Darwin called hereditary variation indeterminate. What is uncertain variability in biology: definition. What is the difference between definite and indefinite variability? Examples of Uncertain Variability

Variability

When comparing many breeds of animals and varieties of plants, Darwin noticed that within any species of animals and plants, and in culture, within any variety and breed there are no identical individuals. Based on the instructions of K. Linnaeus that reindeer herders recognize every deer in their herd, shepherds recognize every sheep, and many gardeners recognize varieties of hyacinths and tulips by bulbs, Darwin concluded that variability is inherent in all animals and plants.

Analyzing the material on the variability of animals, the scientist noticed that any change in living conditions is enough to cause variability. Thus, Darwin understood variability as the ability of organisms to acquire new characteristics under the influence of environmental conditions. He distinguished the following forms of variability:

In his books “On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Breeds in the Struggle for Life” (1859) and “Changes in Domestic Animals and Cultivated Plants” (1868), Darwin described in detail the variety of breeds of domestic animals and analyzed their origin. He noted the diversity of cattle breeds, of which there are about 400. They differ from each other in a number of characteristics: color, body shape, degree of skeletal and muscle development, the presence and shape of horns. The scientist examined in detail the question of the origin of these breeds and came to the conclusion that all European breeds of cattle, despite the great differences between them, originated from two ancestral forms domesticated by humans.

The breeds of domestic sheep are also extremely diverse, there are more than 200 of them, but they come from a limited number of ancestors - mouflon and argali. Various breeds of domestic pigs were also bred from wild forms of wild boar, which, in the process of domestication, changed many features of their structure. The breeds of dogs, rabbits, chickens and other domestic animals are unusually diverse.

Darwin was particularly interested in the question of the origin of pigeons. He proved that all existing breeds of pigeons descended from one wild ancestor - the rock (mountain) pigeon. The breeds of pigeons are so different that any ornithologist, finding them in the wild, would recognize them as independent species. However, Darwin showed their common origin based on the following facts:

• none of the species of wild pigeons, except the rocky one, has any characteristics of domestic breeds;
• many features of all domestic breeds are similar to those of the wild rock pigeon. Domestic pigeons do not build nests in trees, retaining the wild pigeon instinct. All breeds have the same behavior when courting a female;
• when crossing pigeons of different breeds, hybrids sometimes appear with characteristics of a wild rock pigeon;
• all hybrids between any breeds of pigeons are fertile, which confirms that they belong to the same species. It is quite obvious that all these numerous breeds were the result of a change in one original form. This conclusion is also true for most domestic animals and cultivated plants.

Darwin paid much attention to the study of various varieties of cultivated plants. Thus, comparing various varieties of cabbage, he concluded that they were all bred by man from one wild species: they differ in the shape of the leaves with similar flowers and seeds. Ornamental plants, for example, different varieties of pansies, produce a variety of flowers, and their leaves are almost the same. Gooseberry varieties have a variety of fruits, but the leaves are almost the same.

Reasons for variability. Having shown the variety of forms of variability, Darwin explained the material causes of variability, which are environmental factors, the conditions of existence and development of living beings. But the influence of these factors varies depending on the physiological state of the organism and the stage of its development. Among the specific causes of variability, Darwin identifies:

• direct or indirect (through the reproduction system) influence of living conditions (climate, food, care, etc.);
• functional tension of organs (exercise or non-exercise);
• crossing (the appearance in hybrids of characteristics not characteristic of the original forms);
• changes caused by the correlative dependence of parts of the body.

Among the various forms of variability for the evolutionary process, hereditary changes are of paramount importance as the primary material for variety, breed and speciation - those changes that are fixed in subsequent generations.

Heredity

By heredity, Darwin understood the ability of organisms to preserve their species, varietal and individual characteristics in their offspring. This feature was well known and represented hereditary variation. Darwin analyzed in detail the importance of heredity in the evolutionary process. He drew attention to cases of same-suit hybrids of the first generation and splitting of characters in the second generation; he was aware of heredity associated with sex, hybrid atavisms and a number of other phenomena of heredity.

At the same time, Darwin noted that the study of variability and heredity, their immediate causes and patterns is associated with great difficulties. The science of that time could not yet give a satisfactory answer to a number of important questions. The works of G. Mendel were also unknown to Darwin. Only much later did extensive research into variability and heredity begin, and modern genetics made a giant step in the study of the material foundations, causes and mechanisms of heredity and variability, in the causal understanding of these phenomena.

Darwin attached great importance to the presence of variability and heredity in nature, considering them the main factors of evolution, which is adaptive in nature [show] .

Adaptive nature of evolution

Darwin in his work "The Origin of Species..." noted the most important feature of the evolutionary process - the continuous adaptation of species to the conditions of existence and the improvement of the organization of the species as a result of the accumulation of adaptations. However, he noted that the adaptability of a species, developed by selection to the conditions of existence, although it is important for the self-preservation and self-reproduction of species, cannot be absolute; it is always relative and is useful only in those environmental conditions in which species exist for a long time. The body shape, respiratory organs and other features of fish are suitable only for living in water and are not suitable for terrestrial life. The green coloration of locusts camouflages insects on green vegetation, etc.

The process of expedient adaptation can be traced using the example of any group of organisms that has been sufficiently studied in evolutionary terms. A good example is the evolution of the horse.

The study of the horse's ancestors made it possible to show that its evolution was associated with the transition from life in forests on marshy soil to life in open, dry steppes. Changes in the horse's known ancestors occurred in the following directions:

• increased growth due to the transition to life in open spaces (high growth is an adaptation to the expansion of the horizon in the steppes);
• an increase in running speed was achieved by lightening the leg skeleton and gradually reducing the number of toes (the ability to run quickly has a protective value and allows you to more effectively find water bodies and feeding grounds);
• intensification of the grinding function of the dental apparatus as a result of the development of ridges on the molars, which was especially important in connection with the transition to feeding on tough cereal vegetation.

Naturally, along with these changes, correlative ones also occurred, for example, lengthening of the skull, changes in the shape of the jaws, the physiology of digestion, etc.

Along with the development of adaptations, the so-called adaptive diversity appears in the evolution of any group. It lies in the fact that, against the background of unity of organization and the presence of common systematic characteristics, representatives of any natural group of organisms always differ in specific characteristics that determine their adaptability to specific living conditions.

Due to living in similar living conditions, unrelated forms of organisms can acquire similar adaptations. For example, such systematically distant forms as a shark (class Pisces), ichthyosaur (class Reptiles) and dolphin (class Mammals) have a similar appearance, which is an adaptation to the same living conditions in a certain environment, in this case in water. The similarity between systematically distant organisms is called convergence (see below). In sessile protozoa, sponges, coelenterates, annelids, crustaceans, echinoderms, and ascidians, the development of root-like rhizoids is observed, with the help of which they are strengthened in the ground. Many of these organisms are characterized by a stalk-like body shape, which makes it possible, during a sedentary lifestyle, to soften the blows of waves, the impacts of fish fins, etc. All sessile forms are characterized by a tendency to form clusters of individuals and even coloniality, where the individual is subordinate to a new whole - the colony, which reduces the likelihood of death as a result of mechanical damage.

In different living conditions, related forms of organisms acquire different adaptations, i.e. two or more species can arise from one ancestral form. Darwin called this process of divergence of species in different environmental conditions divergence (see below). An example of this is the finches on the Galapagos Islands (west of Ecuador): some feed on seeds, others on cacti, and others on insects. Each of these forms differs from the other in the size and shape of the beak and could have arisen as a result of divergent variability and selection.

The adaptations of placental mammals are even more diverse, among which there are terrestrial forms with fast running (dogs, deer), species leading an arboreal lifestyle (squirrel, monkey), animals living on land and in water (beavers, seals), living in air environment (bats), aquatic animals (whales, dolphins) and species with an underground lifestyle (moles, shrews). All of them descend from a single primitive ancestor - an arboreal insectivorous mammal (Fig. 3).

Adaptation is never absolutely perfect due to the duration of the process of accumulation of adaptations. Changes in relief, climate, composition of fauna and flora, etc. can quickly change the direction of selection, and then adaptations developed in some conditions of existence lose their significance in others, to which new adaptations begin to be developed again. At the same time, the number of some species decreases, while the more adapted ones increase. Newly adapted organisms may retain previous signs of adaptation, which in new conditions of existence are not of decisive importance for self-preservation and self-reproduction. This allowed Darwin to talk about the inexpediency of signs of adaptation, which were found in the organization and behavior of organisms quite often. This is especially clearly seen when the behavior of organisms is not determined by their way of life. Thus, the webbed feet of geese serve as an adaptation for swimming and their presence is advisable. However, mountain geese also have webbed feet, which is clearly impractical given their lifestyle. The frigate bird does not usually land on the surface of the ocean, although, like bar-headed geese, it has webbed feet. It is safe to say that membranes were necessary and useful for the ancestors of these birds, just like modern aquatic birds. Over time, the descendants adapted to new living conditions and lost the habit of swimming, but they retained their swimming organs.

It is known that many plants are sensitive to temperature fluctuations and this is an appropriate response to the seasonal periodicity of vegetation and reproduction. However, such sensitivity to temperature fluctuations can lead to mass plant mortality if temperatures rise in the fall, stimulating the transition to repeated flowering and fruiting. This prevents the normal preparation of perennial plants for winter and they die when cold weather sets in. All these examples indicate relative feasibility.

The relativity of expediency manifests itself when there is a significant change in the conditions of existence of the organism, since in this case the loss of the adaptive nature of one or another characteristic is especially obvious. In particular, the rational design of burrows with exits at the water level of the muskrat is destructive during winter floods. Erroneous reactions are often observed in migratory birds. Sometimes waterfowl fly to our latitudes before the opening of reservoirs, and the lack of food at this time leads to their mass death.

Purpose is a historically arose phenomenon under the constant action of natural selection, and therefore it manifests itself differently at different stages of evolution. In addition, the relativity of fitness provides the possibility of further restructuring and improvement of the adaptations available to a given type, i.e. the infinity of the evolutionary process.

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However, having substantiated the question of variability and heredity as factors of evolution, Darwin showed that by themselves they do not yet explain the emergence of new breeds of animals, plant varieties, species, or their fitness. Darwin's great merit is that he developed the doctrine of selection as the leading and directing factor in the evolution of domestic forms (artificial selection) and wild species (natural selection).

Darwin established that as a result of selection, a change in species occurs, i.e. selection leads to divergence - deviation from the original form, divergence of characteristics in breeds and varieties, the formation of a large variety of them [show] .

Divergent nature of evolution

Darwin developed the principle of divergence, i.e., divergence of characteristics of varieties and breeds, using the example of artificial selection. Subsequently, he used this principle to explain the origin of animal and plant species, their diversity, the emergence of differentiation between species, and substantiation of the doctrine of the monophyletic origin of species from a common root.

The divergence of the evolutionary process is derived from the facts of multidirectional variability, preferential survival and reproduction in a number of generations of extreme variants that compete with each other to a lesser extent. Intermediate forms, whose life requires similar food and habitats, are in less favorable conditions and, therefore, die out faster. This leads to a greater gap between extreme options, the formation of new varieties, which later become independent species.

Divergence under the control of natural selection leads to the differentiation of species and their specialization. For example, the genus of tits unites species that live in different places (biotopes) and feed on different foods (Fig. 2). In butterflies of the white butterfly family, divergence went in the direction of caterpillars adapting to eating different food plants - cabbage, turnips, rutabaga and other wild plants of the cruciferous family. Among buttercups, one species lives in water, others live in swampy places, forests or meadows.

Based on similarity, as well as common origin, taxonomy unites closely related species of plants and animals into genera, genera into families, families into orders, etc. Modern taxonomy is a reflection of the monophyletic nature of evolution.

The principle of divergence developed by Darwin has important biological significance. It explains the origin of the wealth of life forms, the ways of development of numerous and more diverse habitats.

A direct consequence of the divergent development of most groups within similar habitats is convergence - the convergence of characters and the development of outwardly similar traits in forms of different origins. A classic example of convergence is the similarity of body shape and organs of movement in a shark (fish), ichthyosaur (reptile) and dolphin (mammal), i.e., the similarity of adaptations to life in water (Fig. 3). There are similarities between placental and marsupial mammals, between the smallest bird, the hummingbird, and the large butterfly, the hummingbird hawk moth. Convergent similarity of individual organs occurs in unrelated animals and plants, i.e. is built on a different genetic basis.

Progress and regression

Darwin showed that the inevitable consequence of divergent evolution is the progressive development of organic nature from simple to complex. This historical process of increasing organization is well illustrated by paleontological data, and is also reflected in the natural system of plants and animals, combining lower and higher forms.

Thus, evolution can take different paths. The main directions of evolutionary development and morphophysiological patterns of evolution were developed in detail by Academician. A.N. Severtsov (see macroevolution).

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Artificial selection

Analyzing the characteristics of breeds of domestic animals and varieties of cultivated plants, Darwin drew attention to the significant development in them of precisely those characteristics that are valued by humans. This was achieved using the same technique: when breeding animals or plants, breeders left for reproduction those specimens that most fully satisfied their needs and from generation to generation accumulated changes useful for humans, i.e. carried out artificial selection.

By artificial selection, Darwin understood a system of measures to improve existing and create new breeds of animals and plant varieties with useful (economically) hereditary traits and distinguished the following forms of artificial selection:

Purposeful breeding of a breed or variety. When starting work, the breeder sets himself a certain task in relation to the characteristics that he wants to develop in a given breed. First of all, these characteristics must be economically valuable or satisfy the aesthetic needs of humans. The traits with which the breeder works can be both morphological and functional. These may also include the nature of animal behavior, for example, pugnacity in fighting cocks. When solving the task set for himself, the breeder selects from the already available material all the best, in which the characteristics of interest to him are manifested, at least to a small extent. Selected individuals are kept in isolation to avoid unwanted crossbreeding. The breeder then selects pairs to cross. After this, starting from the first generation, he strictly selects the best material and rejects those that do not meet the requirements.

Thus, methodical selection is a creative process leading to the formation of new breeds and varieties. Using this method, the breeder, like a sculptor, sculpts new organic forms according to a pre-thought-out plan. Its success depends on the degree of variability of the original form (the more the characteristics change, the easier it is to find the desired changes) and the size of the original batch (in a large batch there are greater opportunities for choice).

Methodological selection in our time, using the achievements of genetics, has been significantly improved and has become the basis of modern theory and practice of animal and plant breeding.

Unconscious selection carried out by a person without a specific, pre-set task. This is the oldest form of artificial selection, elements of which were already used by primitive people. With unconscious selection, a person does not set a goal to create a new breed, variety, but only leaves it to the tribe and mainly reproduces the best individuals. So, for example, a peasant who has two cows, wanting to use one of them for meat, will slaughter the one that gives less milk; Of the chickens, he uses the worst laying hens for meat. In both cases, the peasant, preserving the most productive animals, carries out directed selection, although he does not set himself the goal of breeding new breeds. It is precisely this primitive form of selection that Darwin calls unconscious selection.

Darwin emphasized the particular importance of unconscious selection from a theoretical point of view, since this form of selection sheds light on the process of speciation. It can be seen as a bridge between artificial and natural selection. Artificial selection was a good model on which Darwin deciphered the process of morphogenesis. Darwin's analysis of artificial selection played an important role in substantiating the evolutionary process: firstly, he finally established the position of variability: secondly, he established the basic mechanisms of morphogenesis (variability, heredity, preferential reproduction of individuals with useful traits) and, finally, showed the ways of development expedient adaptations and divergence of varieties and breeds. These important premises paved the way for a successful solution to the problem of natural selection.

The doctrine of natural selection as a driving and guiding factor in the historical development of the organic world -
central part of Darwin's theory of evolution .

The basis of natural selection is the struggle for existence - the complex relationships between organisms and their connection with the environment.

Struggle for existence

In nature, there is a constant tendency towards unlimited reproduction of all organisms in geometric progression. [show] .

According to Darwin's calculations, one poppy box contains 3 thousand seeds, and a poppy plant grown from one seed produces up to 60 thousand seeds. Many fish annually lay up to 10-100 thousand eggs, cod and sturgeon - up to 6 million.

Russian scientist K. A. Timiryazev gives the following example illustrating this point.

Dandelion, according to rough estimates, produces 100 seeds. Of these, 100 plants can grow next year, each of which will also produce 100 seeds. This means that with unhindered reproduction, the number of descendants of one dandelion could be represented as a geometric progression: the first year - 1 plant; second - 100; third - 10,000; tenth year - 10 18 plants. To resettle the descendants of one dandelion obtained in the tenth year, an area 15 times larger than the area of ​​the globe will be needed.

This conclusion can be reached by analyzing the reproductive ability of a wide variety of plants and animals.

However, if you count, for example, the number of dandelions in a certain area of ​​a meadow over several years, it turns out that the number of dandelions changes little. A similar situation is observed among representatives of the fauna. Those. "geometric progression of reproduction" is never carried out, because between organisms there is a struggle for space, food, shelter, competition when choosing a sexual partner, a struggle for survival with fluctuations in temperature, humidity, lighting, etc. In this struggle, the majority of those born die (are eliminated, removed) without leaving offspring, and therefore in nature the number of individuals of each species on average remains constant. In this case, the surviving individuals turn out to be the most adapted to the conditions of existence.

Darwin laid the discrepancy between the number of individuals born and the number of individuals surviving to adulthood as a result of complex and varied relationships with other living beings and environmental factors as the basis of his doctrine of the struggle for existence or the struggle for life [show] . At the same time, Darwin realized that this term was unsuccessful and warned that he was using it in a broad metaphorical sense, and not literally.

Darwin reduced the various manifestations of the struggle for existence to three types:

1. interspecific struggle - the relationship of an organism with individuals of other species (interspecific relationships);
2. intraspecific struggle - relationships between individuals and groups of individuals of the same species (intraspecific relationships)
3. struggle with the conditions of the inorganic external environment - the relationship of organisms and species with the physical conditions of life, the abiotic environment

Intraspecific relationships are also quite complex (relationships between individuals of different sexes, between parental and daughter generations, between individuals of the same generation in the process of individual development, relationships in a flock, herd, colony, etc.). Most forms of intraspecific relationships are important for the reproduction of the species and maintaining its numbers, ensuring a change of generations. With a significant increase in the number of individuals of a species and restrictions on the conditions for their existence (for example, with dense plantings), acute interaction arises between individual individuals, which leads to the death of some or all individuals or their elimination from reproduction. Extreme forms of such relationships include intraspecific struggle and cannibalism - eating individuals of one's own species.

The fight against inorganic environmental conditions occurs depending on climatic and soil conditions, temperature, humidity, light and other factors affecting the life of organisms. During the process of evolution, animal and plant species develop adaptations to life in a particular environment.

It should be noted that the three named main forms of struggle for existence in nature are not carried out in isolation - they are closely intertwined with each other, due to which the relationships of individuals, groups of individuals and species are multifaceted and quite complex.

Darwin was the first to reveal the content and meaning of such important concepts in biology as “environment”, “external conditions”, “interrelations of organisms” in the process of their life and development. Academician I. I. Shmalgauzen considered the struggle for existence to be one of the main factors of evolution.

Natural selection

Natural selection, in contrast to artificial selection, is carried out in nature itself and consists of the selection within a species of the most adapted individuals to the conditions of a particular environment. Darwin discovered a certain commonality in the mechanisms of artificial and natural selection: in the first form of selection, the conscious or unconscious will of man is embodied in the results, in the second, the laws of nature prevail. In both cases, new forms are created, but with artificial selection, despite the fact that variability affects all organs and properties of animals and plants, the resulting animal breeds and plant varieties retain characteristics that are useful for humans, but not for the organisms themselves. On the contrary, natural selection preserves individuals whose changes are useful for their own existence in given conditions.

In “The Origin of Species,” Darwin gives the following definition of natural selection: “The preservation of beneficial individual differences or changes and the destruction of harmful ones I called natural selection, or the survival of the fittest” (c)-(Darwin Ch. Origin of Species. - M., L.; Selkhozgi, 1937, p. 171). He warns that "selection" should be understood as a metaphor, as a fact of survival, and not as a conscious choice.

So, natural selection is understood as a process constantly occurring in nature, in which the most adapted individuals of each species survive and leave offspring and the less adapted ones die. [show] . The extinction of the unadapted is called elimination.

Consequently, as a result of natural selection, the species that are most adapted to the specific environmental conditions in which their lives take place survive.

Constant changes in environmental conditions over a long period of time cause a variety of individual hereditary changes, which can be neutral, harmful or beneficial. As a result of life competition in nature, there is a constant selective elimination of some individuals and the preferential survival and reproduction of those that, by changing, have acquired useful characteristics. As a result of crossing, a combination of characteristics of two different forms occurs. Thus, from generation to generation, minor useful hereditary changes and their combinations accumulate, which over time become characteristic features of populations, varieties, and species. Moreover, due to the law of correlation, simultaneously with the intensification of adaptive changes in the body, a restructuring of other characteristics also occurs. Selection constantly influences the entire organism, its external and internal organs, their structure and function. This reveals the creative role of selection (see microevolution).

Darwin wrote: “Metaphorically speaking, we can say that natural selection daily, hourly investigates the smallest changes throughout the world, discarding the bad, preserving and adding the good, working silently, invisibly, wherever and whenever the opportunity presents itself, to improve every organic being in relation to the conditions of its life, organic and inorganic" (c)-(Darwin Ch. Origin of Species. - M., Leningrad; Selkhozgi, 1937, p. 174.).

Natural selection is a historical process. Its effect manifests itself after many generations, when subtle individual changes are summed up, combined and become characteristic adaptive characteristics of groups of organisms (populations, species, etc.).

Sexual selection. As a special type of intraspecific natural selection, Darwin identified sexual selection, under the influence of which secondary sexual characteristics are formed (bright colors and various decorations of males of many birds, sexual differences in the development, appearance, behavior of other animals) in the process of active relationships between the sexes of animals, especially during the breeding season .

Darwin distinguished between two types of sexual selection:

1. fight between males for a female
2. active searches, choice of males by females, males only compete with each other in order to excite females, who choose the most attractive males

The results of both types of sexual selection differ. With the first form of selection, strong and healthy offspring appear, well-armed males (the appearance of spurs, horns). During the second, such secondary sexual characteristics of males as the brightness of plumage, characteristics of mating songs, and the smell emitted by the male, which serves to attract a female, are enhanced. Despite the seeming inappropriateness of such traits, since they attract predators, such a male has an increased chance of leaving offspring, which turns out to be beneficial for the species as a whole. The most important result of sexual selection is the appearance of secondary sexual characteristics and associated sexual dimorphism.

Under different circumstances, natural selection can proceed at different rates. Darwin notes circumstances favoring natural selection:

• the number of individuals and their diversity, increasing the likelihood of beneficial changes;
• a fairly high frequency of manifestation of uncertain hereditary changes;
• intensity of reproduction and rate of generation change;
• unrelated crossing, increasing the range of variability in the offspring. Darwin notes that cross-pollination occurs occasionally even among self-pollinating plants;
• isolation of a group of individuals, preventing them from interbreeding with the rest of the organisms of a given population;

  Comparative characteristics of artificial and natural selection
  Comparison indicator	Evolution of cultural forms (artificial selection)	Evolution of natural species (natural selection)
  Material for selection	Individual hereditary variability
  Selective factor	Human	Struggle for existence
  The nature of the selection action	Accumulation of changes in a successive series of generations
  Selection action speed	Acts quickly (methodical selection)	Acts slowly, evolution is gradual
  Selection results	Creation of forms useful to humans; formation of breeds and varieties	Education of adaptations to the environment; formation of species and larger taxa

• wide distribution of the species, since at the boundaries of the range individuals encounter different conditions and natural selection will go in different directions and increase intraspecific diversity.

In its most general form, the scheme of action of natural selection, according to Darwin, comes down to the following. Due to the inherent indefinite variability of all organisms, individuals with new characteristics appear within a species. They differ from ordinary individuals of a given group (species) in their needs. Due to the difference between old and new forms, the struggle for existence leads some of them to elimination. As a rule, less evaded organisms that became intermediate in the process of divergence are eliminated. Intermediate forms find themselves in conditions of intense competition. This means that monotony, which increases competition, is harmful, and evading forms find themselves in a more advantageous position and their numbers increase. The process of divergence (divergence of characteristics) occurs constantly in nature. As a result, new varieties are formed and such separation of varieties ultimately leads to the emergence of new species.

Thus, the evolution of cultural forms occurs under the influence of artificial selection, the components (factors) of which are variability, heredity and human creative activity. The evolution of natural species is carried out thanks to natural selection, the factors of which are variability, heredity and the struggle for existence. Comparative characteristics of these forms of evolution are given in the table.

Darwin's process of speciation

Darwin saw the emergence of new species as a long process of accumulation of beneficial changes, increasing from generation to generation. The scientist took small individual changes as the first steps of speciation. Their accumulation over many generations leads to the formation of varieties, which he considered as steps towards the formation of a new species. The transition from one to another occurs as a result of the cumulative action of natural selection. A variety, according to Darwin, is an emerging species, and a species is a distinct variety.

In the process of evolution, several new ones can arise from one ancestral species. For example, species A, as a result of divergence, can give rise to two new species B and C, which in turn will be the basis for other species (D, E), etc. Of the changed forms, only the most deviated varieties survive and give birth to offspring, each of which again produces a fan of changed forms, and again the most deviated and better adapted survive. Thus, step by step, greater and greater differences arise between extreme forms, finally developing into differences between species, families, etc. The reason for divergence, according to Darwin, is the presence of uncertain variability, intraspecific competition and the multidirectional nature of the action of selection. A new species can also arise as a result of hybridization between two species (A x B).

Thus, C. Darwin in his teaching combines the positive aspects of the doctrine of the species of C. Linnaeus (recognition of the reality of species in nature) and J.-B. Lamarck (recognition of the limitless variability of species) and proves the natural path of their formation on the basis of hereditary variability and selection. They were offered four species criteria - morphological, geographical, ecological and physiological. However, as Darwin pointed out, these characteristics were not sufficient to clearly classify species.

The species is a historical phenomenon; it arises, develops, reaches full development, and then, under changed environmental conditions, disappears, giving way to other species, or itself changes, giving rise to other forms.

Species extinction

Darwin's doctrine of the struggle for existence, natural selection and divergence satisfactorily explains the question of the extinction of species. He showed that in constantly changing environmental conditions, some species, decreasing in number, must inevitably die and give way to others, better adapted to these conditions. Thus, in the process of evolution, the destruction and creation of organic forms are constantly carried out as a necessary condition for development.

The reason for the extinction of species may be various environmental conditions unfavorable for the species, a decrease in the evolutionary plasticity of the species, a lag in the rate of variation of the species or the rate of change in conditions, and narrow specialization. More competitive species displace others, as the fossil record clearly demonstrates.

Assessing Charles Darwin's evolutionary theory, it should be noted that he proved the historical development of living nature, explained the paths of speciation as a natural process, and actually substantiated the formation of adaptations of living systems as a result of natural selection, revealing for the first time their relative nature. Charles Darwin explained the main causes and driving forces of the evolution of plants and animals in culture and the wild. Darwin's teaching was the first materialist theory of the evolution of living things. His theory played a major role in strengthening the historical view of organic nature and largely determined the further development of biology and all natural science.

To clarify the question of what driving forces in the process of evolution lead to the formation of new species, Darwin turned to the study of the phenomena of variability and heredity.

Variability. Variability is the process by which differences arise between individuals of the same species.

Due to variability, even between closely related individuals there are differences. In the offspring of one pair of animals or among plants grown from the seeds of the same fruit, it is impossible to find exactly the same ones. In a herd of sheep of the same breed, an experienced shepherd distinguishes each animal by subtle features: body size, length of legs, head, color, length and density of hair curl, voice, habits.

Let us present some facts of variability that are easy for everyone to observe. The leaves on a birch or other tree appear the same, but if we place any two leaves from the same tree side by side, we will see subtle differences between them. The number of marginal reed flowers in the inflorescences of the golden rod (family Asteraceae) ranges from 5 to 8. The number of petals of the oak anemone (family Ranunculaceae) is 6, sometimes 7 and 8. The branching of the “horns” of the stag beetle and the length of the “whiskers” are variable » longhorned beetle, etc. In a flock of black jackdaws, single specimens of light and even white color sometimes appear.

Forms and causes of variability. Darwin distinguished two main forms of variability: definite (group) and indefinite (individual).

Darwin called certain (group) variability of animals and plants mass variability, when all individuals of a given breed, or variety, or species, under the influence of a certain cause, change in the same way in one direction.

Darwin gave such examples. Cultivated plant varieties lose their qualities when transferred to new conditions. White cabbage, when cultivated in hot countries, does not form a head. Horse breeds brought to the mountains or islands, where the food is not nutritious, become stunted over time. Northern breeds of sheep in hot countries lose their thick wool after several generations.

Darwin called indefinite (individual) variability the appearance of infinitely varied insignificant differences in individual individuals within one variety, one breed, one species. For example, he noted that in peacock pigeons the number of tail feathers varies from 14 to 42. In different individuals of the same breed of pigeons, the shape of the beak, the number of scutes on the fingers, etc., are very variable. In addition to Darwin's examples, one can cite other. Remember the examples of variability indicated on page 22, and say whether they should be classified as definite variability or uncertain. Darwin also noted the facts of very sharp uncertain variability in plants and animals. So, in 1791 in North America, a lamb with very short legs was born from normal sheep.

Yellow and red fruits appear on the same gooseberry branch. Potato tubers have eyes that change color. Facts of bud changes are known in grapes and some fruit trees.

Darwin found that a change in one part of an organism often causes changes in other parts. He called this variability Correlative. Darwin cited a number of interesting facts. Long limbs in animals are almost always accompanied by an elongated neck. Hairless dogs have underdeveloped teeth. Pigeons with long beaks have long legs, and pigeons with short beaks have short legs; pigeons with feathered feet have webbed toes.

In table beet varieties, the color of the root crop, petioles and underside of the leaves changes consistently. In the snapdragon plant, light-colored flower corollas are accompanied by green coloration of the stem and leaves; dark-colored - the dark color of these organs. By achieving a change in one sign, a person involuntarily receives changes in other signs associated with it. The reasons for this kind of facts remained unknown to Darwin and were discovered by science much later.

Darwin considered the main reason for the variability of domestic animals and cultivated plants to be the influence of new living conditions compared to those under which their ancestors existed. Man constantly changes these conditions, as a result of which domestic animals and cultivated plants are characterized by increased variability compared to wild species. Darwin believed that crossing different breeds and varieties also contributed to variability.

The variability of organisms in a natural environment occurs under the influence of the same reasons that cause changes in domestic animals and cultivated plants.

Variability occurs not only during sexual reproduction, but also during vegetative reproduction. Darwin gave examples of the variability of the buds of plants that reproduce vegetatively.

Based on extensive materials, Darwin concluded: variability-- a universal property of organisms.

Heredity. Heredity is the general property of all organisms to preserve and transmit signs of structure and development from ancestors to offspring.

Everyone knows that an oak tree grows from an acorn, and its chicks hatch from a cuckoo’s eggs. From the seeds of cultivated plants of a certain variety, plants of the same variety grow. Animals pass on to their descendants the properties of their breed.

Darwin emphasized that the transmission of traits by inheritance is associated, first of all, with the reproductive reproductive system, which is characterized by exceptional sensitivity to external conditions. But the change can only be revealed in the next generation if it turns out to be hereditary. The influence of the same living conditions can have different effects on different organisms, since their heredity is different.

Heredity is preserved during vegetative propagation. Methods of propagating plants by layering, cuttings, tendrils, and tubers are widely known, and species and varietal characteristics are passed on to the offspring. The growth of poplar, aspen, willow, etc. produces trees and bushes of the same species.

Thus, variability and heredity-- general properties of organisms.

Hereditary and non-hereditary variability. Darwin distinguished between hereditary and non-hereditary variability.

He considered hereditary variability to be indefinite (individual) variability, when changes that once appeared are preserved in subsequent generations.

Darwin considered non-hereditary variability to be certain (group) variability when the changes that have arisen are not preserved in subsequent generations.

In the process of evolution, according to Darwin, only hereditary, individual variability plays a role.

Darwin drew attention to the fact that the laws governing heredity are still unknown. He correctly demonstrated the role of hereditary individual variability in the process of evolution and attracted the attention of scientists to it. Darwin repeatedly emphasized the need for a thorough development of the problem of hereditary variability. Later, this problem became the subject of science - genetics.

So, we ended with the fact that the nature of VARIABILITY and HERITANCE (the two main factors of evolution) is understood in neo-Darwinism (as if “modern Darwinism”, but only as it were) not as it was understood by Darwin, but natural selection (the third factor proposed by Darwin evolution - “provoking” and “directing” changes) is either not recognized at all, or is recognized declaratively (after all, the brand must somehow be consistent), but not at all in the sense in which Darwin understood it.

3.1. Variability and heredity.

Darwin titled his work "The Origin of Species by Means of Natural Selection." But I could call it differently, focusing attention on the nature of variability and heredity - two most important components of the study of biological diversity and two factors of evolution, which, unlike natural selection, were and are accepted by everyone (although they understand “heredity” differently and interpret multi-levelness differently variability) and to which many chapters are devoted in Darwin’s work.

Darwin (and not only him in his time) accepted the slow nature of the “restructuring” of form, i.e. changes in the structure of living organisms, in a long series of generations, from species to species. He denied saltationism (again, he was not alone), since in terms of conformity with the environment, the “great leap” will always be a “finger in the sky” (and the more, the stronger), which is quite logical, IF it is meant “as a firm condition of the problem” , the very need for a very subtle conformity of structure to the habitat (this condition is already controversial). Darwin saw the change in form in the complex interaction of the body's ability to change ("at the output" we have VARIABILITY) and the ability not to change ("at the output" we have HEREDITARY - i.e. inertia in change, retention of the previous state, or in modern language - possession system stability, system memory in relation to each previous state; all this is the same thing, just expressed in different words.

3.2. Definite and indefinite variability.

In some sense, “interaction” (see 3.1) also implies “resistance to variability” (one of the signs of the concept of “heredity” in Darwin’s time). Hegel would call this the “struggle of opposites” and (to generalize) dialectics; otherwise, our system of perception is apparently incapable of perceiving (and language reflecting) the integrity of phenomena and processes in their multidirectional components; we have to classify them as “opposite-struggling” " elements.

We won’t talk about heredity for now (because this is the most difficult question, more complicated than natural selection). Let's talk about variability. Darwin knew nothing about chromosomes and genes, but like other biologists of his time (and also experienced practicing breeders), he knew - from many, many examples - that there is DIFFERENT variability. Firstly, the variability of different species and genera is very different (some have no varieties at all despite a fairly large range) and this difference is reflected in the meaningfulness of breeders’ work with them - it is useless to work with some species, they are not opened at all through “limited crossings” hidden variability.

Young Charles Darwin. Collage with a portrait by G.Richmond.

Secondly, there is DEFINITE variability - when an individual always develops in exactly the same way in some limited range of conditions external to development (for example, in the mountains a dandelion always grows low, and on the plain high; under water, a growing arrow leaf gives one form, another above water) - although the cells from which they develop may be genetic clones. And there is UNDEFINED variability (puppies and kittens of the same litter will all be a little different; we will too), when small deviations from the parental appearance appear and disappear, and there is no end to it, and it was not clear why this happens in each specific case. [It must be said that in terms of the causal algorithm for the development of each visible individual characteristic, this is still not very clear: the decoding of human DNA has not clarified this issue at all. Strict splitting of “symmetrical states” of characters - three to one (3:1) according to Mendel - is not a common case, even almost an exception to the rule, and what is important is not the splitting of alleles (slightly different “symmetrical” pieces of DNA into paired (from dad and mom) chromosomes in the nucleus), - as interpreted in school textbooks and in any STE courses. Alleles do split, but this is often not reflected in traits. Cases of coincidence, as was the case with Mendel’s peas, are interpreted in another system of views (more about it later) only as a particular version of the holistic response of the development system. Mendel, like Darwin, knew nothing about genes as “particles” and fractional sections of DNA; he only understood that there was a certain factor that determined the normalized combination of characteristics.]

Subsequently, these forms of variability, different in nature and dynamics (in one and different generations), were called MODIFICATIONS (the first, modifications of the development norm) and MUTATIONS (= mutant individuals; even in the time of Schmalhausen, “mutation” and “mutant” were used as synonyms; this must be kept in mind when reading Schmalhausen, because it can be confusing; in different cases different things are meant). Darwin devoted a lot of time to the problem of the relationship between modifications and mutations (mutants) in an attempt to understand the tools for changing form. In the “synthesis” of Darwinism and genetics of the early 20th century, this problem disappears, because “mutations” (=mutants; = unstable trajectories of egg development) are recognized as the “alpha and omega” of an evolutionarily significant restructuring of morphology (morphogenesis), and modifications (= already stable trajectories egg development, in some cases linked to signals from the external environment) - were thrown out of consideration as “not playing a role” in evolution. Meanwhile, it is precisely the analysis of the causal relationship between modifications and mutations (both are extreme expressions of the same “hereditary base”, i.e. they are extreme manifestations of the same causal factor - maybe we will get to Schmalhausen, Goldschmidt and Waddington) makes it possible to understand much in the dialectic of the “mechanics of development” of form, which is not reducible to genes (to the DNA molecule), just as the variability of forms is not reducible to the combination of genes. Darwin could not understand this question in his time, and this question is the golden key that opens the door to the “new theater”, on the stage of which an old play called “What controls the change in form in a series of generations?” is performed in a new way? I’ll sleep until I find out.”

Having abandoned modifications as “non-hereditary” (and this is a key mistake in understanding the relationship between “change” and “not change”), the founders of neo-Darwinism (“new Darwinism”) sharply narrowed their capabilities in understanding ontogenesis (embryogenesis), i.e. the entire path of form formation, since they reduced both variability and heredity to the fact of transmission of “magic wands” (genes), making them causally one and the same in two conditional manifestations. It is as if we reduced the perception of a word in a living language to the number and sequence of letters from which this word consists (sometimes it has no meaning at all; French especially “suffers from this”). Heredity has become synonymous with the very fact of transferring “particles” (genes) from parents to offspring, like passing a baton in a race - passed on and “already won” (or lost) upon the fact of the transfer. And now there is no need for a causal explanation of the stability of the form (= heredity), in conditions of never-ending genetic variability (fact) in a long series of generations, including those measured over many millions of years (fact). But this stability is the very phenomenon (of the paleontological record of organisms, including) that should cause the greatest surprise and even difficult feelings about its misunderstanding in any person who decided to read it at night - instead of watching the “evening Urgant” “, - a school textbook on genetics (of the same “old woman Auerbach”), gathering dust in a barn at the dacha. The inability to explain this phenomenon convincingly puts an end to any system of deductive constructions that wants to present itself to the world as a theory of evolution.

Very educated and thoughtful biologists, primarily K. Waddington (in Britain), tried to stop the vigorous march towards the “bright genetic future of humanity” (slightly overshadowed in the 1930s - 1940s by the shadow of eugenics - many geneticists do not like to remember this). and I.I. Shmalhausen (in Russia).

Left - K.H. Waddington (1905 - 1975), right - I.I. Schmalhausen (1884 - 1963). Both are true biologist-thinkers of the mid-20th century, holisticists (systemists), opponents of “corpuscular-genetic” thinking in the understanding of heredity. Independently of each other, the achievements of genetics and embryology in the development of evolutionary ideas were combined into a meaningful whole. The results of this synthesis are known as the doctrine of epigenesis (not to be confused with modern molecular epigenetics - these concepts are consonant and fit well in terms of basic ideas about the complexity of control over the development of the embryo, but still are not the same thing; see also below about R. Goldschmidt). [Interesting from the intersection of destinies: F.G. Dobzhansky married a student and employee of I.I. Shmalhausen; I spent my entire American life working in Morgan’s laboratory, where C. Waddington also began his activities. Both, it turns out, took different things from Morgan’s thoughts (in general, Morgan, in a number of his statements, it seems to me, was no less an epigeneticist than Waddington) ].

Of course, not only them (we remember, knowledge is a systemic process; there were Gurvich, and Kamshilov, and many others only in our country. The very serious geneticist Goldschmidt (in Germany) immediately realized that “the process has begun” and it cannot be stopped, and accepted everything as an “accomplished inevitability”, stopping even to explain to his colleagues the meaning of his concept of “systemic mutations” - but wrote a couple of significant lines in his justification for future generations). As a result, the development of evolutionary theory, precisely on the path of a real synthesis of the ideas of Darwin (but also of his opponents! - there is no other way) and molecular biology, as well as embryology, histology, cytology (sciences from the cycle of “developmental biology”) was delayed almost century, and only in our time they started talking loudly again about the fact that there was no synthesis (well, “there was no boy” - only an illusion), but that such a synthesis is possible and necessary. Fortunately, the factual and theoretical heritage of Waddington, Schmalhausen and Goldschmidt was not lost, and there remained people who were still able to understand their language and their wise thoughts. These people, although they themselves are already aged, held the rope (again in Britain and Russia, but not only) and preserved the possibility and even the probability for a “new”, or rather a long-promised, but not accomplished in the 20th century, synthesis.

Richard Goldschmidt (1878 - 1958) - German and American geneticist, one of the “synthesizers” of the achievements of genetics and embryology (see Waddington and Schmalhausen above). The founder of the doctrine of systemic mutations, which was ambiguously understood by his followers. Having become convinced from his work with Drosophila that ordinary intraspecific genetic variability and selection, understood as a sieve for mutations, cannot in itself lead to the formation of a new taxon (species, etc.), unlike Waddington and Schmalhausen (who were independently convinced in the same), abandoned altogether the role of natural selection as a factor in taxon formation, and translated the idea of ​​systemic mutations into the idea of ​​macromutations, which immediately lead to the emergence of a new taxon (this is set out in his book “Material Foundations of Evolution” (1940) , mainly devoted to the genetic causes of macroevolution. Goldschmidt's ideas were picked up by the modern founders of Evo Devo. It is interesting that three powerfully thinking scientists (U, Sh, G), with pronounced synthetic thinking, at the peak of the crisis of their previous ideas about genetic evolution, “chose” different ways of “coming out of the crisis” with different understanding of the systematic nature of embryogenesis (but all three increased the level of understanding of the systematic nature of control in comparison with the axiomatics on which the “genetic theory of evolution” (= STE) was formed). Some of Goldschmidt's famous sayings are: “The facts of genetics may, of course, be described in terms of genes, but the theory of germ plasm must free itself entirely from the concept of genes as units” (Goldschmidt, 1938, p. 311). “For many geneticists it is clearly difficult to think in such terms, since most of them are so bound by an axiomatic belief in the atomistic gene theory that they are unable to think otherwise...” (Goldschmidt, 1940, p. 218) (written back in 1938-1940!! !).

What Darwin wrote about and how he thought is no one’s business anymore, but yet it’s still interesting (well, it’s interesting!) how he thought, especially against the backdrop of how other smart and intelligent people thought and understood the same facts (!) knowledgeable biologists of his time. So, at the end of this “lesson,” let’s at least briefly get acquainted with how these same scientists opposed Darwin on the issue of his understanding of the nature and patterns of VARIABILITY in animals and plants (based on the work of N.Ya. Danilevsky, 1885). I will not be a judge in this matter (from the position of the level of knowledge and facts of modern biology), and not in all cases it is possible to say so confidently who is right and who is wrong. What is more interesting is the train of thought itself and the clear vision of those “other equal conditions” (“conditions of the problem”), without the acceptance of which and without the implementation of which, a seemingly beautiful and correct formulation still “does not work,” that is, cannot be realized in nature. But sometimes I will still allow myself accompanying comments.

3.3. Criticism of Darwin's understanding of the properties and scope of variability by his contemporaries(based on the work of N.Ya. Danilevsky, 1885).

1. Darwin, transferring the logic of artificial selection to nature, -
(A) accepted that variability in domestic animals and cultivated plants was comparable to that in the wild, and (B) believed that wild forms can change indefinitely in some direction. He was objected to (including Danilevsky, a specialist in the theory and practice of selection:
- the scope of variability in domesticated forms cannot be correctly transferred to wild forms.
- the variability of varieties “fluctuates” within certain boundaries and nothing more.
- No (1) known facts and no (2) inferences from known facts show that in the natural state the changes of organisms ever cross the boundary of species.

As for (1), the question hangs in the air to this day (even though the domestic cat and dog were called special species in Latin, they would not have become such for Danilevsky and Darwin, since both cats and dogs interbreed freely with "wild progenitors" and produce fertile offspring (this, as a criterion for the species, was accepted even then). But still, there were direct experiments (in our time, for example, Shaposhnikov's experiments with aphids), when "transferred" to other food plants groups of aphids have almost reached the reproductive break, i.e., non-crossbreeding (the experiments, in my opinion, were not completed).

As for (2), this is debatable. Many series of the fossil record, traced along the time scale in nearby layers ("known facts") allow us to draw "conclusions" (at the level of simple common sense, - according to the direction and continuity of changes from more ancient to younger layers, traced step by step), that some species (and genera) were transformed over time into others and therefore “the changes crossed the species boundary” (i.e. such a transition is possible, in principle). With regard to families and orders, and even more so classes and types, such a clear picture of the transition in the chronicle is usually not possible to identify (but with reservations: for example, the transition of therapsids into mammals, by several parallel lines, was traced in great detail and looks convincing in the picture) . In any case, the absence of such a clear picture does not mean that this could not have happened (the literature of discussion on the issue is enormous); on the other hand, the presence of a clear picture of a “normalized” transition (the same therapsids into mammals) in itself can be interpreted “against Darwin’s ideas”, for example, as evidence of nomogenesis (development according to some existing “internal law”, including expediency inherent in organisms, i.e. “the desire to realize a certain goal in generations”).

2. Darwin himself accepted: in order for species to change as he thought (slowly, through barely noticeable deviations in structure), it is necessary that variability have the following properties: gradualism, uncertainty, boundlessness, mosaic.
- Opponents pointed out to Darwin that the observed variability is often normalized (remember the same Vavilov homological series that we know about now): it does not occur “in all possible directions,” but follows some direction (remember geographic clines). And if so, then the organism changes not by chance, but according to some “norm of development.” - Again, a lot of literature is devoted to this issue (and many copies have been broken). It should be noted that the worst way to explain normalization in changes in phenotypic traits is within the framework of the genetic theory of evolution (neo-Darwinism, STE), where all variability is reduced to genetic variability (although the latter is conditionally discrete, it is very fractional and in this regard chaotic, limitless." In the so-called epigenetic theory of evolution, the founders of which - in essence, their doctrine of the mechanism of change in form - are Waddington and Schmalhausen (and partly Goldschmidt), normalization is easily explainable: the transition from changes in genes to developmental deviations in ontogenesis is a sharp reduction in "degrees" freedom" - variability becomes two orders of magnitude more discrete and takes on the appearance (image) of a river bed with a limited number of channels, and the channels have different depths, where depth is the probability of the trait occurring in ontogenesis: the trait is, as it were, linked to the movement of water along a given channel in a dry period: there is little water, - small channels will not be flooded (aberration variability), deep channels are the main development paths, creodes).

The Ussuri rivers flowing through the virgin forest in many channels (the Bikin River in the frame) are a good image of Waddington's epigenetic landscape model.

The most interesting point is the requirement for “mosaic” variability. By mosaic, Danilevsky meant the unlinkedness of signs, namely, that changes in individual organs should (for everything to be “according to Darwin”) occur independently and not simultaneously (if everything is actually the other way around, this will be “corresponding variability” in Timiryazev’s translation) . Here Danilevsky essentially repeats the philosopher Spencer’s counter-arguments to Darwin, to which, according to Schmalhausen, Darwin was never able to give a proper answer (and was greatly upset by them). The essence of the objection is that the organism (at all levels - from cells to tissues and organs) is a single whole in its development (the doctrine of Severtsov-Schmalhausen correlations), and if there are countless relationships between all elements of the “hardware” and the “interface” (contact-spatial and functional), then almost all “urges” for changes in development will be blocked by development itself (this is what is now called autoregulation, self-assembly), and what does “pass” will be more determined precisely by the very structure of the whole ( where it allows some kind of play - those very “channels in the river valley and their depth - see the picture above) than by the original “urges”, be it the requirements of the environment (violations of the “external umwelt”) or mutations, recombinations and other changes genome (violations of the “internal umwelt”) (umwelt is “world”).

“No adaptation could appear if it contradicted this connection, and in it, therefore, the main mystery of the various forms of organisms would lie. In a word, a norm would again appear instead of chance.”(Danilevsky, 1885).

The historical vicissitudes of resolving this issue are amazing. Neo-Darwinism removed it from the agenda, because, having absolutized the level of genetic variability and accepted de facto a linear, specific connection of special fractional genes with special fractional traits (the organism is literally represented as a huge puzzle), allowed a priori an infinite mosaic of variability, i.e. independence of changing individual characteristics (with a bunch of “amendments to the constitution” in each specific inconvenient case). The epigenetic theory accepts Spencer’s arguments, and Schmalhausen and Waddington already remove the contradiction in the concept of correlative development identified by Danilevsky through a visual model of the epigenetic landscape (the very river beds), but considers (the theory) this solution to the question to be “Darwinian” (which is hardly entirely correct, although this can probably be considered a development of Darwin’s views in the most general terms). But morphologists and paleontologists who deal with variability at the level of macrotaxa are terribly uncomfortable in the condition of a “correlative whole” (there are few degrees of freedom in the possibilities of inferring “one from the other” in articles - in macro-morphology when constructing specific phylogenies) and therefore they have always looked for and will clutch at any straw that will allow them to get rid of the bonds of correlative connections of the whole in development. Now such a straw has turned out to be the idea of ​​“independent modes of development”, developed in the bosom of EvoDevo on the basis of “new” experimental embryology. Therefore, paleontologists and macro-morphologists will inevitably love Evo Devo (and many already do), inevitably treating its axiomatics uncritically.
With this I will end this long “lesson”. We'll continue next time (maybe tomorrow - the weather is bad, so the reading hut won't be empty).

C. Darwin established that between individuals of the same species there are always both clearly noticeable differences (sports) and subtle ones. Charles Darwin believed that it was the small differences between individuals that provided the material for selection.

Charles Darwin called these differences variability and identified several forms of variability: definite, indefinite, combinative and correlative.

Non-hereditary variability is certain, since by changing the conditions of development of organisms, it is possible to predict the direction of variability. At the same time, she is also group, since the entire group of individuals subjected to the same change in development conditions changes in the same direction. [In the 20th century, such variability was called modification for a long time.]

For example, if a group of piglets of the same breed is raised in good conditions, then after six months they will all be characterized by similar features: large weight (about 200 kg), elongated body, shortened limbs, poorly developed coat, calm behavior, good appetite. If a group of piglets of the same breed is raised in poor conditions, then the adults will also be similar to each other: low weight (about 50 kg), short body, elongated limbs, highly developed coat, angry disposition, poor appetite.

Hereditary variability exactly the opposite of non-hereditary. This variability is uncertain. For example, we cannot predict in advance: when and in what herd a sheep with sharply shortened limbs will appear. Hereditary variability is individual: a change in a trait is observed in only one individual out of many. [In the 20th century, this form of variability was long called mutational.]

C. Darwin contrasted hereditary variability with non-hereditary variability and believed that only hereditary variability leads to selection: “Non-hereditary change is unimportant for us.”

Correlative variability. An example with a change in a whole complex of traits in piglets shows that when development conditions change, not just one trait changes, but a whole complex of traits. The simultaneous change of several characteristics was called by Charles Darwin correlative, or correlative variability.

Hereditary variability can also be correlative: a change in one trait entails a change in other traits. For example, all white, blue-eyed cats are deaf; hairless dogs of a certain breed have underdeveloped teeth. [It has now been established that hereditary correlative variability can be caused by the multiple actions of genes, as well as by the linkage of genes - true linkage or quasi-linkage.]

Combinative variability. In Darwin's time (even before Mendel's experiments), it was known that different hereditary traits can form different combinations (for example, various combinations of color and hair length in animals). Charles Darwin called independent variability of characters combinative variability.

[It has now been established that correlative variability is often combined with combinative variability, i.e. joint variability of two or more characteristics is often of a combinative-correlative nature.]

The lack of genetic theory in the 19th century led Charles Darwin to some erroneous ideas about heredity. For example, C. Darwin believed that the degree of manifestation of traits in offspring is equal to the arithmetic mean of this trait in parents. This premise (“Jenkins' nightmare”) was a serious obstacle to the further development of Darwinism.

In addition, Charles Darwin admitted the possibility of inheritance of acquired characteristics, supporting the theory of pangenesis. According to this theory, there are special particles in the blood - gemmules(literally “rudiments”, “buds”), which carry information from all parts of the body to the gonads. As a result, information about the ontogenesis of an individual can be transmitted through the blood to the germ cells. Note that Darwin's gemmules are similar in their properties to Lamarck's fluids. Charles Darwin's Lamarckism was also manifested in the partial recognition of the law of exercise and non-exercise. For example, Charles Darwin explained the reduction of the organs of vision in troglobionts by the uselessness of this feature in the dark.

In 1900, the laws of I.G. were rediscovered. Mendel, the formation of ideas about the discrete nature of heredity and variability. Thus, the “Jenkins nightmare” was eliminated. In 1901, G. de Vries created the mutation theory. By 1908, basic ideas about the genetic structure of populations were formed (Hardy-Weinberg law). The doctrine of heredity has not yet been completed. However, modern genetics claims that any trait is inherited to one degree or another. The evolutionary role of non-hereditary changes (modifications) cannot be denied. “Everything that is not hereditary in its occurrence is connected with the hereditary. The modification of an organism is always determined by its hereditary structure” (Schmalhausen).

Questions in ecology, evolutionary theory, population biology, and other sciences often touch on the concept of variation (both defined and undefined). to understand their ability to adapt to changing environmental conditions. These principles underlie modern breeding and molecular biology. Let's try to figure out what these concepts mean.

Types of variability

These concepts are also called non-hereditary and hereditary variability. What is the difference between definite and indefinite variability? The first occurs in a group of individuals as a response to external factors. It is regulated by the value of the reaction norm. As an example, we can recall the hibernation of a bear, the thickness of a dog’s fur, or the length of a dandelion stem. If environmental conditions change, these signs may not appear. So, if you artificially create an abundance of food and warm temperatures all year round, the bear will not sleep through the winter. A dog that lives inside a house in winter will have much less undercoat than a chained yard dog. With constant mowing of the lawn, the dandelion will grow with a shorter stem length, which will allow it to form a peduncle and avoid cutting. Of course, such traits are not inherited genetically.

Hereditary variability occurs within a group of individuals and is inherited through generations. However, not all mutations are beneficial. Most of them become useless or harmful. Only some changes will be supported by natural selection. This property is the basis of evolution, as it allows the body to adapt to changes in the environment and acquire qualities that promote survival. Let's take a closer look at this type.

History of the study of uncertain variability

When mentioning the factors influencing the origin of species, one cannot fail to mention the author of the book of the same name and the theory of evolution, Charles Darwin. Of course, at the moment this theory has been refined and is called synthetic. However, the description of the basic concepts and principle remained unchanged.

According to Darwin, indefinite variability - these are “the infinitely varied and insignificant peculiarities which distinguish individuals of the same species and which cannot be explained by inheritance from one of the parents or from more distant ancestors.” He also spoke about the direct and indirect influence of living conditions on the formation of a living organism, about the correlation of characteristics. At the same time, the concept of a gene did not yet exist, and the reasons for the appearance of these features were not clear to this scientist. It is now known that inheritance is genetic in nature, and mutations occur in DNA constantly.

How does this mechanism work?

Errors constantly occur during DNA replication. Normally, they should be removed by the immune system or the cellular apoptosis (programmed death) system. If these systems fail, this cell can survive and create copies of itself. If the organism is single-celled or the changes affected the germ cells, this defect will be inherited and passed on to other generations. This creates population diversity and guarantees the survival of the species and evolution as a whole.

Types of mutations

Meaning

Definition: what is uncertain variability in biology

Summarizing the above, let us summarize what this concept means in science. Uncertain variability in biology is a concept synonymous with mutations. It is hereditary in nature (as opposed to definite), with minor changes in the genome in the first generation accumulating and intensifying in subsequent ones. This type of variability is also associated with changes in environmental factors, but not in the form of adaptations, but indirectly. Thus, it helps to adapt not to a specific organism, but to the taxon as a whole.

Examples of Uncertain Variability

Earlier in the article, specific examples of mutations that help adapt to the environment were discussed. Let us consider several broad types of such variability in nature:

Summarizing

This type of variability does not guarantee the survival of the organism, but ensures the survival of the species in constantly changing environmental conditions. Uncertain variability is necessary for humans as a tool for breeding work. It promotes the natural and artificial origin of new taxa. This is why indefinite variability represents the basis of evolution.