Charles Darwin comes down to a line of logical propositions confirmed by experiments and other research. Thus, he proved that all types of living organisms are characterized by individual hereditary variability for any characteristics; they all multiply exponentially; within species there is a struggle for existence due to limited vital resources; In this struggle, only adapted individuals survive and further reproduce.
3. Natural selection - identifies the mechanism of survival of units with the necessary hereditary changes and their further reproduction. Selection is the result of the struggle for existence. The following mechanisms are distinguished:
a) formation of hereditary changes;
b) survival and preservation of individuals with these changes in the corresponding habitat;
c) the reproduction of these units, the growth of their numbers and the spread of useful hereditary changes.
The driving forces of evolution, interacting with each other, make it possible to explain the formation of other species in nature. Materials accumulated in different branches of biology have a logical conclusion only when they correspond to the principle of evolution.
The great merit of Charles Darwin lies in explaining the process of development and formation of species. It was this fact that made Darwin's evolutionary theory a generally accepted theory.
The preconditions of evolution by themselves cannot lead to evolution. For the evolutionary process to occur, leading to the appearance of adaptations and the formation of new species and other taxa, the driving forces of evolution are necessary. Currently, the doctrine created by Darwin about the driving forces of evolution (the struggle for existence and natural selection) has been supplemented with new facts thanks to the achievements of modern genetics and ecology.
The struggle for existence and its forms
According to the concepts of modern ecology, individuals of the same species are united in populations, and populations of different species exist in certain ecosystems. The relationships of individuals within populations and with individuals of populations of other species, as well as with environmental conditions in ecosystems, are considered as struggle for existence.
Darwin believed that the struggle for existence is the result of species multiplying exponentially and the emergence of an excess number of individuals with limited food resources. That is, the word “fight” essentially meant competition for food in conditions of overpopulation. According to modern ideas, elements of the struggle for existence can be any relationship - both competitive and mutually beneficial (caring for offspring, mutual assistance). Overpopulation is not a necessary condition for the struggle for existence. Consequently, at present the struggle for existence is understood more broadly than according to Darwin, and is not reduced to competitive struggle in the literal sense of the word.
There are two main forms of struggle for existence: direct struggle and indirect struggle.
Direct fight- any relationship in which there is physical contact to one degree or another between individuals of the same or different species within their populations. The consequences of this struggle can be very different for the interacting parties. Direct struggle can be either intraspecific or interspecific.
Examples of straight intraspecific struggle There may be: competition between families of rooks for nesting sites, between wolves for prey, between males for territory. This is also feeding the young with milk in mammals, mutual assistance in building nests in birds, protection from enemies, etc.
Indirect struggle- any relationship between individuals of different populations that use common food resources, territory, environmental conditions without direct contact with each other. Indirect control can be intraspecific, interspecific and with abiotic environmental factors.
Examples of indirect control could be the relationship between individual birch trees in a dense birch grove ( intraspecific struggle), between polar bears and arctic foxes, lions and hyenas for prey, light-loving and shade-loving plants ( interspecific struggle). Also, an indirect struggle is the different resistance of plants to the supply of soil with moisture and minerals, and of animals to temperature conditions ( combating abiotic environmental factors).
The result of the struggle for existence is the success or failure of these individuals in surviving and leaving offspring, i.e. natural selection, as well as changes in territories, changes in environmental needs, etc.
Natural selection and its forms
According to Darwin, natural selection is expressed in the preferential survival and leaving of offspring by the most fit individuals and the death of the less fit. Modern genetics has expanded this idea. The diversity of genotypes in populations, arising as a result of the preconditions of evolution, leads to the appearance of phenotypic differences between individuals. As a result of the struggle for existence in each population, individuals with phenotypes and genotypes useful in the given environment survive and leave offspring. Consequently, the action of selection is the differentiation (selective preservation) of phenotypes and the reproduction of adaptive genotypes. Since selection occurs according to phenotypes, this determines the significance phenotypic (modification) variability in evolution. Diversity modifications influences the degree of diversity of phenotypes analyzed by natural selection and allows the species to survive in changing environmental conditions. However, modification variability cannot be a prerequisite for evolution, since it does not affect the gene pool of the population.
Natural selection is a directed historical process of differentiation (selective preservation) of phenotypes and reproduction of adaptive genotypes in populations.
Depending on the environmental conditions of populations in nature, two main forms of natural selection can be observed: driving and stabilizing.
Driving selection operates in environmental conditions gradually changing in a certain direction. It preserves useful deviant phenotypes and removes old and useless deviant phenotypes. In this case, there is a shift in the average value of the reaction norm of the characteristics and a shift in their variation curve in a specific direction without changing its limits.
If selection acts in this way in a series of generations (F 1 → F 2 → F 3), then it leads to the formation of a new norm of reaction of characters. It does not overlap with the previous reaction norm. As a result, new adaptive genotypes are formed in the population. This is the reason for the gradual transformation of the population into a new species. It was this form of selection that Darwin considered the driving force of evolution.
Stabilizing selection operates under constant and optimal environmental conditions for populations. It maintains the same phenotype and removes any phenotypes that deviate from it. In this case, the average value of the reaction norm of the traits does not change, but the limits of their variation curve are narrowed. Consequently, the genotypic and phenotypic diversity that arises as a result of the preconditions of evolution is reduced. This helps to consolidate previous genotypes and preserve the existing species. The result of this form of selection is the current existence of ancient ( relict) organisms. Relic(from lat. relictum- remainder) kinds- living organisms preserved in modern flora and fauna or in a certain region as a remnant of an ancestral group. In past geological eras they were widespread and played a large role in ecosystems.
The driving forces of evolution are natural selection and the struggle for existence. There are two forms of struggle for existence: direct and indirect struggle. There are two main forms of natural selection in nature: driving and stabilizing.
The modern theory of organic evolution differs from Darwin’s in a number of important points:
She clearly highlights elementary structure, from which evolution begins. Currently, such a structure is considered to be a population, and not an individual or species, which includes several populations;
Modern theory considers sustainable change as an elementary phenomenon or process of evolution genotype populations;
It interprets the factors and driving forces of evolution more broadly and deeply, distinguishing between them the main and non-basic factors.
Charles Darwin and subsequent theorists considered variability, heredity and the struggle for existence to be the main factors of evolution. Currently, many other additional, non-basic factors are added to them, which, nevertheless, influence the evolutionary process. The main factors themselves are now understood in a new way and, accordingly, the leading factors now include mutation processes, population waves and isolation. Before moving on to their characteristics, we note that the difficulties that Charles Darwin encountered in explaining the hereditary transmission of useful traits to offspring can easily be overcome with the power of those laws of heredity that were established by the Austrian scientist Gregor Mendel (1822-1884). Indeed, one of his laws states that individual hereditary characteristics of parents do not merge during crossing, but are transmitted to the offspring in their original form. Therefore, there is no “dissolution”* of the hereditary substance that critics spoke about| C. Darwin, doesn't actually happen. Further! These ideas were developed in the interpretation of the processes of change and heredity in the modern theory of evolution.
Mutations are those hereditary changes that either separately or jointly determine changes in the properties, characteristics, characteristics or norms of reaction of organisms. Taken together, they represent what Charles Darwin called individual or indeterminate variability. Since mutations occur randomly, their outcome is truly uncertain. However, random change becomes necessary, when it turns out useful for the body, helps it survive in the struggle for existence. Fixed and repeated over a number of generations, such random changes cause restructuring in the structure of living organisms and their populations and thus lead to the emergence of new species.
Although mutations are the main suppliers of evolutionary material, they are random changes that obey probabilistic, or statistical, laws. Therefore, they cannot serve as a guiding force in the evolutionary process. True, some scientists consider the mutation process as the determining force of evolution, forgetting that in this case it is necessary to recognize the initial usefulness and suitability of all random changes that arise, which contradicts observations in living nature and the practice of selection. In reality, other than selection - natural or artificial - there is no other means of regulating hereditary variability. Only random changes that prove beneficial under certain environmental conditions are are selected in nature or artificially by man for further evolution.
It has been established that small and numerous populations are not favorable for the evolution and emergence of new forms of living organisms. In large populations, new hereditary changes are much more difficult to manifest, and in small populations such changes are subject to the influence of random processes. Therefore, the most suitable for the evolution and emergence of new species are medium-sized populations, in which the number of individuals constantly changes.
This feature was also pointed out by Charles Darwin, who believed that in order to form a new species, a certain group of organisms of the old species must separate, but he could not explain the need for this requirement from the point of view of heredity. It has now been established that the separation and isolation of a certain group of organisms is necessary so that it cannot interbreed with other species and thereby transmit to them and receive genetic information from them. Isolation of various groups of organisms in nature, as well as in the practice of selection, is carried out in a variety of ways, but their goal is the same - to exclude the exchange of genetic information with other species. This can be achieved by a geographic boundary (insurmountable water environment, swamps, high mountains, etc.), environmental conditions (preferences in choosing an ecological niche or habitat), different mating periods, behavioral characteristics of different groups and species of organisms, and much more.
To these main factors of evolution, the frequency of generational changes in populations, the pace and nature of mutation processes, and some others are often added. It should be emphasized that all of the listed basic and non-basic factors do not act in isolation, but in interconnection and interaction with each other.
The most important thing is that although all the factors of evolution are its necessary prerequisites, by themselves, neither individually nor collectively, they can explain the mechanism of the evolutionary process and its driving force. This force lies in the action of natural selection, which results from the interaction of populations and their environment. Populations constitute elementary objects for selection, and the environment limits the possibilities of such selection, since the potential for reproduction is extremely high, characterized by geometric progression, and the food, territorial, geographical, climatic and ecological capabilities of the environment are very limited. It is the struggle of such opposing tendencies as, on the one hand, the desire to preserve life and reproduction, and on the other, the influence of the external environment aimed at limiting reproduction, that constitute the internally contradictory content of the evolutionary process.
Internal contradictions at different levels of organization of living systems constitute the source of their development and determine the nature of the “struggle for existence.” At the population level, these contradictions appear in the form of unity and struggle of individuals within a population, at the species level - the unity of the populations that make up the species, and at the same time, competition between them, which can lead to the formation first of varieties, and then of a new species. The result of this complex process is the elimination from reproduction of individual organisms, populations, species and other levels of organization of living systems. Natural selection is often characterized as a process survival of the fittest organisms. This formulation was first used by the famous English philosopher Herbert Spencer (1820-1903), from whom Charles Darwin himself borrowed it. Subsequently, it became widespread among biologists.
If you think about it, such a characteristic cannot be considered correct, since the expression “adaptability” allows for different degrees, verbally defined using the terms “greater or lesser adaptability.” Indeed, how can one assess which species is more adapted to the conditions of existence, for example, an elephant or a tiger? In addition, even with a lower degree of adaptation, the possibility of reproduction is allowed. In contrast to this, elimination, or elimination from reproduction, has an unambiguous meaning and precisely determines the result of natural selection. After all, the results of natural selection can only be judged retrospectively, that is, in hindsight. This is why English biologist Julian Huxley (1887-1975) recommends using the term “destruction of the unadapted” instead of the term “survival of the fit.” However, natural selection is not only negative, but also creative. In fact, through such selection not only old forms of life are eliminated, but new, more perfect forms are created.
The modern theory of evolution also reveals specific types of natural selection mechanisms:
At stabilizing selection all noticeable deviations from some average norm are eliminated, as a result of which new species do not arise. Such selection plays a minor role in evolution, since it preserves already established forms of living organisms, including such ancient ones as, for example, lobe-finned fish.
leading (driving) form of selection is one that picks up the smallest changes that contribute to progressive transformations of living systems and the emergence of new, more advanced species;
At destructive selection, which usually occurs when there is a sharp change in the living conditions of organisms, a large group of individuals of the average type finds themselves in unfavorable conditions and dies;
It is more complex balancing selection, when it comes to the existence and change of adaptive, or adaptive, forms.
When selecting from increased variability selection advantage goes to those populations that are most diverse in certain traits.
It should be noted, however, that the listed types of selection are very rarely found in their “pure” form. As a rule, complex types of selection are observed in living nature, and special efforts are required to isolate simpler types from them.
Prerequisites and driving forces of evolution according to Charles Darwin
The concept of variability and its forms. 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). Darwin, analyzing the forms of variability, identified three among them: definite, indefinite and correlative.
Certain, or group, variability - this 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 non-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. it is 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.
At correlative or correlative, variability a change in 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 this selection artificial). Variation is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.
Struggle for existence. The evolution of species in nature, according to Darwin, is determined by factors similar to those that determine the evolution of cultural forms.
Darwin considered the explanation of the historical variability of species possible only through revealing the reasons for adaptability to certain conditions. He came to the conclusion that the fitness of natural species, as well as cultural forms, is the result of selection, which was carried out not by man, but by environmental conditions.
How does natural selection work? Darwin considers one of its most important conditions in the natural environment to be overpopulation of species, which arises as a consequence of the geometric progression of reproduction. Darwin noticed that individuals of species that produce even relatively few actual offspring end up reproducing quite intensively. For example, the roundworm produces up to 200 thousand eggs per day, the female perch lays 200-300 eggs, and the cod produces up to 10 million eggs. The same can be observed in plants: one sow thistle plant produces up to 19 thousand seeds, shepherd’s purse - more than 70 thousand, broomrape - 143 thousand, henbane - more than 400 thousand, etc. Even an elephant, bringing not more than six cubs can give rise to a generation that in 750 years will amount to 19 million individuals. Thus, the fertility of organisms as a whole is very high, but in fact in nature the number of individuals of any species of animals and plants that could be expected is never observed. A significant part of the offspring die for various reasons. Darwin concludes that overpopulation is the main (though not the only) reason for the struggle for existence between organisms. He puts a broad and metaphorical meaning into the concept of “struggle for existence.” In The Origin of Species, Darwin writes: “I must warn that I use this term in a broad and metaphorical sense, including here the dependence of one creature on another, and also including (more importantly) not only the life of one individual, but also its success in leaving behind offspring."
The struggle of organisms occurs both among themselves and with the physicochemical conditions of the environment. It is in the nature of direct clashes between organisms or, more often observed, indirect conflicts. Competing organisms may not even come into contact with each other and nevertheless be in a state of fierce struggle (for example, spruce and wood sorrel growing under it).
Among the factors that limit the number of species (this means causing a struggle for existence), Darwin includes the amount of food, the presence of predators, various diseases and unfavorable climatic conditions. These factors can influence species abundance directly and indirectly through a chain of complex relationships. Mutual contradictions between organisms play a very important role in limiting the number of species. For example, sprouted seeds most often die because they sprouted on soil that is already densely overgrown with other plants. These contradictions become especially acute in cases where organisms have similar needs and a similar organization. Therefore, the struggle for existence between species of the same genus is more severe than between species of different genera. Even more intense are the contradictions between individuals of the same species (intraspecific struggle).
The natural result of contradictions between organisms and the external environment is the extermination of some individuals of a species (elimination). The struggle for existence, therefore, is the eliminating factor.
If some of the individuals of each species die in the struggle for existence, then the rest are able to overcome unfavorable conditions. The question arises: why alone individuals die while others survive?
In each individual case the reasons are different. But this phenomenon is subject to general laws. As a result of the constantly occurring variability of individuals in the population of each species, heterogeneity arises, the consequence of which is the inequality of individuals in relation to the environment, i.e., their biological diversity. Thus, some individuals or their groups are more suitable to the environment than others, which ensures their success in the struggle for existence. As a result, individuals that are most suitable for the environment (the most adapted) survive, while those less adapted die.
Selection occurs continuously over an endless 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:
- Variation is common to every group of animals and plants, and organisms differ from each other in many ways.
- 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 a species survived and reproduced, they would soon replace all other species on the globe.
- As 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.
- 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.
- 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 dissimilar with your ancestors, that their it would be advisable separate into a separate type.
It may also happen that some members of a given group of individuals acquire certain changes and find themselves adapted to the environment in one way, while other members, possessing a different set of changes, turn out to 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.
Sexual selection. Natural selection, which constantly occurs in nature, usually leads to the survival of the most adapted individuals under given conditions and their successful reproduction. They become the most adaptable thanks to the appearance of small useful changes that allow them to win the struggle for existence. In the future, such beneficial changes can intensify, accumulate and combine. At the same time, as C. Darwin notes, natural selection does not always lead to the death of less adapted individuals. An example is a special form of selection - sexual selection.
Sexual selection determined by the competition of individuals of one sex during the mating season for mating with individuals of the other sex. Darwin distinguishes two forms of sexual selection: 1) when females remain passive and direct competition between males occurs; 2) when males do not engage in direct struggle with each other, but “compete” with the goal of “charming” females who choose the most attractive males.
Tournament fights are typical for many species of animals: moose, deer, seals, birds, insects, etc. At the grouse current, the winning rooster dominates the majority of the females. This form of sexual selection, when weak, sick males are excluded from reproduction, leads to the appearance of healthy and strong offspring, which contributes to the prosperity of the species.
With the second form of selection, there is an increase in secondary sexual characteristics of males; bright plumage, loud mating songs, smells, etc. As a result of such selection, males of many species become especially noticeable, which puts them in a seemingly dangerous position in difficult living conditions. However, this is probably aimed at stimulating the reproductive processes (such males will be more likely to be noticed and chosen by females) and at diverting predators and enemies from the plain females responsible for reproducing and raising the young. Thus, this form of sexual selection leads to the accumulation of negative traits for individual individuals, but has great positive significance for the species as a whole: it increases the chances of males to leave offspring and contributes (even at the cost of life) to the preservation of the majority of females.
Associated with the processes of sexual selection is the phenomenon of sexual dimorphism - a more or less sharp discrepancy in the characteristics of males and females.
Source : ON THE. Lemeza L.V. Kamlyuk N.D. Lisov "A manual on biology for those entering universities"
Evolution is a biological factor. It concerns all changes in the system of living organisms that have occurred during the life of our planet. All manifestations of evolution occur under the influence of certain factors. Which ones have the biggest impact, and how does it manifest itself? Let's consider the main factors of evolution.
1. One of them is heredity. This is the ability to copy from generation to generation certain properties of the body that relate to metabolism or other features of individual development as a whole. This guiding factor of evolution is carried out due to the self-reproduction of gene units that accumulate in the structure, namely, in the chromosomes and cytoplasm. These genes are decisive in ensuring the constancy and species diversity of various life forms. Heredity is considered the main factor that forms the foundation of the evolution of all living nature.
2. Variability, in contrast to the first factor, is the manifestation in living organisms of various signs and properties that do not depend on family ties. This property is characteristic of all individuals. It is divided into the following categories: hereditary and non-hereditary, group and individual, directed and non-directed, qualitative and quantitative. Hereditary variability is a consequence of mutations, and non-hereditary variability is the influence of evolution; heredity and variability can be called decisive in this process.
3. The struggle for existence. It determines the relationships between living organisms or the influence of abiotic traits on them. As a result of this process, organisms that are weaker die. Those that have higher viability rates remain.
4. It is a consequence of the previous factor. This is a process by which the survival of the strongest individuals occurs. The essence of natural selection is the transformation of populations. As a result, new types of living organisms appear. It can be called one of the engines of evolution. Like many other factors of evolution, it was discovered by Charles Darwin.
5. Adaptability. This includes features of body structure, coloring, behavior patterns, methods of raising offspring and much more. There are a lot of these factors, so they have not yet been fully studied.
6. The essence of this factor lies in some fluctuation in the number of certain types of living organisms. As a result, a rare species may become more numerous and vice versa.
7. Isolation. It implies the emergence of obstacles to the spread of living organisms and their interbreeding. There may be various reasons for its occurrence: mechanical, environmental, territorial, morphological, genetic, etc. One of the main reasons is often an increase in differences between previously close organisms.
8. Mutations. These environmental factors can arise from natural or artificial features. When changes are made to the genetic nature of an organism, mutational changes occur. This factor underlies hereditary changes.
9. Situations arise when the population declines sharply. This can occur under the influence of various circumstances (flood, fire). The remaining representatives of living organisms become a determining link in the formation of new populations. As a result, some characteristics of this species may disappear and new ones may appear.
Human development has passed its course. But the factors are similar to those described above.
