Examples of modification variability. Combinatorial, mutational and modification variability

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

Modification variability - ability of organisms with the same genotype develop differently under different conditions environment. In a population of such organisms, a certain set of phenotypes. However, the organisms must be the same age.

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

Modification examples widely known and numerous.

The morphology of the leaves water buttercup And arrowhead depends on what environment, air or underwater, they develop.

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

If the aerial part of the stem potatoes artificially shut out the light, tubers hanging in the air develop on it.

At flounders , leading a benthic lifestyle, the upper side of the body is dark, which makes it invisible to approaching prey, and the lower side is light. But if the aquarium has a glass bottom and is illuminated not from above, but from below, then the lower surface of the body becomes dark.

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

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

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

Mod Properties

S. M. Gershenzon describes the following modification properties :

1. The degree of severity of the modification proportional to strength and duration effect on the body of a factor that causes modification. This regularity radically distinguishes modifications from mutations, especially gene ones.

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

3. Only those modifications that are caused by normal changes in nature these conditions , which this species has encountered many times before. If the body enters unusual , extreme circumstances , then there are modifications devoid of adaptive meaning - morphoses .

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

4. Unlike mutations, modifications reversible , i.e. the change that has arisen gradually disappears if the effect that caused it is eliminated. So, a person’s tan disappears when the skin ceases to be exposed to insolation, the volume of muscles decreases after the cessation of training, etc.

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

The first serious blow to the idea of ​​the inheritance of acquired traits came from A. Weisman . For 22 generations, he chopped off the tails of white mice and crossed them with each other. A total of 1,592 mice were examined, and no shortening of the tail was found in newborn mice. The results of the experiment were published in 1913, but there was no particular need for it, since intentional injury to humans, made for ritual or "aesthetic" reasons - circumcision, ear piercing, mutilation of the feet, skull, etc., are also known not to be inherited.

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

Change, 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), which differ from each other in a complex of signs, properties and qualities. The reasons for this may be different. In some cases, these differences (with the same genotypes in organisms) are determined by the environmental conditions in which the development of individuals occurs. In others, differences are due to unequal 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 environmental conditions in organisms of the same species, genotypically the same, a change in signs (phenotype) is observed. These changes are individual and are not inherited, that is, they are not transmitted to individuals of the next generations. Let us consider the manifestation of such a regularity on 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 lowland and in the mountains. By the end of the season, completely different plants grew from these seedlings. 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 undersized, with small leaves and a flower (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, did not differ from each other in any way. Therefore, phenotypic changes are not inherited.

Rice. 1.Dandelion change under the influence of external environmental conditions (according to Bonnier): a - a plant grown in a lowland; á - in the mountains; both plants are layers of one individual

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

Let's consider another example. Imagine that two brothers, identical twins (i.e., 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 an athlete will not have a newborn son with "athletic" characteristics.

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

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

2.49. Combinative variability and its mechanism

Combination variability has two main components; 1) random, equiprobable divergence of chromosomes in meiosis (it provides recombination of parental chromosomes and serves as a cytological justification for 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 a prerequisite for it, in particular, and for combinative variability in general, is the organism's heterozygosity for one or more genes. This heterozygosity, and hence recombination, occurs in eu- and prokaryotes in different ways: for their implementation in prokaryotes, there are conjugation, transformation and transduction, as well as joint infection (in viruses). In eukaryotes, heterozygosity is ensured by the diploidy of the genome, and recombination itself can occur both in sex and somatic cells. Recombination ultimately results in the transfer of DNA segments from one molecule to another. In the case of reciprocal recombination, this transfer is mutual, and in non-reciprocal recombination, it is unilateral.

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

Three types can be distinguished 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).

Variation in biology is the occurrence of individual differences between individuals of the same species. Due to variability, the population becomes heterogeneous, and the species has a better chance of adapting to changing environmental conditions.

In a science like biology, heredity and variation go hand in hand. There are two types of variability:

• Non-hereditary (modification, phenotypic).
• Hereditary (mutational, genotypic).

Non-hereditary variability

Modification variability in biology is the ability of a single living organism (phenotype) to adapt to environmental factors within its genotype. Due to this property, individuals adapt to changes in climate and other conditions of existence. underlies the adaptation processes occurring in any organism. So, in outbred animals, with the improvement of conditions of detention, productivity increases: milk yield, egg production, and so on. And the animals brought to the mountainous regions grow undersized and with a well-developed undercoat. Changes in environmental factors and cause variability. Examples of this process can be easily found in everyday life: human skin becomes dark under the influence of ultraviolet rays, muscles develop as a result of physical exertion, plants grown in shaded places and in the light have different leaf shapes, and hares change coat color in winter and summer.

For not hereditary variability are characterized by the following properties:

• group character of changes;
• not inherited by offspring;
• change in trait within the genotype;
• the ratio of the degree of change with the intensity of the impact of an external factor.

hereditary variability

In biology, hereditary or genotypic variability is the process by which the genome of an organism changes. Thanks to her, the individual acquires features that were previously unusual for her species. According to Darwin, genotypic variation is the main engine of evolution. There are the following types of hereditary variability:

• mutational;
• combinative.

Occurs as a result of the exchange of genes during sexual reproduction. At the same time, the traits of the parents are combined in different ways in a number of generations, increasing the diversity of organisms in the population. Combinative variability obeys the rules of Mendelian inheritance.

An example of such variability is inbreeding and outbreeding (closely related and unrelated crossing). When the traits of an individual producer want to be fixed in the breed of animals, then inbreeding is used. Thus, the offspring becomes more uniform and reinforces the qualities of the founder of the line. Inbreeding leads to the manifestation of recessive genes and can lead to the degeneration of the line. To increase the viability of the offspring, outbreeding is used - unrelated crossing. At the same time, the heterozygosity of the offspring increases and the diversity within the population increases, and, as a result, the resistance of individuals to the adverse effects of environmental factors increases.

Mutations, in turn, are divided into:

• genomic;
• chromosomal;
• genetic;
• cytoplasmic.

Changes affecting sex cells are inherited. Mutations in can be transmitted to offspring if the individual reproduces vegetatively (plants, fungi). Mutations can be beneficial, neutral or harmful.

Genomic mutations

Variation in biology through genomic mutations can be of two types:

• Polyploidy - a mutation often found in plants. It is caused by a multiple increase in the total number of chromosomes in the nucleus, is formed in the process of violation of their divergence to the poles of the cell during division. Polyploid hybrids are widely used in agriculture- in crop production there are more than 500 polyploids (onion, buckwheat, sugar beet, radish, mint, grapes and others).
• Aneuploidy is an increase or decrease in the number of chromosomes in individual pairs. This type of mutation is characterized by low viability of the individual. A widespread mutation in humans - one in the 21st pair - causes Down's syndrome.

Chromosomal mutations

Variability in biology by way appears when the structure of the chromosomes themselves changes: loss of the terminal section, repetition of a set of genes, rotation of a single fragment, transfer of a chromosome segment to another place or to another chromosome. Such mutations often occur under the influence of radiation and chemical pollution of the environment.

Gene mutations

A significant part of these mutations does not appear externally, as it is a recessive trait. Gene mutations are caused by a change in the sequence of nucleotides - individual genes - and lead to the appearance of protein molecules with new properties.

Gene mutations in humans cause the manifestation of some hereditary diseases - sickle cell anemia, hemophilia.

Cytoplasmic mutations

Cytoplasmic mutations are associated with changes in the structures of the cell cytoplasm containing DNA molecules. These are mitochondria and plastids. Such mutations are transmitted through the maternal line, since the zygote receives all the cytoplasm from the maternal egg. An example of a cytoplasmic mutation that has caused variability in biology is plant pinnateness, which is caused by changes in chloroplasts.

All mutations have the following properties:

• They appear suddenly.
• Passed down by inheritance.
• They don't have any direction. Mutations can be subjected to both an insignificant area and a vital sign.
• Occur in individuals, that is, individual.
• In their manifestation, mutations can be recessive or dominant.
• The same mutation can be repeated.

Each mutation is caused by specific causes. In most cases, it cannot be accurately determined. Under experimental conditions, to obtain mutations, a directed factor of the external environment is used - radiation exposure and the like.

Non-hereditary (phenotypic) variability is not associated with a change in genetic material. It is the body's response to specific changes in the environment. The study of the influence of new conditions on a person showed that such signs as the type of metabolism, predisposition to certain diseases, blood type, skin patterns on the fingers and others are determined by the genotype and their expression depends little on environmental factors. Other characteristics, such as intelligence, weight, height, etc., have a wide range of changes, and their manifestation is largely determined by the environment. Those external differences that are caused by the environment are called modifications. Modifications are not associated with a change in the genetic structures of an individual, but are only a particular reaction of the genotype to specific changes in the environment (temperature, oxygen content in the inhaled air, nutrition, upbringing, training, etc.). However, the limits of these trait changes in response to environmental influences are determined by the genotype. Specific changes are not inherited, they are formed during the life of an individual. The genotype is inherited with its specific rate of reaction to a change in the environment. Thus, the set of traits 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 of individual development, starting from the moment of fertilization. The physical, mental and mental health of a person is the result of the interaction of the characteristics inherited by a person with environmental factors that affect him throughout his life. Neither heredity nor the human environment is immutable. This important principle underlies modern understanding processes of variability and heredity. There are no two people in the world, except for identical twins (developing from the same fertilized egg) that have the same set of genes. It is also impossible to find two people who have lived their lives in the same conditions. Heredity and environment are not opposed to each other: they are one and inconceivable one without the other.

Modification variability

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

Modification variability is phenotypic differences in genetically identical individuals.

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

As you know, the evolutionary theory developed by J.B. Lamarck (1744-1829), was based on the erroneous postulate of the inheritance of changes acquired during life, i.e. about modification inheritance. In itself, the representation of J.B. Lamarck on the evolution of organic forms was undoubtedly progressive for his time, but his explanation of the mechanism of evolutionary progress was incorrect and reflected a common misunderstanding characteristic of eighteenth-century biologists.

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

One of the first researchers who studied modification variability was K. Naegeli (1865), who reported that if alpine forms of plants, such as hawksbill, are transferred to the rich soil of the Munich botanical garden, then they show an increase in power, abundant flowering, and some plants change beyond recognition. If the forms are again transferred to poor stony soils, then 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 rigorous quantitative approach to the study of modification variability from the standpoint of genetics was applied by V. Johansen. He studied the inheritance of the weight and size of bean seeds, traits that largely change 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 by sex in such a way that in next generation organisms changed adequately to those modifications that the parents underwent during their ontogeny.

Illustrating this situation, A. Weisman set up the following experiment, which proved the non-inheritance of acquired traits. 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 shortening of the tail 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; 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 example of 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 are manifested in the form of a genetically determined change in 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. The ermine rabbit is shaved on its back certain area(the back of an 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, a dark spot appears on the back. 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. 1).

Fig. 1. Map of temperature thresholds of wool pigmentation in the 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 would be various forms the leaf of the arrowhead, due to the influence of the environment: swept-back surface, wide floating, ribbon-shaped underwater.

An arrowhead plant that produces three types of leaves: underwater, floating and above water. Photo: Udo Schmidt

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.

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

In 1895, the French botanist G. Bonnier conducted an experiment that became a classic example of ecological modification. He divided one dandelion plant into two parts and grew them in different conditions: on the plain and high in the mountains. The first plant reached normal height, 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 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 on Drosophila of many chemical compounds, 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 a 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 the change in the moisture content in the leaves of plants in arid and humid regions, the color of a chameleon, the shape of a leaf in an arrowhead, depending on environmental conditions.

3. Reversibility within one generation, i.e. with a change in external conditions in adults, the degree of expression of certain signs changes. For example, in 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 with 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 generations until reproduction was asexual. When the conditions of development change, long-term modifications are not inherited. Therefore, the opinion is erroneous that education and external influence a new trait can be fixed 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 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

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

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 prolonged exposure to 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 variability - changes in the phenotype of the organism, which in most cases are adaptive in nature and are formed as a result of the interaction of the genotype with the environment. Changes in the body, or modifications, are not inherited. In general, the concept of "modification variability" corresponds to the concept of "variability determined", which was introduced by Darwin.

Conditional classification of modification variability

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

The mechanism of modification variability

Gene → protein → change in the organism's phenotype Environment

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

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

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

Another example is the seasonal change in fur color in animals (molting). Shedding and subsequent coloring are due to the action of temperature indicators on the pituitary gland, which stimulates the production of thyroid-stimulating hormone. This causes an effect on the thyroid gland, under the action of hormones of which molting occurs.

reaction rate

The reaction rate is the spectrum of gene expression with an unchanged genotype, from which the most appropriate level of activity of the genetic apparatus is selected, and forms a specific phenotype. For example, there is an allele of the X a gene, which causes the production of more ears of wheat, and an allele of the Y b gene, which produces a small number of ears of wheat. The expression of alleles of these genes is interrelated. The entire expression spectrum is located between the maximum expression of the a allele and the maximum expression of the b allele, and the intensity of the expression of these alleles depends on environmental conditions. Under favorable conditions (with a sufficient amount of moisture, nutrients), the allele "dominates" and under unfavorable conditions, the manifestation of the b allele predominates.

The reaction rate has a limit of manifestation for each species - for example, increased feeding of animals will cause an increase in its mass, however, it will be within the range of detection of this trait for a given species. The reaction rate is genetically determined and inherited. For various changes, there are different facets of the manifestation of the reaction norm. For example, the amount of milk yield, the productivity of cereals (quantitative changes) vary greatly, the color intensity of animals varies slightly, etc. (qualitative changes). In accordance with this, the reaction rate can be narrow (qualitative changes - the color of the pupae and adults of some butterflies) and wide (quantitative changes - the size of the leaves of plants, the size of the body of insects, depending on the nutrition of their pupae. However, for some quantitative changes a narrow reaction rate is characteristic (fat content of milk, number of toes in porpoises), and for some qualitative changes it is wide ( seasonal changes colors in animals of northern latitudes). In general, the reaction rate and the intensity of gene expression based on it predetermine the dissimilarity of intraspecific units.

Characteristics of modification variability

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

Analysis and patterns of modification variability

Displays of the manifestation of modification variability are ranked - a variation series - a series of modification variability of an organism's property, consisting of individual interconnected properties of the organism's phenotype, arranged in ascending or descending order of the quantitative expression of the property (leaf size, changes in fur color intensity, etc.). A single indicator of the ratio of two factors in a variation series (for example, the length of the fur and the intensity of its pigmentation) is called a variant. For example, wheat growing in one field can vary greatly in the number of spikelets and ears due to different soil parameters. Comparing the number of spikelets in one spikelet and the number of ears, you can get the following variation series:

Variation curve

A graphical representation of the manifestation of modification variability - a variation curve - reflects both the range of power variation and the frequency of occurrence of individual variants.

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

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

Graph of the standard deviation, built on the basis of the variation curve "modification variability of wheat"

Forms of modification variability

Phenocopies

Phenocopies - changes in the phenotype under the influence of adverse environmental factors, similar to mutations. The genotype does not change. Their causes are teratogens - certain physical, chemical (drugs, etc.) and biological agents (viruses) with the occurrence of morphological anomalies and malformations. Phenocopies often look like hereditary diseases. Sometimes phenocopies originate from embryonic development. But more often examples of phenocopies are changes in ontogeny - the spectrum of phenocopies depends on the stage of development of the organism.

morphoses

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

Long-term modification variability

Most modifications are not inherited and are only a reaction of the genotype to environmental conditions. Of course, the offspring of an individual that has been exposed to certain factors that have formed a wider reaction rate can also have the same wide changes, but they will only appear when exposed to certain factors, which, by acting on genes that cause more intense enzymatic reactions. However, in some protozoa, bacteria, and even eukaryotes, there is a so-called long-term modification variability due to cytoplasmic heredity. To elucidate the mechanism of long-term modification variability, let us first consider the regulation of the trigger by environmental factors.

Trigger regulation by modifications

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

The effect of switching triggers can be observed in some non-cellular life forms, such as bacteriophages, and in prokaryotes, such as Escherichia coli. Let's consider both cases.

colibacillus - a set of species of bacteria that interact with certain organisms with a common benefit (mutualism). They have a high enzymatic activity against sugars (lactose, glucose), moreover, they cannot simultaneously break down glucose and lactose. The regulation of the ability to cleave lactose is performed by the lactose operon, which consists of a promoter, operator, and terminator, as well as a gene encoding a repressor protein for the promoter. In the absence of lactose in the environment, the repressor protein binds to the operator and transcription stops. If lactose enters a bacterial cell, it combines with the repressor protein, changes its conformation, and dissociates the repressor protein from the operator.

Bacteriophages are viruses that infect bacteria. When entering a bacterial cell, under adverse environmental conditions, bacteriophages remain inactive, penetrating into the genetic material and being transferred to daughter cells during the binary separation of the mother cell. When favorable conditions appear in the bacterial cell, the trigger switches to the bacteriophage as a result of the ingestion of nutrients-inducers, and the bacteriophages multiply and break out of the bacterium.

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

Cytoplasmic inheritance

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

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

Also, cytoplasmic inheritance associated with long-term modification variability found in eukaryotic representatives such as the Colorado potato beetle and Habrobracon wasps. Under the action of intense thermal indicators in the pupae of the Colorado potato beetle, the color of the beetles changed. Under the obligatory condition that the female beetle also experienced the effects of intense thermal indicators, in the descendants of such beetles the present manifestation of the trait persisted for several generations, and then the previous norm of the trait returned. This continued modification variability is also an example of cytoplasmic inheritance. The reason for inheritance is the autoreproduction of those parts of the cytoplasm that have undergone changes. Let us consider the mechanism of autoreproduction as the cause of cytoplasmic heredity in detail. In the cytoplasm, organelles that have their own DNA and RNA and other plasmogens can self-reproduce. Organelles that are able to self-reproduce are mitochondria and plastids that are capable of self-duplication and protein biosynthesis through replication and the stages of transcription, processing and translation. Thus, the continuity of the autoreproduction of these organelles is ensured. Plasmogenes are also capable of self-reproduction. If, under the influence of the environment, the plasmogen has undergone changes that determined the activity of this gene, for example, during the dissociation of a repressor protein or associations encoding a protein, then it begins to produce a protein that forms a certain trait. Since plasmogens are able to be transported across the membrane of female eggs and thus inherited, their specific state is also inherited. At the same time, the modifications that the gene caused by activating its own expression are also preserved. If the factor that caused the activation of gene expression and protein biosynthesis by it is preserved during ontogenesis to the offspring of the individual, then the trait will be transmitted to the next offspring. Thus, a long-term modification persists as long as there is a factor that causes this modification. With the disappearance of the factor, the modification slowly fades away over several generations. This is where long-term modifications differ from regular modifications.

Modification variability and theories of evolution

Natural selection and its influence on modification variability

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

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

driving selection. This form of selection leads to: a) the disclosure of mobilization reserves, consisting of neutralized mutations, b) the selection of neutralized mutations and their compounds, and c) the formation of a new phenotype and genotype. As a result of this selection, organisms with a new average reaction rate dominate, which is more in line with the changing environmental conditions in which they live.

Disruptive selection. This form of selection brings about the same processes as in motive selection, but it is not aimed at the formation of a new average reaction rate, but at the survival of organisms with extreme reaction rates.

sexual selection. This form of selection results in facilitating the encounter between the sexes, limiting the participation in the reproduction of the species of individuals with less developed sexual characteristics.

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

Darwinism and Neo-Darwinism

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

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

2) Then she is able or unable to leave descendants;

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

Epigenetic theory of evolution

The epigenetic theory of evolution considers the phenotype as a substratum of natural selection, and selection not only fixes beneficial changes, but also takes part in their creation. The main influence on heredity is not the genome, but the epigenetic system - a set of factors acting on ontogeny. With morphosis, which is one of the types of modification variability, a stable developmental trajectory (creod) is formed in an individual - an epigenetic system that adapts to morphosis. This system of development is based on the genetic assimilation of organisms, which consists in the modification of a certain mutation - a modification gene copy, due to an epigenetic change in the structure of chromatin. This means that a change in gene activity can be the result of both mutations and environmental factors. Those. on the basis of a certain modification under the intense influence of the environment, mutations are selected that adapt the body to new changes. This is how a new genotype is formed, which forms a new phenotype. Natural selection, according to et, consists of the following stages:

1) Extreme environmental factors lead to morphosis;

2) morphosis leads to destabilization of ontogeny;

3) Destabilization of ontogeny leads to the appearance of an abnormal phenotype, which most closely matches the morphosis;

4) With a successful match of the new phenotype, the modifications are copied, which leads to stabilization - a new reaction norm is formed;

Comparative characteristics of hereditary and non-hereditary variability

Comparative characteristics of the forms of variability

Property	Non-hereditary (modification)	hereditary
Object of change	Phenotype within normal limits	Genotype
Occurrence factor	Changes in environmental conditions	Gene recombination resulting from gamete fusion, crossing over, and mutation
trait inheritance	Not inherited (reaction rate only)	Inherited
Significance for an individual	Adapt to environmental conditions, improve vitality	Beneficial changes lead to survival, harmful changes lead to death.
View value	Promotes survival	Leads to the emergence of new populations, species as a result of divergence
Role in evolution	Adaptation of organisms	Material for natural selection
Shape of variability	group	Individual, combined
regularity	Statistical (variation series)	Law of homologous series of hereditary variability

Modification variability in human life

Man, in general, has long used the knowledge of modification variability, for example, in the economy. With knowledge of certain individual characteristics of each plant (for example, the need for light, water, temperature conditions), it is possible to plan maximum level use (within the reaction norm) of this plant - to achieve the highest fruitfulness. That's why different types people place plants for their formation in different conditions - in different seasons etc. The situation is similar with animals - knowledge of the need, for example, cows causes increased production of milk and, as a result, an increase in milk yield.

Since the functional asymmetry of the cerebral hemispheres is formed with the achievement of a certain age, and in illiterate uneducated people it is less, it can be assumed that the asymmetry is a consequence of modification variability. Therefore, at the stages of training, it is very advisable to identify the child's abilities in order to most fully realize its phenotype.

Examples of modification variability

• In insects and animals
• An increase in red blood cells when climbing mountains in animals (homeostasis)
  • Increased skin pigmentation with intense exposure to ultraviolet radiation
  • The development of the motor apparatus as a result of training
  • Scars (morphosis)
  • Change in coloration of Colorado potato beetles with prolonged exposure to high or low temperatures on their pupae
  • Changing the color of the fur in some animals with changing weather conditions
  • The ability of butterflies from the genus Vanessa (Vanessa) to change their color with changes in temperature
• In plants
  • The different structure of the underwater and emersed leaves in water ranunculus plants
  • Development of undersized forms from seeds of lowland plants grown in the mountains

• In bacteria
  • work of the genes of the lactose operon of Escherichia coli