Distinguish age, seasonal and environmental modifications. They come down to changing only the degree of expression of the trait; violation of the structure of the genotype does not occur with them. It should be noted that it is impossible to draw a clear boundary between age, seasonal and ecological modifications.

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

Ontogenetic variability, like ontogeny itself, is determined by the genotype, where the development program of the individual is encoded. However, the features of the formation of the phenotype in ontogeny are due to 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 of traits (for example, a change in coat color, the appearance of a down in animals), which occurs as a result of seasonal changes in climatic conditions [Kaminskaya E.A.].

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

The Soviet biologist Ilyin showed that ambient temperature is more important in the development of pigment in the ermine rabbit, and for each area of the body there is a temperature threshold, above which white hair grows, and below - black (Fig. 9).

Fig 9.

ermine rabbit (from Ilyin according to S.M. Gershenzon, 1983)

Seasonal modifications can be attributed to the group environmental modifications. The latter are adaptive changes in the phenotype in response to changes in environmental conditions. Ecological modifications are phenotypically manifested in a change in the degree of expression of a trait. They can appear early in development and persist throughout life. An example is the various forms of the leaf of the arrowhead, due to the influence of the environment (Fig. 10): swept-back surface, wide floating, ribbon-shaped underwater.

Rice. ten.

underwater, floating and surface

Environmental modifications affect quantitative (number of petals in a flower, offspring of animals, weight of animals, plant height, leaf size, etc.) and qualitative (flower color in lungwort, forest rank, primrose; human skin color under the influence of ultraviolet rays, etc.). ) signs. So, for example, Levakovsky, when growing a blackberry branch in water until it blooms, found significant changes in the anatomical structure of its tissue. In a similar experiment, Constantin revealed phenotypic differences in the structure of the surface and underwater parts of the leaf in buttercup (Fig. 11).

Rice. eleven.

A - immersed in water;

B - surface

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 plains and high in the mountains. The first plant reached normal height, and the second turned out to be dwarfed. Such changes occur in animals as well. 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 by them with a change of generations, provided that changes in the external environment can manifest themselves. 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 crooked legs due to rickets has quite normal offspring. If, however, conditions do not change over a number of generations, the degree of expression of the trait in the offspring is preserved, it is often mistaken for a persistent hereditary trait (long-term modifications).

With the intensive action of many agents, non-heritable 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 investigated the effects of many chemical compounds on Drosophila, showing that, for example, antimony compounds are brown (brown eyes); arsenic acid and some other compounds - changes in wings, body pigmentation; boron compounds - eyeless (eyelessness), aristopredia (turning aristas into legs), silver compounds - yellow (yellow body), etc. At the same time, some morphoses, when exposed to a certain stage of development, were induced with high frequency(up to 100%).

Characteristics of modification variability:

1. Adaptive changes (example, arrowhead).

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

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

4. Modifications are adequate, i.e. the degree of manifestation of the symptom is directly dependent on the type and duration of the action of a particular factor. Thus, improving the maintenance of livestock contributes to an increase in the 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 is due to the fact that the same factor causes approximately the same change in individuals that are genotypically similar.

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

7. Rate of reactions (modification limit). It is the reaction rate, and not the modifications themselves, that are inherited, i.e. the ability to develop one or another trait is inherited, and the form of its manifestation depends on the conditions of the external environment. The reaction rate is a specific quantitative and quality characteristics genotype, i.e. a certain combination of genes in the genotype and the nature of their interaction.

Property	Non-hereditary (adaptive modifications)	hereditary
Object of change	Phenotype in the reaction range
Occurrence factor	Changes in environmental conditions	Gene recombination due to gamete fusion, crossing over, mutation
Property Inheritance	Not inherited	Inherited
Values for an individual	Increases vitality, adaptability to environmental conditions	Beneficial changes lead to survival, harmful - to the death of the organism.
View value	Promotes survival	Leads to the emergence of new populations, species as a result of divergence
Role in evolution	Adaptation of organisms to environmental conditions	Material for natural selection
Shape of variability	group	Individual
regularity	Statistical regularity of variation series	Law of homologous series of hereditary variability

Rice. 12. ? Comparative characteristics of hereditary and

non-hereditary variability

Examples of modification variability

In a person:

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 change in the Colorado potato beetle due to lasting influence on their pupae of high or low temperatures.

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

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

In plants:

The different structure of underwater and surface leaves of the water buttercup, arrowhead, etc.

Development of undersized 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 the processing of this carbohydrate).

Modification (phenotypic) variability- changes in the body associated with a change in the phenotype due to the influence of the environment and, in most cases, are adaptive in nature. The genotype does not change. Generally modern concept"adaptive modifications" corresponds to the concept of "certain variability" introduced into science by Charles Darwin.

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Subtitles

Conditional classification of modification variability

According to the changing signs of the body:
- morphological changes
- physiological and biochemical adaptations - homeostasis (increase in the level of red blood cells in the mountains, etc.)
By the range of the reaction norm
- narrow (more characteristic of qualitative traits)
- wide (more typical for quantitative traits)
By value:
- modifications (beneficial to the body - appear as an adaptive reaction to environmental conditions)
- morphoses (non-hereditary changes in the phenotype under the influence of extreme environmental factors or modifications that occur as an expression of newly emerging mutations that do not have an adaptive nature)
- phenocopies (various non-hereditary changes that copy the manifestation of various mutations)
By duration:
- there is only an individual or group of individuals that have been influenced by the environment (not inherited)
- long-term modifications - last for two or three generations

The mechanism of modification variability

Environment as a reason for modifications

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

Under the influence of certain environmental conditions on the body, the course of enzymatic reactions (enzyme activity) changes and specialized enzymes can be synthesized, 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 correspond to specific environmental factors.

Melanin is produced by four genes located on different chromosomes. Nai large quantity dominant alleles of these genes - 8 - is found in people of the Negroid race. When exposed to a specific environment, such as intense exposure to ultraviolet rays, epidermal cells are destroyed, which leads to the release of endothelin-1 and eicosanoids. They cause the activation of the tyrosinase enzyme and its biosynthesis. Tyrosinase, in turn, catalyzes the oxidation of the amino acid tyrosine. Further formation of melanin takes place without the participation of enzymes, however, a larger amount of the enzyme causes more intense pigmentation.

reaction rate

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

However, some quantitative traits are characterized by a narrow reaction rate (fat content of milk, number of toes in guinea pigs), and for some qualitative characters - wide (for example, seasonal color changes in many animal species of northern latitudes). In addition, the boundary between quantitative and qualitative features 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 - a variation series - a series of modification variability of an organism's property, which consists of individual modifications placed in order of increasing or decreasing quantitative expression of the property (leaf size, change in coat color intensity, etc.). A single indicator of the ratio of 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 vary greatly in the number of ears and spikelets due to different soil indicators and moisture in the field. Compiling the number of spikelets in one spike and the number of ears, you can get a variation series in a statistical form:

Variation curve

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

When constructing a variation curve, you can calculate the value of the standard deviation and, on the basis of this, build a graph of the standard deviation from the median - the most common feature value.

Modification variability in the theory of evolution

Darwinism

In 1859, Charles Darwin published his work on the subject of evolution entitled The Origin of Species by Natural Selection, or the Maintenance 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 it leaves offspring and its 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 a change 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 on the genotype, mutations can be selected, 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 high percentage of melanin pigment in the skin of the Negroid and Mongoloid races compared to the Caucasoid.

Darwin called modification variability definite (group).

A certain variability is manifested in all normal individuals of the species, subjected to a certain influence. A certain variability expands the limits of the existence and reproduction of the 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 non-hereditary variation. It is stabilizing, moving and disruptive selection.

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

Disruptive selection is characterized by the discovery of reserves with neutralized mutations and the subsequent selection of these mutations to form a new genotype and phenotype that are suitable for the environment. As a result, organisms with an extreme rate of reaction survive. For example, insects with strong wings are more resistant 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 develop resistance 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 influence of environmental conditions on its ontogeny (epigenetic system). At the same time, a stable developmental trajectory based on morphoses (creod) is formed - a stable epigenetic system is formed that is adaptive to morphoses. This system of development is based on the genetic assimilation of organisms (modification genocopy), which consists in the correspondence of any modification to a certain mutation. That is, it 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. Under the action of a new environment on the body, mutations are selected that adapt the body to new conditions, so the body, first adapting to the environment with the help of modifications, then becomes adapted to it genetically (motor selection) - a new genotype arises, on the basis of which a new phenotype. For example, with congenital underdevelopment of the motor apparatus of animals, a restructuring of the supporting and motor organs occurs in such a way that underdevelopment turns out to be adaptive. Further, this trait is fixed by hereditarily stabilizing selection. Subsequently, a new mechanism of behavior appears, aimed at adapting to adaptation. Thus, in the epigenetic theory of evolution, postembryonic morphosis based on special environmental conditions is considered as the driving lever of evolution. Thus, natural selection in the epigenetic theory of evolution consists of the following stages:

Thus, the 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 genocopy of modifications if the original mutations did not determine this ) theories.

Forms of modification variability

In most cases, modification variability contributes to a positive adaptation of organisms to environmental conditions - the reaction of the genotype to the environment improves and a rearrangement of the phenotype occurs (for example, the number of erythrocytes in a person who climbs mountains increases). However, sometimes, under the influence of adverse environmental factors, for example, the influence of teratogenic factors on pregnant women, phenotype changes occur that are similar to mutations (not hereditary changes similar to hereditary ones) - phenocopies. Also, under the influence of extreme environmental factors, morphoses may appear in organisms (for example, a disorder of the motor apparatus due to injury). Morphoses are irreversible and non-adaptive in nature, and in the 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 and is only a reaction of the genotype of a given individual to environmental conditions, followed by a change in the phenotype. However, long-term modifications are also known, which have been 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 bound to 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. With 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 bacterial cytoplasm, lactose (an inducer substance) attaches to the repressor protein, changing its conformation, which leads to dissociation of the repressor from the operator. This causes the beginning of the synthesis of an enzyme for the breakdown of lactose.

In bacteria, when dividing, an 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 (lactase) activity for the breakdown of lactose in sticks 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 state can be inherited by subsequent generations of bacteria. Molecular triggers can provide long-term modifications in eukaryotes as well. This can occur, for example, by cytoplasmic inheritance of cytoplasmic inclusions in bacteria during their reproduction.

The effect of trigger switching can be observed in non-cellular life forms, such as bacteriophages. When introduced into a cell, bacteria with a lack of nutrients they remain inactive, invading 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 the forms of variability

Property	Non-hereditary (adaptive modifications)	hereditary
Object of change	Phenotype in the reaction range	Genotype
Occurrence factor	Changes in environmental conditions	Gene recombination due to gamete fusion, crossing over, mutation
Property Inheritance	Not inherited	Inherited
Values for an individual	Increases vitality, adaptability to environmental conditions	Beneficial changes lead to survival, harmful - to the death of the organism.
View value	Promotes survival	Leads to the emergence of new populations, species as a result of divergence
Role in evolution	Adaptation of organisms to environmental conditions	Material for natural selection
Shape of variability	group	Individual
regularity	Statistical regularity of variation series	Law of homologous series of hereditary variability

Together, hereditary and modificational variation provide the basis for natural selection. At the same time, qualitative or quantitative changes in the manifestations of the genotype in the signs 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 the patterns of modification variability is of great importance in plant growing and animal husbandry, as it makes it possible to foresee and plan in advance the maximum use of the capabilities of each plant variety and animal breed (for example, individual indicators of a sufficient amount of light for each plant). Creation of known optimal conditions for the implementation of the genotype ensures their high productivity.

It 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, even at school age, are trying to determine the inclinations of children and their abilities for one or another professional activity, increasing the level of implementation of genetically determined children's abilities.