Hereditary and non-hereditary variability. Hereditary variability
LECTURE
SUBJECT:Heredity and variability
LECTURE PLAN:
Non-hereditary variability
Heredity
Variability
1. Heredity
Development organic world, largely depends on factors such as heredity and variability.Heredity called general property All organisms store and transmit their characteristics to their offspring. Thanks to heredity, the specific qualities of each biological species are preserved from generation to generation.
The connection between parents and offspring in organisms occurs mainly through reproduction. Although the offspring are like parents and ancestors, they are not theirs. an exact copy. The mechanism of heredity has long interested humanity. In 1866 G. Mendel expressed the opinion that the characteristics of organisms are determined by heritable units, which he called “elements”. Later they began to be calledhereditary factors and finallygenes . Genes are located on chromosomes and they are passed on from one generation to the next.
Despite the fact that much is now known about chromosomes and the structure of DNA, give precise definition gene is still difficult. As a result of studying the nature of a gene, it can be defined as a unit of recombination, mutation and function.Gene is a factor of heredity, a functionally indivisible unit of genetic material in the form of a section of a nucleic acid molecule (DNA or RNA).It encodes a specific protein structure, t-RNA or r-RNA molecules, or interacts with biologically active substances (for example, enzymes). A gene is an integral functional unit, and any violation of its structure changes the information encoded in it or leads to its loss.
As a result of heredity, an organism receives a set of genes from its parents, which is commonly calledgenotype . The genome of eukaryotes is more complex than that of prokaryotes because it has a larger amount of nuclear DNA, structural and regulatory genes. In addition to the hereditary material located in the nucleus, there is alsocytoplasmic inheritance , orextra-nuclear . It lies in the ability of certain cytoplasmic structures to store and transmit to descendants part of the hereditary information of the parents. Although the leading role in the inheritance of most traits of an organism belongs to nuclear genes, the role of cytoplasmic inheritance is also significant. It is associated with two types of geneticphenomena:
Inheritance of traits that are encodedextranuclear genes located in certain organelles (mitochondria, plastids);
The manifestation in descendants of traits predetermined by nuclear genes, the formation of which is influenced byegg cytoplasm .
The existence of genes in organelles (mitochondria, plastids) capable of self-duplication became known at the beginning of the twentieth century. while studying green and colorless plastids in some flowering plants with mosaic leaf colors. Extranuclear genes, interacting with nuclear ones, influence the formation of the trait. For example, cytoplasmic heredity associated with plastid genes affects such a trait as variegation in plants (begonia, snapdragon, etc.). And this trait is transmitted through the maternal line.
The reason for variegation is the loss of the ability of some plastids to form the pigment chlorophyll. After cell division with colorless plastids, white spots appear in the leaves, which alternate with green areas. The transmission of this trait through the maternal line is explained by the fact that during the formation of gametes, plastids reach the eggs, and not the sperm. When new plastids are formed, green plastids give rise to green ones, and colorless ones give rise to colorless ones. During cell division, plastids are distributed randomly, resulting in cells with colorless, green, or both plastids..
The phenomenon of cytoplasmic inheritance associated with mitochondrial genes can be observed in yeast. In these microorganisms, genes were found in the mitochondria that determine the absence or presence of respiratory enzymes, as well as resistance to certain antibiotics. The influence of the nuclear genes of the mother's body through the cytoplasm of the egg on the formation of characteristics can be traced using the example of the pond snail. This freshwater mollusk has forms with different directions of twisting of the shell - left or right. The allele that determines the twisting of the shell to the right dominates the left-handed allele, but this trait is determined by the genes of the maternal individual. For example, individuals homozygous for a recessive gene (left-handed) may have a right-handed shell if the maternal organism had the dominant allele.
2. Variability
Variabilityname the entire set of differences in one or another characteristic between organisms that belong to the same population or species. There are two main forms of variability:hereditaryAndnon-hereditary.
2.1. Hereditary variability
Hereditary variability is the variability that is transmitted from parents to offspring, i.e. inherited. This variability is associated with changes in genetic material caused by mutations. That is why hereditary variability is also calledgenotypic , genetic ormutational .
Mutation is a change in chromosomes that occurs under the influence of environmental factors. The concept of mutations was introduced into science by the Dutch botanist Hugo Frieze. He also formulatedmutation theory , a number of provisions of which belong to the famous Russian botanist S.I. Korzhinsky.
Basic provisions of modern mutation theory :
Mutations occur suddenly, spasmodically and appear in the form of discrete symptoms;
Mutations are not lost and are passed on from generation to generation;
Mutations manifest themselves in different ways and can be dominant or recessive, beneficial or harmful, differ in the strength of their effect on the body, cause minor changes in the functioning of the body or affect vital signs and be lethal;
The probability of detecting mutations depends on the number of individuals examined;
Mutations can occur repeatedly;
Mutations can be caused by the influence of potent physical or chemical agents on the body, but the appearance of a particular mutation is not related to the type of agent;
Mutations are always spontaneous, independent of one another, and do not have a group orientation. Any part of a chromosome can mutate.
Mutational variability, in contrast to modification, is an important source of evolutionary transformations. Thanks to genetic variability, organisms with new properties and characteristics are formed, and a high level of phenotypic variability is also maintained.
Depending on the nature of the influence on the viability of organisms, they are distinguishedlethal , sublethal Andneutral mutations. Lethal, as a rule, entail the death of organisms even before birth or before the onset of sexual maturity. Sublethal - reduce viability, leading to the death of some (from 10 to 50%). Neutral mutations under normal living conditions for organisms do not affect their viability. And in some cases, such mutations can even become useful, especially when the living conditions of the organism change.
Based on the nature of hereditary changes in genetic material, three types of mutations are distinguished: gene, chromosomal, and genomic.
Genetic ( point ) mutations – are qualitative changes in individual genes. These mutations occur at the level of the primary DNA strand and lead to disruption of the amino acid sequence in proteins. Such changes can have negative consequences for the body. After all, the amino acid sequence in each protein is strictly specific, and replacing even one of them can lead to disruption of the spatial structure of the protein and, accordingly, functions.
The most common case of point mutation is the replacement of a nucleotide pair from GA to GC or vice versa. If these changes occur within structural genes, then as a result, instead of the AGA triplet, an AHC triplet may appear in the polypeptide chain, respectively, instead of a negatively charged amino acidarginine will be an uncharged amino acidserine. Such a mutation can lead to a change in the charge of the protein, disruption of its conformation, and if it is an enzyme, then to a decrease in the rate of the chemical reaction that it catalyzes. As a result, disruptions in the metabolism of the entire body may begin.
Substitutions can also be neutral, for example, substitutions of amino acids with the same properties. Stop codon mutations or mutations of loss or insertion of one of the nucleotides lead to extremely negative consequences. As a result, part or all of the sequence of triplets is changed, which contributes to a serious violation of the amino acid structure of the protein and this is almost always incompatible with the normal functioning of the body.
Chromosomal mutations – mutations associated with visible transformations of chromosomes. This can be a movement of one part of a chromosome to another, a rotation of a section of a chromosome by 180°, the insertion of extra parts of a chromosome, or, conversely, the loss of some sections. In most cases, chromosomal rearrangements do not occur without consequences for the body. Most often they lead to fatal outcome even at very early stages of embryonic development. If chromosomal changes do not affect genes that are responsible for important functions of the body, then they usually lead to meiosis disorders, and therefore to infertility of the individual. However, there are also completely neutral chromosomal mutations(chromosomal polymorphisms).
Genomic mutations associated with changes in the number of chromosomes. They are caused by gross violations of meiosis. One type of chromosomal mutation isaneuploidy- an increase in homologous chromosomes by one or more or, on the contrary, a deficiency, most often of one chromosome. Typically in animals, such disorders are incompatible with the normal functioning of the body and lead either to death in the early stages or to numerous developmental disorders. The hereditary human disease, the so-called Down syndrome, is caused by the appearance of a third additional chromosome in the 21st pair. And the appearance of the third chromosome in the 15th pair causes another hereditary human anomaly - polydactyly - the appearance of a sixth finger on the limbs.
Genomic mutations associated with a multiple increase in the number of chromosomes are calledpolyploidy (from Greekpolyploethia many, large number). If the number of chromosome sets increases by one, then it is a triploid, if by two, it is a tetraploid, etc. The greatest increase in the number of chromosome sets found in organisms is those with a tenfold chromosome set.
Polyploidy contributes to an increase in the size of the body, accelerates vital processes, and can cause disturbances in the process of reproduction. This is especially true for polyploid forms with an unpaired set of chromosomes, which can reproduce only by parthenogenesis or vegetatively.
Polyploidy is very common in nature. For the most part, it is represented by paroploid (tetra- or octoploid) forms in which meiosis occurs normally. There are a lot of polyploid species among plants and much fewer among animals. Quite often they are found among invertebrates (crustaceans, mollusks, worms). There are polyploids among vertebrates. In fish, for example, there are even entire families (sturgeons) and orders (salmonids), the species of which are exclusively polyploid. Polypoids occur less frequently in amphibians and reptiles, and in birds and mammals such individuals die on early stages development.
Somatic mutations – mutations that occur only in individual somatic cells. In organisms that reproduce sexually (most animals), such mutations are not inherited. It’s a different matter in plants - vegetative propagation allows you to preserve the change that has arisen and make it hereditary.
Most mutations that occur in the body, as a rule, are recessive, and the wild type (the so-called common phenotype characteristic of individuals living in natural conditions) is dominant. For example,albinism(from lat.albus– white) is a recessive trait that manifests itself in the homozygous (aa) state as the absence of pigment in the skin, hair, and iris of the eyes. As it turned out, the enzyme tyrosinase, which catalyzes the formation of the melanin pigment, does not function in albino individuals. Heterozygous individuals (Aa) have a wild color.
Dominant mutations also appear in the heterozygous state, but they occur much less frequently than recessive ones. The consequence of such mutations is, for example, the majority of cases of the appearance of melanistic animals, in which, unlike non-mutated individuals, a lot of melanin is synthesized. Usually such organisms have a darker color.
One more important factor genetic variability isrecombination (from lat.re– a prefix that indicates a repeated action andcombinare,– connection) – redistribution of genetic material in the offspring. The main reasons for gene recombination are:
The combination of gametes from different parents in the case of random crossing in animals and cross-pollination in plants;
Independent distribution of chromosomes after the first meiotic division;
Crossing over is the exchange of sections of homologous chromosomes during conjugation in metaphaseMeiosis I.
As a result of sexual reproduction, recombination leads to the formation of offspring with a wide variety of genotypic combinations. As a result, it is impossible to find two genetically identical individuals in one population. Recombination plays an important role in the evolution of organisms. Its properties are used in the process of breeding new varieties of plants and animal breeds.
2.2. Non-hereditary variability
The development of an organism's phenotype occurs through the interaction of its hereditary basis - genotype - with environmental conditions. Signs of an organism vary to varying degrees under the influence of various environmental factors. Some of them are very plastic and changeable, others are less changeable, and others practically do not change under the influence of environmental conditions. For example, milk yield cattle largely depends on the conditions of detention (feeding, care). While milk fat content is largely breed dependent and difficult to change, although some results can be achieved by changing the diet. An even more permanent feature is the suit. Under all possible conditions, it remains almost unchanged.
Modification (from lat.modulus– measure, type andfacies- shape, appearance)variability – These are changes in the characteristics of an organism (its phenotype) caused by changes in environmental conditions and not associated with changes in the genotype.Due to the fact that modification variability is not associated with changes in the genotype, it is not inherited.
Actually modificationchanges (modifications)– these are the reactions of organisms to changes in the intensity of the action of certain environmental factors. They are the same for all genotypes of closely related organisms. For example, all arrowhead plants immersed in water develop long and thin leaves, while those growing on dry land have arrow-shaped leaves. Arrowhead plants that are partially submerged in water produce both types of leaves.
In the diurnal butterfly, the variable wing color depends on the temperature at which the pupae developed. From those pupae that overwintered, butterflies emerge with a brick-red color, and from those that developed in the summer in conditions of elevated temperatures, butterflies emerge with a black background of wings. The degree of severity of modifications directly depends on the intensity and duration of action of a certain factor on the body. Thus, in the small brine shrimp, the degree of hairiness of the rear part of the abdomen depends on the salinity of the water: the lower the salt concentration, the greater it is.
As numerous studies have shown, modifications can disappear during the life of one individual if the action of the factor that caused them ceases. For example, a tan acquired by a person in the summer gradually disappears during the autumn-winter period. If an arrowhead plant is transplanted from water to dry land, the new leaves will not have an elongated, but an arrow-shaped shape. The resulting modifications can persist throughout the life of the individual, especially those that arose in the early stages individual development. But they are not passed on to descendants. For example, curvature of the bones of the lower extremities as a result of rickets persists throughout life. But to parents who suffered from rickets in childhood, children are born normal if during their development they receive the required amount of vitamin D. Another example of modifications that persist throughout life is the differentiation of honey bee larvae into queens and workers. The larvae, which develop in special large cells of honeycombs and feed only on “royal jelly”, which is produced by the special glands of worker bees, develop into queens. And those who are fed with beebread (a mixture of honey and pollen) subsequently become workers - underdeveloped females, incapable of reproduction. Therefore, differentiation of female honey bee larvae depends on the food they receive during their development. If at the early stages of development the larvae are swapped, from which the queen and worker bee should subsequently develop, then the nature of their nutrition and subsequent differentiation will change accordingly. However, at later stages of development this becomes impossible.
Modification variability plays an exceptional role in the life of organisms, ensuring their adaptability to changes in environmental conditions. Thus, changing the shape of arrowhead leaves from arrow-shaped to ribbon-shaped (linear) when this plant is immersed in water protects it from damage by the current. The change to a thicker coat of mammals during autumn molting provides protection from low temperatures, and a human tan provides protection from the harmful effects of solar radiation. All this gives reason to believe that such modifications arose in the process historical development species as certain adaptive reactions to changing environmental conditions that organisms constantly encounter. However, not all modifications are adaptive. For example, if you shade the lower part of a potato stem, above-ground tubers will begin to form on it. Modifications devoid of adaptive significance arise when organisms find themselves in unusual conditions that their ancestors did not have to face.
Modification variability is subject to certainstatistical patterns . In particular, any characteristic can vary only within certain limits. Such limits of modification variability (from min to max) of traits are predetermined by the genotype of the organism and are calledreaction norm . Consequently, a specific allelic gene does not predetermine the development of a specific state of the trait it encodes, but only the limits within which it can vary depending on the intensity of the action of certain environmental factors. Among the characters there are those whose different states are almost entirely determined by the genotype (for example, the location of the eyes, the number of fingers on the limbs, blood type, the pattern of leaf venation, etc.). But the degree of manifestation of the states of other characteristics (height and weight of organisms, the size of the puff plate, etc.) is significantly influenced by environmental conditions. For example, the development of fur colors of some animals (for example, ermine rabbits, Siamese cats) depends on temperature. If you shave an area of the body covered with white hair in such animals and apply ice to it, then in low temperature conditions black hair will grow in this area.
The reaction norm for different characteristics has its own limits. Signs that determine the viability of organisms (for example, the relative position of internal organs), and for signs that do not carry important vital significance, it can be much wider (body weight, height, hair color).
Usually a single manifestation of a trait is calledoption . To study the variability of a particular trait, i.e. option, make upvariational (from lat.variatio –change)row – a sequence of quantitative indicators of manifestations of states of a certain characteristic (variant), arranged in ascending or descending order.
The length of the variation series indicates the scope of modification variability (reaction norm). It is predetermined by the genotype of the organism, but depends on environmental conditions: the more stable they are, the shorter the variation series will be, and vice versa. If you trace the distribution of individual options within the variation series, you will notice that the largest number of them are located in its middle part, i.e. they have the average value of this characteristic.
This distribution is explained by the fact that the minimum and maximum values of trait development are formed when the majority of environmental factors act in one direction: the most or least favorable. But the body, as a rule, feels their different influences: some factors contribute to the development of the trait, while others, on the contrary, inhibit it. That is why the degree of development of a certain trait in most individuals of the same species is averaged. Yes, most people have average height, and only a small part of them are giants or dwarfs. The distribution of variants within a variation series can be graphically depicted in the form of a variation curve.Variation curve is a graphical representation of the dependence of possible variants of a trait on the frequency of occurrence.Using a variation curve, you can establish the average indicators and reaction norm of a certain trait.
GENERALIZATION
The manifestation of the phenotype of each organism depends on heredity and variability. Thanks to heredity, an individual receives a genetic set from its parental forms, thus preserving the specific characteristics of each species, and variability violates this pattern - thanks to variability, it is impossible to meet two genetically identical individuals in the world.
There are two types of variability: non-hereditary (phenotypic, modification) and hereditary (genotypic, genetic). Factors of genetic variability are mutations and recombinations of genetic material. Therefore, hereditary variability is also called mutational. Modifying variability is caused by the body's reactions to environmental factors. And since the conditions for the formation of each organism are largely different, individuals, even if they are representatives of the same species, have their own unique phenotype.
Heredity and variability play an important role in the evolution of organisms. Their properties are also used in the process of breeding new varieties of plants and animal breeds.
QUESTIONS FOR CONTROL
1. What is a gene from a biochemical and genetic point of view?
2. Why are heredity and variability called alternative phenomena? Define heredity and variability.
3. What is cytoplasmic inheritance and what causes it?
4. What are mutations? What types of mutations do you know?
5. What are aneuploidy and polyploidy?
6. Why do mutations associated with a multiple decrease in the chromosome set negatively affect the viability of organisms compared to those caused by a multiple increase in the genome?
7. Are most mutations recessive or dominant?
8. What is the difference between modification and mutational variability?
9. What is called the reaction norm of modification variability?
10. What is included in the statistical processing of modification variability data?
Variability call the general property of all living organisms to acquire differences between individuals of the same species.
Charles Darwin identified the following main types of variability: definite (group, non-hereditary, modification), indefinite (individual, hereditary, mutation) and combined. Hereditary variability includes such changes in the characteristics of living beings that are associated with changes in (i.e. mutations) and are transmitted from generation to generation. The transfer of material from parents to offspring must occur very precisely, otherwise species cannot survive. However, sometimes quantitative or qualitative changes occur in the DNA, and the daughter cells receive distorted genes compared to the parental genes. Such errors in the hereditary material are passed on to the next generation and are called mutations. An organism that has acquired new properties as a result of mutations is called a mutant. Sometimes these changes are clearly visible phenotypically, for example, the absence of pigments in the skin and hair - albinism. But most often mutations are recessive and appear in the phenotype only when they are present in a homozygous state. The existence of hereditary variations was known. All of it follows from the doctrine of hereditary changes. Hereditary variability is a necessary prerequisite for natural and... However, at the time of Darwin there was still no experimental data on heredity and the laws of inheritance were not known. This did not make it possible to strictly distinguish different shapes variability.
Mutation theory was developed at the beginning of the twentieth century by the Dutch cytologist Hugo de Vries.
have a number of properties:
Mutations occur suddenly, and any part of the genotype can mutate.
Mutations are more often recessive and less often dominant.
Mutations can be harmful, neutral or beneficial for the body.
Mutations are passed on from generation to generation.
Mutations can occur under the influence of both external and internal influences.
Mutations are divided into several types: Point (gene) mutations
Chromosomal mutations represent changes in individual genes. This can occur when one or more nucleotide pairs are replaced, dropped, or inserted in a DNA molecule.
are changes in parts of a chromosome or entire chromosomes. Such mutations can occur as a result of deletion - the loss of part of a chromosome, duplication - doubling of any part of a chromosome, inversion - turning a part of a chromosome by 1800, translocation - tearing off a part of a chromosome and moving it to a new position, for example, joining to another chromosome. consist in changing the number of chromosomes in the haploid set. This can occur due to the loss of a chromosome from the genotype, or, conversely, an increase in the number of copies of any chromosome in the haploid set from one to two or more. A special case of genomic mutations is polyploidy - a multiple increase in the number of chromosomes. The concept of mutations was introduced into science by the Dutch botanist de Vries. In the plant evening primrose (evening primrose), he observed the appearance of sharp, abrupt deviations from the typical form, and these deviations turned out to be hereditary. Further studies on various objects - plants, animals, microorganisms - showed that the phenomenon of mutational variability is characteristic of all organisms.
The material basis of the genotype is chromosomes. Mutations are changes that occur in chromosomes under the influence of external or external factors. Mutational variability is newly emerging changes in the genotype, while combinations are new combinations of parental genes in the zygote. Mutations affect various aspects of the structure and functions of the body. For example, in Drosophila there are known mutational changes in the shape of the wings (up to their complete disappearance), body color, development of bristles on the body, eye shape, their color (red, yellow, white, cherry), as well as many physiological characteristics (life expectancy, fertility ).
They occur in different directions and in themselves are not adaptive, beneficial changes for the body.
Many mutations that occur are unfavorable for the body and can even cause its death. Most of these mutations are recessive.
Most mutants have reduced viability and are eliminated through the process of natural selection. For the evolution of new breeds and varieties, those rare individuals that have favorable or neutral mutations are needed. The significance of mutations is that they create hereditary changes, which are the material for natural selection in nature. Mutations are also necessary for individuals with new properties that are valuable to humans. Artificial mutagenic factors are widely used to obtain new breeds of animals, plant varieties and strains of microorganisms.
Combinative variability also refers to hereditary forms of variability. It is caused by the rearrangement of genes during the process of gamete fusion and the formation of a zygote, i.e. during sexual intercourse.
IN evolutionary theory Darwin's prerequisite for evolution is hereditary variability, and driving forces evolution - the struggle for existence and natural selection. When creating an evolutionary theory, Charles Darwin repeatedly turned to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believes that the causes of variability are the impact of environmental factors on organisms (direct and indirect), as well as the nature of the organisms themselves (since each of them specifically reacts to the influence of the external environment). Variability serves as the basis for the formation of new characteristics in the structure and functions of organisms, and heredity consolidates these characteristics. Darwin, analyzing the forms of variability, identified three among them: definite, indefinite and correlative.
Specific, or group, variability is variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability include an increase in body weight in animal specimens with good feeding, change hairline under the influence of climate, etc. A certain variability is widespread, covers the entire generation and is expressed in each individual in a similar way. It is not hereditary, i.e., in the descendants of the modified group under other conditions, the characteristics acquired by the parents are not inherited.
Uncertain, or individual, variability manifests itself specifically in each individual, i.e. singular, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is uncertain, i.e., a trait under the same conditions can change in different directions. For example, one variety of plants produces specimens with different colors of flowers, different intensities of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Uncertain variability has hereditary character, i.e., it is stably transmitted to offspring. This is its importance for evolution.
With correlative, or correlative, variability, a change in any one organ causes changes in other organs. For example, dogs with poorly developed coats usually have underdeveloped teeth, pigeons with feathered feet have webs between their toes, and pigeons with a long beak usually have long legs, white cats with blue eyes usually deaf, etc. From the factors of correlative variability, Darwin draws an important conclusion: a person, selecting any structural feature, is almost “likely to unintentionally change other parts of the organism on the basis of mysterious laws of correlation.”
Having determined the forms of variability, Darwin came to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors of evolution cultural forms- this is hereditary variability and selection made by man (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.
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4. ROLE OF HEREDITARY VARIATION IN THE EVOLUTION OF SPECIES AND ITS FORM
In Darwin's evolutionary theory, the prerequisite for evolution is hereditary variability, and the driving forces of evolution are the struggle for existence and natural selection. When creating an evolutionary theory, Charles Darwin repeatedly turned to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believes that the causes of variability are the impact of environmental factors on organisms (direct and indirect), as well as the nature of the organisms themselves (since each of them specifically reacts to the influence of the external environment). Variability serves as the basis for the formation of new characteristics in the structure and functions of organisms, and heredity consolidates these characteristics. Darwin, analyzing the forms of variability, identified three among them: definite, indefinite and correlative.
Specific, or group, variability is variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability include an increase in body weight in animal individuals with good feeding, changes in hair coat under the influence of climate, etc. A certain variability is widespread, covers the entire generation and is expressed in each individual in a similar way. It is not hereditary, i.e., in the descendants of the modified group under other conditions, the characteristics acquired by the parents are not inherited.
Uncertain, or individual, variability manifests itself specifically in each individual, i.e. singular, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is uncertain, i.e., a trait under the same conditions can change in different directions. For example, one variety of plants produces specimens with different colors of flowers, different intensities of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Uncertain variability is hereditary in nature, that is, it is stably transmitted to offspring. This is its importance for evolution.
With correlative, or correlative, variability, a change in any one organ causes changes in other organs. For example, dogs with poorly developed coats usually have underdeveloped teeth, pigeons with feathered feet have webbing between their toes, pigeons with a long beak usually have long legs, white cats with blue eyes are usually deaf, etc. Of the factors of correlative variability, Darwin makes an important conclusion: a person, selecting any structural feature, will almost “probably unintentionally change other parts of the body on the basis of mysterious laws of correlation.”
Having determined the forms of variability, Darwin came to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors in the evolution of cultural forms are hereditary variability and selection made by humans (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.
CONCLUSION
Thus, Darwin for the first time in the history of biology constructed the theory of evolution. This was of great methodological importance and made it possible not only to substantiate the idea of organic evolution clearly and convincingly for contemporaries, but also to test the validity of the theory of evolution itself. This was a decisive phase in one of the greatest conceptual revolutions in natural science. The most important thing in this revolution was the replacement of the theological idea of evolution as the idea of primordial purposiveness with the model of natural selection. Despite fierce criticism, Darwin's theory quickly gained recognition due to the fact that the concept of the historical development of living nature explained the observed facts better than the idea of \u200b\u200bthe immutability of species. To substantiate his theory, Darwin, unlike his predecessors, drew on a huge amount of facts available to him from the most different areas. The highlighting of biotic relationships and their population-evolutionary interpretation was the most important innovation of Darwin's concept of evolution and gives the right to the conclusion that Darwin created his own concept of the struggle for existence, fundamentally different from the ideas of his predecessors. Darwin's doctrine of the evolution of the organic world was the first theory of development created “naturally historical materialism in the depths of natural science, the first application of the principle of development to an independent field of natural sciences.” This is the general scientific significance of Darwinism.
Darwin's merit lies in the fact that he revealed the driving forces of organic evolution. The further development of biology deepened and complemented his ideas, which served as the basis for modern Darwinism. In all biological disciplines leading place now occupies historical method research that allows one to study specific paths of evolution of organisms and deeply penetrate into the essence of biological phenomena. The evolutionary theory of Charles Darwin has found wide application in modern synthetic theory, where the only guiding factor of evolution remains natural selection, the material for which is mutations. A historical analysis of Darwin's theory inevitably gives rise to new methodological problems of science, which can become the subject of special research. The solution to these problems entails an expansion of the field of knowledge, and, consequently, scientific progress in many areas: both in biology, medicine, and in psychology, on which the evolutionary theory of Charles Darwin had no less influence than on natural Sciences.
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Some particles transmitted from parents to offspring. Now we call these particles genes. The idea of corpuscular heredity is of great importance for understanding how natural selection operates in populations. Evolution can be thought of as changes in any property of a given population over time. In a certain general philosophical sense, this is the essence of evolution. ...
They would strive to preserve themselves to changed conditions, and natural selection would have full scope for its improving action. 1. NATURAL SELECTION AS AN ELEMENTARY EVOLUTIONARY FACTOR I called the preservation of favorable individual differences and changes and the destruction of harmful ones natural selection or the survival of the fittest C. Darwin In the modern understanding...
Preservation and accumulation of small hereditary changes, each of which is beneficial for the being preserved. Circumstances favoring the formation of new forms through natural selection. A significant part of the variability, of course, and individual differences, will obviously be a favorable circumstance. A large number of individuals, increasing the chances of appearing in...
And therefore they play a more important role in evolution. Of fundamental importance is the fact that these mutations are random, in other words, they are not directed. 3. The central dogma and Weismann's principle are accepted. 4. Evolution occurs by changing gene frequencies. 5. These changes can occur as a result of mutations, the entry of genes into the population and their outflow from it, random drift and...
In Darwin's evolutionary theory, the prerequisite for evolution is hereditary variability, and the driving forces of evolution are the struggle for existence and natural selection. When creating an evolutionary theory, Charles Darwin repeatedly turned to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believes that the causes of variability are the impact of environmental factors on organisms (direct and indirect), as well as the nature of the organisms themselves (since each of them specifically reacts to the influence of the external environment). Variability serves as the basis for the formation of new characteristics in the structure and functions of organisms, and heredity consolidates these characteristics. Darwin, analyzing the forms of variability, identified three among them: definite, indefinite and correlative.
Specific, or group, variability is variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability include an increase in body weight in animal individuals with good feeding, changes in hair coat under the influence of climate, etc. A certain variability is widespread, covers the entire generation and is expressed in each individual in a similar way. It is not hereditary, i.e., in the descendants of the modified group under other conditions, the characteristics acquired by the parents are not inherited.
Uncertain, or individual, variability manifests itself specifically in each individual, i.e. singular, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is uncertain, i.e., a trait under the same conditions can change in different directions. For example, one variety of plants produces specimens with different colors of flowers, different intensities of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Uncertain variability is hereditary in nature, that is, it is stably transmitted to offspring. This is its importance for evolution.
With correlative, or correlative, variability, a change in any one organ causes changes in other organs. For example, dogs with poorly developed coats usually have underdeveloped teeth, pigeons with feathered feet have webbing between their toes, pigeons with a long beak usually have long legs, white cats with blue eyes are usually deaf, etc. Of the factors of correlative variability, Darwin makes an important conclusion: a person, selecting any structural feature, will almost “probably unintentionally change other parts of the body on the basis of mysterious laws of correlation.”
Having determined the forms of variability, Darwin came to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors in the evolution of cultural forms are hereditary variability and selection made by humans (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.
Forms of natural selection
Selection occurs continuously over an infinite series of successive generations and preserves mainly those forms that are more consistent with given conditions. Natural selection and the elimination of some individuals of a species are inextricably linked and are a necessary condition for the evolution of species in nature.
The scheme of the action of natural selection in a species system according to Darwin comes down to the following:
1) Variation is characteristic of any group of animals and plants, and organisms differ from each other in many respects;
2) The number of organisms of each species that are born exceeds the number of those that can find food and survive. However, since the number of each species is constant under natural conditions, it should be assumed that most of the offspring die. If all the descendants of any species survived and reproduced, they would very soon supplant all other species on the globe;
3) Since more individuals are born than can survive, there is a struggle for existence, competition for food and habitat. This may be an active life-and-death struggle, or less obvious, but no less effective competition, as, for example, for plants during periods of drought or cold;
4) Among the many changes observed in living beings, some facilitate survival in the struggle for existence, while others lead to the death of their owners. The concept of "survival of the fittest" is the core of the theory of natural selection;
5) Surviving individuals give rise to the next generation, and thus “successful” changes are passed on to subsequent generations. As a result, each subsequent generation turns out to be more adapted to its environment; as the environment changes, further adaptations arise. If natural selection operates over many years, then the latest offspring may turn out to be so different from their ancestors that it would be advisable to separate them into an independent species.
It may also happen that some members of a given group of individuals acquire certain changes and find themselves adapted to environment in one way, while its other members, possessing a different set of changes, will be adapted in a different way; In this way, from one ancestral species, provided that similar groups are isolated, two or more species can arise.
Driving selection
Natural selection always leads to an increase in the average fitness of populations. Changes in external conditions can lead to changes in the fitness of individual genotypes. In response to these changes, natural selection, drawing on the enormous pool of genetic diversity for many different traits, leads to significant shifts in the genetic structure of the population. If the external environment is constantly changing in a certain direction, then natural selection changes the genetic structure of the population in such a way that its fitness in these changing conditions remains maximum. At the same time, the frequencies of individual alleles in the population change. The average values of adaptive traits in populations also change. In a series of generations, their gradual shift in a certain direction can be traced. This form of selection is called driving selection.
Classic example driving selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of the lichen-covered bark of trees on which it spends the daylight hours. Obviously, such a protective coloration was formed over many generations of previous evolution. However, with the beginning of the industrial revolution in England, this device began to lose its importance. Air pollution has led to massive death of lichens and darkening of tree trunks. Light butterflies against a dark background became easily visible to birds. Beginning with mid-19th century, mutant dark (melanistic) forms of butterflies began to appear in birch moth populations. Their frequency increased rapidly. TO end of the 19th century centuries, some urban populations of the birch moth consisted almost entirely of dark forms, while in rural populations light forms continued to predominate. This phenomenon was called industrial melanism. Scientists have found that in polluted areas, birds are more likely to eat light-colored forms, and in clean areas, dark ones. The introduction of air pollution restrictions in the 1950s caused natural selection to reverse course again, and the frequency of dark forms in urban populations began to decline. They are almost as rare these days as they were before the Industrial Revolution.
Driving selection brings the genetic composition of populations into line with changes in external environment so that the average fitness of populations is maximized. On the island of Trinidad, guppy fish live in different bodies of water. Many of those that live in the lower reaches of rivers and ponds die in the teeth of predatory fish. In the upper reaches, life for guppies is much calmer - there are few predators there. These differences in external conditions led to the fact that the “top” and “bottom” guppies evolved in different directions. “Grassroots”, under constant threat of extermination, begin to multiply in more early age and produce many very small fry. The chance of survival for each of them is very small, but there are a lot of them and some of them manage to reproduce. The “mountains” reach sexual maturity later, their fertility is lower, but their offspring are larger. When researchers transferred “low-growth” guppies to uninhabited reservoirs in the upper reaches of rivers, they observed a gradual change in the type of development of the fish. Eleven years after the move, they became significantly larger, began breeding later, and produced fewer but larger offspring.
The rate of change in allele frequencies in a population and the average values of traits under the influence of selection depends not only on the intensity of selection, but also on the genetic structure of the traits for which turnover occurs. Selection against recessive mutations turns out to be much less effective than against dominant ones. In a heterozygote, the recessive allele does not appear in the phenotype and therefore escapes selection. Using the Hardy-Weinberg equation, one can estimate the rate of change in the frequency of a recessive allele in a population depending on the intensity of selection and the initial frequency ratio. The lower the allele frequency, the slower its elimination occurs. In order to reduce the frequency of recessive lethality from 0.1 to 0.05, only 10 generations are needed; 100 generations - to reduce it from 0.01 to 0.005 and 1000 generations - from 0.001 to 0.0005.
The driving form of natural selection plays a decisive role in the adaptation of living organisms to external conditions that change over time. It also ensures the widespread distribution of life, its penetration into all possible ecological niches. It is a mistake to think, however, that in stable conditions of existence natural selection ceases. Under such conditions, it continues to act in the form of stabilizing selection.
Stabilizing selection
Stabilizing selection preserves the state of the population that ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value for adaptive traits are removed.
Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them is. As a result, individuals with average fertility are the most fit.
Selection toward the mean has been found for a variety of traits. In mammals, very low- and very-high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. A study of the size of the wings of birds that died after the storm showed that most of them had wings that were too small or too large. And in this case, the average individuals turned out to be the most adapted.
What is the reason for the constant appearance of poorly adapted forms in constant conditions of existence? Why is natural selection not able to once and for all clear a population of unwanted deviant forms? The reason is not only and not so much the constant emergence of more and more new mutations. The reason is that heterozygous genotypes are often the fittest. When crossed, they constantly split and their offspring produce homozygous offspring with reduced fitness. This phenomenon is called balanced polymorphism.
Sexual selection
Males of many species display clearly expressed secondary sexual characteristics that at first glance seem non-adaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features complicate the life of their carriers and make them easily noticeable to predators. It would seem that these characteristics do not provide any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their emergence and spread?
It is known that the survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. Charles Darwin called this phenomenon sexual selection. He first mentioned this form of selection in On the Origin of Species and then analyzed it in detail in The Descent of Man and Sexual Selection. He believed that “this form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by competition between individuals of the same sex, usually males, for the possession of individuals of the other sex.”
Sexual selection is natural selection for reproductive success. Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival. A male who lives short but is liked by females and therefore produces many offspring has much higher overall fitness than one who lives long but produces few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition arises between males for females. This competition can be direct, and manifest itself in the form of struggle for territory or tournament battles. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition manifests itself through displays of flamboyant appearance or complex courtship behavior. Females choose the males they like best. As a rule, these are the brightest males. But why do females like bright males?
The fitness of a female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.
Two main hypotheses about the mechanisms of sexual selection have been proposed.
According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, then it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored genes and will be attractive to females in the next generation. Thus, there is a positive Feedback, which leads to the fact that from generation to generation the brightness of the plumage of males increases more and more. The process continues to grow until it reaches the limit of viability. Let's imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases because females choose males not with a certain tail size, but with a larger than average size. Eventually, the tail reaches a length where its detriment to the male's vitality is balanced by its attractiveness in the eyes of females.
In explaining these hypotheses, we tried to understand the logic of the actions of female birds. It may seem that we expect too much from them, that such complex calculations of fitness are hardly possible for them. In fact, females are no more or less logical in their choice of males than in all their other behavior. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to a watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much with this self-sacrifice she increases the overall fitness of her sisters - she follows instinct. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. All those to whom instinct suggested a different behavior, all of them did not leave offspring. Thus, we were discussing not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all the amazing diversity of shapes, colors and instincts that we observe in the world of living nature .
