The ability of a population to adapt to new factors. Disturbances of the equilibrium state of populations: mutations, natural selection, migrations, isolation
Factors in the genetic dynamics of a population that disrupt its equilibrium state include: mutation process, selection, genetic drift, migration, isolation.
Mutations and natural selection
In each generation, the gene pool of the population is replenished with newly emerging mutations. Among them there can be both completely new changes and mutations already existing in the population. This process is called mutation pressure. The magnitude of mutation pressure depends on the degree of mutability of individual genes, on the ratio of direct and reverse mutations, on the efficiency of the repair system, on the presence of mutagenic factors in the environment. In addition, the magnitude of mutation pressure is affected by the extent to which the mutation affects the viability and fertility of the individual.
Research shows that natural populations are saturated with mutant genes, which are mainly in a heterozygous state. The mutation process creates the primary genetic variability of the population, which must then be dealt with natural selection. In the event of a change in external conditions and a change in the direction of selection, the reserve of mutations allows the population to quickly adapt to the new situation.
The effectiveness of selection depends on whether the mutant trait is dominant or recessive. Clearing a population of individuals with a harmful dominant mutation can be achieved in one generation if its carrier does not leave offspring. At the same time, harmful recessive mutations escape the action of selection if they are in a heterozygous state, and especially in cases where selection acts in favor of heterozygotes. The latter often have a selective advantage over homozygous genotypes due to a wider reaction norm, which increases the adaptive potential of their owners. When heterozygotes are preserved and reproduced, the probability of separating recessive homozygotes simultaneously increases. Selection in favor of heterozygotes is called balancing.
A striking example of this form of selection is the situation with the inheritance of sickle cell anemia. This disease is widespread in parts of Africa. It is caused by a mutation in the gene encoding the synthesis of the hemoglobin b-chain, in which one amino acid (valine) is replaced by another (glutamine). Homozygotes for this mutation suffer from a severe form of anemia, almost always leading to death at an early age. The red blood cells of such people are shaped like a sickle. Heterozygosity for this mutation does not lead to anemia. Red blood cells in heterozygotes have a normal shape, but contain 60% normal and 40% altered hemoglobin. This suggests that in heterozygotes both alleles—normal and mutant—function. Since homozygotes for the mutant allele are completely eliminated from reproduction, one would expect a decrease in the frequency of the harmful gene in the population. However, in some African tribes the proportion of heterozygotes for this gene is 30-40%. The reason for this situation is that people having the heterozygous genotype are less susceptible to contracting dengue, which causes high mortality in these areas, compared to the norm. In this regard, selection preserves both genotypes: normal (dominant homozygote) and heterozygous. The reproduction from generation to generation of two different genotypic classes of individuals in a population is referred to as balanced polymorphism. It has adaptive value.
There are other forms of natural selection. Stabilizing selection preserves the norm as the genotype variant that best meets the prevailing conditions, eliminating any deviations from it that arise. This form of selection usually operates when a population has been in relatively stable conditions of existence for a long time. In contrast, driving selection preserves a new trait if the resulting mutation turns out to be beneficial and gives its carriers some advantage. Selection disruptive(disruptive) acts simultaneously in two directions, preserving extreme variants of the development of the trait. A typical example of this form of selection was given by Charles Darwin. It concerns the preservation of two forms of insects on the islands: winged and wingless, which live on different sides of the island - leeward and windless.
The main result of the activity of natural selection comes down to an increase in the number of individuals with characteristics in the direction of which selection occurs. At the same time, traits linked to them and traits that are in a correlative relationship with the former are also selected. For genes that control traits not affected by selection, the population can be in a state of equilibrium for a long time, and the distribution of genotypes for them will be close to the Hardy-Weinberg formula.
Natural selection operates widely and simultaneously affects many aspects of the life of an organism. It is aimed at preserving traits beneficial to the organism, which increase its adaptability and give it an advantage over other organisms. In contrast, the effect of artificial selection that occurs in populations of cultivated plants and domestic animals is narrower and most often affects traits that are beneficial to humans rather than to their carriers.
Genetic drift
The effect of random causes has a great influence on the genotypic structure of populations. These include: fluctuations in population size, age and sex composition of populations, quality and quantity of food resources, the presence or absence of competition, the random nature of the sample giving rise to the next generation, etc. Changes in gene frequencies in a population for random reasons, American geneticist S. Wright named genetic drift, and N.P. Dubinin - a genetic-automatic process. A particularly noticeable effect on the genetic structure of populations is exerted by sharp fluctuations in population size - population waves, or waves of life. It has been established that in small populations dynamic processes occur much more intensely, and at the same time the role of chance in the accumulation of individual genotypes increases. When the population size decreases, some mutant genes may be accidentally retained in it, while others may also be randomly eliminated. With subsequent population increases, the number of these surviving genes can increase rapidly. The rate of drift is inversely proportional to population size. At the moment of population decline, the drift is especially intense. With a very sharp reduction in population size, there may be a threat of extinction. This is the so-called “bottleneck” situation. If the population manages to survive, then as a result of genetic drift, a change in their frequencies will occur, which will affect the structure of the new generation.
Genetic-automatic processes occur especially clearly in isolates, when a group of individuals stands out from a large population and forms a new settlement. There are many such examples in the genetics of human populations. Thus, in the state of Pennsylvania (USA) there lives a Mennonite sect numbering several thousand people. Marriages here are only allowed between members of the sect. The isolate was started by three married couples who settled in America at the end of the 18th century. This group of people is characterized by an unusually high concentration of a pleiotropic gene, which in the homozygous state causes a special form of dwarfism with polydactyly. About 13% of members of this sect are heterozygous for this rare mutation. It is likely that there was a “progenitor effect” here: by chance, one of the founders of the sect was heterozygous for this gene, and closely related marriages contributed to the spread of this anomaly. No such disease has been found in other Mennonite groups scattered throughout the United States.
Migrations
Another reason for changes in gene frequencies in a population is migration. When groups of individuals move and crossbreed with members of another population, genes are transferred from one population to another. The effect of migration depends on the size of the migrant group and the differences in gene frequencies between the exchanging populations. If the initial frequencies of genes in populations are very different, then a significant frequency shift can occur. As migration progresses, genetic differences between populations become equalized. The end result of migration pressure is the establishment throughout the entire system of populations between which individuals are exchanged of a certain average concentration for each mutation.
An example of the role of migration is the distribution of genes that determine the human blood group system AB0. Europe is characterized by the predominance of the group A, for Asia - groups IN. The reason for the differences, according to geneticists, lies in the large population migrations that occurred from East to West in the period from 500 to 1500. ad.
Insulation
If individuals of one population do not completely or partially interbreed with individuals of other populations, such a population experiences a process isolation. If separation is observed over a number of generations, and selection acts in different directions in different populations, then a process of differentiation of populations occurs. The process of isolation operates both at the intrapopulation and interpopulation levels.
There are two main types of insulation: spatial, or mechanical, insulation and biological insulation. The first type of isolation occurs either under the influence of natural geographical factors (mountain building; the emergence of rivers, lakes and other bodies of water; volcanic eruption, etc.), or as a result of human activity (plowing land, draining swamps, forest planting, etc.). One of the consequences of spatial isolation is the formation of a discontinuous range of the species, which is characteristic, in particular, of the blue magpie, sable, grass frog, sedge, and common loach.
Biological isolation is divided into morpho-physiological, environmental, ethological and genetic. All these types of isolation are characterized by the emergence of reproductive barriers that limit or exclude free interbreeding.
Morpho-physiological isolation occurs mainly at the level of reproductive processes. In animals, it is often associated with differences in the structure of the copulatory organs, which is especially typical for insects and some rodents. In plants, a significant role is played by such characteristics as the size of the pollen grain, the length of the pollen tube, and the coincidence of the maturation times of pollen and stigmas.
At ethological isolation In animals, the obstacle is differences in the behavior of individuals during the reproductive period, for example, unsuccessful courtship of a male with a female is observed.
Environmental insulation can manifest itself in different forms: in preference for a certain reproductive territory, in different periods of maturation of germ cells, reproduction rates, etc. For example, in marine fish that migrate to rivers to reproduce, a special population develops in each river. Representatives of these populations may differ in size, color, time of onset of sexual maturity and other characteristics related to the reproductive process.
Genetic isolation includes different mechanisms. Most often it occurs due to disturbances in the normal course of meiosis and the formation of non-viable gametes. The causes of disorders may be polyploidy, chromosomal rearrangements, and nuclear-plasma incompatibility. Each of these phenomena can lead to a limitation of panmixia and infertility of hybrids, and, consequently, to a limitation of the process of free combination of genes.
Isolation is rarely created by any one mechanism. Typically, several different forms of isolation occur simultaneously. They can act both at the stage preceding fertilization and after it. In the latter case, the insulation system is less economical, because a significant amount of energy resources is wasted, for example, on the production of sterile offspring.
The listed factors of genetic dynamics of populations can act individually and jointly. In the latter case, either a cumulative effect can be observed (for example, mutation process + selection), or the action of one factor can reduce the effectiveness of another (for example, the appearance of migrants can reduce the effect of genetic drift).
The study of dynamic processes in populations allowed S.S. Chetverikov (1928) formulate the idea genetic homeostasis. By genetic homeostasis he understood the equilibrium state of a population, its ability to maintain its genotypic structure in response to the action of environmental factors. The main mechanism for maintaining an equilibrium state is the free crossing of individuals, the very conditions of which, according to Chetverikov, contain the apparatus for stabilizing the numerical ratios of alleles.
The genetic processes we have considered, occurring at the population level, create the basis for the evolution of larger systematic groups: species, genera, families, i.e. For macroevolution. The mechanisms of micro- and macroevolution are in many ways similar, only the scale of the changes that occur is different.
Goals: form the concept of a population as an elementary unit of evolution; show the role of hereditary variability as one of the factors of evolution, the reason for the variability of species.
Move lesson
I. Check of knowledge.
1. Testing.
1) The presence of similar structural features of organisms determines
criterion:
a) genetic;
b) morphological;
c) physiological;
d) environmental.
2) The commonality of ancestors proves the criterion:
a) historical;
b) morphological;
c) genetic d) geographical.
3) The karyotype of organisms studies the criterion:
a) genetic:
b) physiological;
c) morphological; d) historical.
4) The influence of biotic environmental factors on organisms considers the criterion:
a) geographical; b) environmental;
c) physiological;
d) historical.
5) The distribution of species in nature considers the criterion:
a) environmental;
b) geographical; c) historical;
d) physiological.
6) Distinction between species based on the set of enzymes is carried out in accordance with:
a) with morphological criterion;
b) physiological criterion;
c) biochemical criterion;
d) genetic criterion.
7) the ability of organisms to produce fertile offspring
serves as the basis:
a) for morphological criterion; b) physiological criterion;
c) genetic criterion;
d) environmental criterion.
8) The similarity of the processes of nutrition and breathing is studied by the criterion:
a) environmental;
b) physiological;
c) biochemical;
d) genetic.
9) The totality of environmental factors is the basis:
a) genetic criterion;
b) geographical criterion;
c) environmental criterion;
d) historical criterion.
2. Written response on the card.
Exercise.
Fill in the blanks in the following phrases:
1) The totality of environmental factors in which a species exists is... a criterion for the species
2) The main reason for separating a group of individuals into a population is...
3) Individuals of two populations of the same species...
5) The similarity of the body’s reactions to external influences, rhythms of development and reproduction is studied... criterion
II. Learning new material.
1 Populations.
Living organisms in nature, as a rule, do not live alone but form more or less permanent groups. There are many reasons for the formation of such groups, but the main ones are that organisms belonging to the same species accumulate in places that are most favorable for their existence and reproduction.
A set of individuals of the same species that inhabit a certain space for a long time, reproduce by free crossing and, to one degree or another, isolated from each other, is called a population.
The existence of species in the form of populations is a consequence of the heterogeneity of external conditions. Populations remain stable in time and space, although their numbers may change from year to year due to emerging differences in the conditions of reproduction and development of organisms. Within populations there are even smaller troupes into which individuals with similar behavior or based on family ties can unite. However, they are not able to sustain themselves sustainably.
The organisms that make up a population are related to each other through various relationships. They compete with each other for certain types of resources. Internal relationships in populations are complex and contradictory. Within each population of organisms that reproduce sexually, there is a constant exchange of genetic material.
Crossbreeding of individuals from different populations occurs less frequently, so genetic exchange between different populations is limited. As a result, each population is characterized by its own specific set of genes with a ratio of frequencies of occurrence of different alleles unique to this population. The existence of species in the form of populations increases their resistance to local changes in living conditions.
2. Population genetics.
In Darwin's time, genetics did not exist. It began to develop as a science in the twentieth century. It became known that genes are carriers of hereditary variability. The ideas of genetics introduced deep explanations into the theories of natural selection of Charles Darwin. The synthesis of genetics and classical Darwinism led to the birth of population genetics, which made it possible to explain from a new perspective the processes of changes in the genetic composition of populations, the emergence of new properties of organisms and their consolidation under the influence of natural selection.
A population is a collection of organisms of the same species, each of which has a specific genotype. The totality of genotypes of all individuals in a population is called the gene pool of the population. The richness of the gene pool depends on allelic diversity. This means that in a population where there is no allelic diversity for a particular gene, all individuals have an identical genotype for this AA gene. Genes in which two or more allelic variants are found in a population are called polymorphic. With two alleles there are three genotypes (AA, Aa, aa), with three alleles there are six genotypes, and then their number quickly increases.
The richness of the gene pool of a species is determined not only by allelic diversity, that is, the polymorphism of loci, but also by the diversity of allele combinations. A sharp decrease in the number of species leads to a reduction in allelic diversity and the number of combinations. Therefore, it is important to preserve the gene pools of wild species and avoid sudden depletion. The intensity of processes occurring in populations largely depends on the level of genetic diversity.
The mutation process is a source of hereditary variability. In a population of several million individuals, several mutations of each gene present in this population can occur in each generation. Thanks to combinative variability, mutations spread throughout the population.
The constantly ongoing mutation process and free crossing lead to the accumulation within the population of a large number of externally invisible, qualitative changes (the vast majority of emerging mutations are recessive). These facts were established by the Russian scientist S.S. Chetverikov.
Genetic studies of natural populations of plants and animals have shown that, despite their relative phenotypic homogeneity, they are saturated with a variety of recessive mutations. Chromosomes in which mutations have arisen, as a result of doubling during cell division, gradually spread among populations. Mutations do not manifest themselves phenotypically as long as they remain heterozygous.
Once a sufficiently high concentration of mutations is reached, crossing of individuals carrying allelic recessive genes becomes possible.
In these cases, mutations manifest themselves phenotypically and will fall under the direct control of natural selection, and this is precisely where the population’s ability to adapt lies, that is, to adapt to new factors - climate change, the emergence of a new predator or competitor, and even human pollution.
III. Consolidation.
Laboratory work
Topic: IDENTIFYING VARIABILITY IN INDIVIDUALS OF THE SAME SPECIES
Goals: form the concept of variability of organisms, continue to develop the skills to observe natural objects, and find signs of variability.
Equipment: handouts illustrating the variability of organisms (plants of 5-6 species, 2-3 specimens of each species, sets of seeds, fruits, leaves, etc.)
Progress
1. Compare 2-3 plants of the same species (or their individual organs: leaves, seeds, fruits, etc.). Find signs of similarity in their structure. Explain the reasons for the similarity of individuals of the same species.
2. Identify signs of difference in the plants under study. Answer the question: what properties of organisms determine differences between individuals of the same species? 3. Reveal the significance of these properties of organisms for evolution. Which differences, in your opinion, are due to hereditary variability, and which are not due to hereditary variability? Explain how differences could arise between individuals of the same species.
Homework: § 54, 55.
1. What is natural selection?
Answer. Natural selection is a process originally defined by Charles Darwin as leading to the survival and preferential reproduction of individuals more adapted to given environmental conditions and possessing useful hereditary traits. In accordance with Darwin's theory and the modern synthetic theory of evolution, the main material for natural selection is random hereditary changes - recombination of genotypes, mutations and their combinations.
2. What is a genotype?
Answer. The term “genotype” was introduced into science by Ioganson in 1909. Genotype (genotype, from the Greek genos - genus and typos - imprint, form, sample) is the totality of the genes of an organism, in a broader sense - the totality of all hereditary factors of the organism, both nuclear , and non-nuclear. The combination of the unique genomes (sets) received from each parent creates the genotype that underlies genetic individuality. The concepts of genotype and phenotype are very important in biology. As stated above, the totality of all the genes of an organism constitutes its genotype. The totality of all the characteristics of an organism (morphological, anatomical, functional, etc.) constitutes a phenotype. Throughout the life of an organism, its phenotype may change, but the genotype remains unchanged. This is explained by the fact that the phenotype is formed under the influence of the genotype and environmental conditions. The word genotype has two meanings. In a broad sense, it is the totality of all the genes of a given organism. But in relation to experiments of the type that Mendel performed, the word genotype refers to the combination of alleles that control a given trait (for example, organisms can have the genotype AA, Aa or aa).
Thus, the genotype is: - the entire set of genetic (genomic) characteristics characteristic of a given individual and the characteristics of certain pairs of alleles that the individual has in the studied region of the genome.
Questions after § 55
1. What is the gene pool of a population?
Answer. Each population is characterized by a certain gene pool, i.e., the total amount of genetic material, which is made up of the genotypes of individual individuals.
Necessary prerequisites for the evolutionary process are the occurrence of elementary changes in the apparatus of heredity - mutations, their distribution and consolidation in the gene pools of populations of organisms. Directed changes in the gene pools of populations under the influence of various factors represent elementary evolutionary changes.
As already noted, natural populations in different parts of a species' range are usually more or less different. Within each population, free interbreeding of individuals occurs. As a result, each population is characterized by its own gene pool with the ratios of various alleles unique to this population.
2. Why do most mutations not appear externally?
Answer. Natural populations are saturated with a wide variety of mutations. This was brought to the attention of the Russian scientist Sergei Sergeevich Chetverikov (1880–1959), who established that a significant part of the variability of the gene pool is hidden from view, since the vast majority of mutations that arise are recessive and do not appear externally. Recessive mutations are, as it were, “absorbed by the species in a heterozygous state,” because most organisms are heterozygous for many genes. Such hidden variability can be revealed in experiments with crossing closely related individuals. With such a crossing, some recessive alleles that were in a heterozygous and therefore latent state will become homozygous and will be able to appear. Significant genetic variability in natural populations is also easily detected during artificial selection. In artificial selection, those individuals are selected from a population in which any economically valuable traits are most strongly expressed, and these individuals are crossed with each other. Artificial selection proves effective in almost all cases where it is resorted to. Consequently, in populations there is genetic variability for literally every trait of a given organism.
3. What is the ability of a population to adapt (adapt) to new conditions?
Answer. Since any population is usually well adapted to its environment, large changes usually reduce this fitness, just as large random changes in the mechanism of a watch (removing a spring or adding a wheel) lead to a malfunction. Populations have large reserves of alleles that do not bring any benefit to it in a given place or at a given time; they remain in the population in a heterozygous state until, as a result of changes in environmental conditions, they suddenly turn out to be useful. Once this happens, their frequency begins to increase under the influence of selection, and ultimately they become the main genetic material. This is where the population’s ability to adapt lies, that is, to adapt to new factors - climate change, the emergence of a new predator or competitor, and even human pollution.
An example of such adaptation is the evolution of insect species that are resistant to insecticides. In all cases, events develop in the same way: when a new insecticide (poison acting on insects) is introduced into practice, a small amount is sufficient to successfully combat an insect pest. Over time, the concentration of the insecticide has to be increased until it is finally ineffective. The first report of insecticide resistance in an insect appeared in 1947 and concerned housefly resistance to DDT. Resistance to one or more insecticides has subsequently been found in at least 225 species of insects and other arthropods. Genes capable of conferring insecticide resistance were apparently present in each of the populations of these species; their action ultimately ensured a decrease in the effectiveness of poisons used to control pests
4. How can recessive alleles be identified?
Answer. A recessive allele (recessive allele, from the Latin recessus - deviation) is an allele whose phenotype is not manifested in heterozygotes, but is manifested in a homozygous or hemizygous genotype for this allele. If recessive alleles are in a homozygous state, they will manifest themselves in the phenotype. If it is necessary to find out whether they are present in the genotype of an organism with a dominant phenotype, then analytical crossing is used. To do this, the organism being tested is crossed with a carrier of the recessive phenotype. If there are recessive individuals in the offspring, then the organism being tested is a carrier of the recessive gene.
Current page: 15 (book has 26 pages total) [available reading passage: 18 pages]
§ 53. Type, its criteria
1. What is a species?
2. What types of plants and animals do you know?
View. With the development of biology came the understanding that, in comparison with the infinite variety of conditions in which life occurs, the variety of forms of organisms is finite; it is, as it were, collected into “nodes” - biological species.
Biological species - this is a set of individuals that have the ability to interbreed with the formation of fertile offspring; inhabiting a certain area; possessing a number of common morphological and physiological characteristics and similarities in relationships with the biotic and abiotic environment.
A biological species is not only a systematic category. This is a holistic element of living nature, isolated from other species. The integrity of the species is manifested in the fact that its individuals can live and reproduce only by interacting with each other thanks to the mutual adaptations of organisms developed in the process of evolution: the peculiarities of the coordination of the structure of the maternal body and the embryo, signaling and perception systems in animals, common territory, similarity of life habits and reactions to seasonal climate changes, etc. Species adaptations ensure the preservation of the species, although sometimes they can harm individual individuals. River perch, for example, feeds on its own young, due to which the species survives when there is a lack of food, even despite the loss of part of the offspring. Each species exists in nature as a historically emerged integral formation.
The isolation of a species is maintained by reproductive isolation (see § 59), which prevents it from mixing with other species during reproduction. Isolation is ensured by differences in the structure of the genital organs, fragmentation of habitats, differences in the timing or location of reproduction, differences in behavior, ecological fragmentation, and other mechanisms that you will learn about in subsequent sections. The isolation of species prevents the emergence of intermediate forms. Warty birch, for example, does not grow in moss swamps where dwarf birch usually grows. Thanks to isolation, species do not mix with each other.
Type criteria. The characteristic features and properties by which some species differ from others are called criteria kind.
Morphological criterion - This is the similarity of the external and internal structure of organisms. Carl Linnaeus, for example, defined species as integral groups of organisms that differ from other life forms based on structural characteristics. In other words, the presence of structural features that make a certain group of organisms similar to each other and at the same time different from all other groups is the criterion for classifying them as a given species.
Individuals within a species are sometimes so variable that it is not always possible to determine the species based on morphological criteria alone. There are species that are morphologically similar. These are twin species that are discovered in all systematic groups. For example, two twin species are known in black rats - with 38 and 49 chromosomes; the malaria mosquito has 6 twin species, and the small spined lance fish, widespread in fresh water bodies, has 3 similar species. Twin species are found among a wide variety of organisms: fish, insects, mammals, plants, but individuals of such twin species do not interbreed (Fig. 72).
Genetic criterion – this is a set of chromosomes characteristic of each species; their strictly defined number, sizes and shapes, DNA composition. The chromosome set is the main species characteristic. Individuals of different species have different sets of chromosomes, so they cannot interbreed and are reproductively limited from each other in natural conditions.
Rice. 72. Twin species: tetraploid (left) and diploid (right) spined loach species
Physiological criterion – similarity of the body’s reactions to external influences, rhythms of development and reproduction. This criterion is based on the similarity of all life processes, and above all reproduction. Representatives of different species, as a rule, do not interbreed or their offspring are infertile. However, there are exceptions. For example, dogs can produce offspring by mating with wolves. Hybrids of some species of birds (canaries, finches), as well as plants (poplars, willows) can be fertile. Consequently, the physiological criterion is also insufficient to determine the species identity of individuals.
Ecological criterion - this is a characteristic position of a species in natural communities, its connections with other species, sets of environmental factors necessary for existence.
Geographical criterion – area of distribution, a certain area occupied by a species in nature.
Historical criterion – community of ancestors, a single history of the origin and development of the species.
The criteria of a species are interconnected and determine the qualitative feature of the species. But none of them are absolute. For example, two different species may not differ in anatomical structure and have the same chromosome sets. But if they differ in behavior, then they do not interbreed and, therefore, are isolated from one another. Only taken together, the listed criteria make it possible to establish with sufficient reliability that an organism belongs to a particular species.
Species represent a certain level of organization of living matter - species.
Biological species. Species criteria: morphological, genetic, physiological, ecological, geographical, historical.
1. Define a biological species.
2. What species criteria do you know?
3. What is the integrity of the species, how is it manifested?
4. Why is it important to preserve species in nature?
Make lists of plant and animal species you know. Try to group the species known to you according to the degree of similarity: a) morphological; b) environmental.
§ 54. Populations
1. Why do organisms of most of the species known to us live in groups in nature?
2. Why are groups of single-species organisms (for example, thickets of plants such as buttercup, nettle, sedge, etc.) not found everywhere, but only in certain areas? What areas are these?
In reality, a species is a much more complex entity than just a collection of similar individuals interbreeding. It breaks up into smaller natural groups of individuals - populations, inhabiting separate, relatively small areas of the range of this species.
Population is a group of single-species organisms that occupy a certain area of territory within the species’ range, freely interbreeding and partially or completely isolated from other populations.
The existence of species in the form of populations is a consequence of the heterogeneity of external conditions.
Populations remain stable in time and space, although their numbers may change from year to year due to changes in the conditions of reproduction and development of organisms. Within populations, there are even smaller groups into which individuals with similar behavior or based on family ties can unite (for example, flocks of fish or sparrows, prides of lions). However, such groups may disintegrate under the influence of external factors or mix with others. They are unable to sustain themselves sustainably.
Relationships between organisms in populations. The organisms that make up a population are related to each other through various relationships. They compete with each other for certain types of resources, they can eat each other or, on the contrary, together defend themselves from a predator. Internal relationships in populations are very complex and contradictory. The reactions of individuals to changes in living conditions and population reactions often do not coincide. The death of individual weakened organisms (for example, from predators) can improve the qualitative composition of the population (including the quality of the hereditary material available to the population), and increase its ability to survive in changing environmental conditions.
Within each population of sexually reproducing organisms, there is a constant exchange of genetic material; Crossing of individuals from different populations occurs much less frequently, so genetic exchange between different populations is limited. As a result, each population is characterized by its own specific set of genes (gene pool - see below) with a ratio of frequencies of occurrence of different alleles unique to this population. Under the influence of this, properties may arise in individual populations that distinguish them from each other. Thus, existence in the form of populations increases the internal diversity of the species, its resistance to local changes in living conditions, and allows it to gain a foothold in new conditions. The direction and speed of evolutionary changes occurring within a species largely depend on the properties of populations. The processes of formation of new species originate in changes in the properties of individual populations.
Population.
1. What is a population?
2. Why do species exist in the form of populations?
3. What properties of populations contribute to the sustainable existence of a species?
§ 55. Genetic composition of populations
1. What is natural selection?
2. What is a genotype?
Population genetics. In Darwin's time, the science of genetics did not yet exist. It began to develop at the beginning of the 20th century. It became known that genes are carriers of hereditary variability. The ideas of genetics introduced additional in-depth explanations into the theory of natural selection of Charles Darwin. The synthesis of genetics and classical Darwinism led to the birth of a special direction of research - population genetics, which made it possible to explain from a new perspective the processes of changes in the genetic composition of populations, the emergence of new properties of organisms and their consolidation under the influence of natural selection.
Gene pool. Each population is characterized by a certain gene pool, i.e., the total amount of genetic material that is made up of the genotypes of individual individuals.
Necessary prerequisites for the evolutionary process are the occurrence of elementary changes in the apparatus of heredity - mutations their distribution and consolidation in the gene pools of populations of organisms. Directed changes in the gene pools of populations under the influence of various factors represent elementary evolutionary changes.
As already noted, natural populations in different parts of a species' range are usually more or less different. Within each population, free interbreeding of individuals occurs. As a result, each population is characterized by its own gene pool with the ratios of various alleles unique to this population.
The mutation process is a constant source of hereditary variability. In a population of several million individuals, several mutations of literally every gene present in this population can occur in each generation. Thanks to combinative variability, mutations spread throughout the population.
Natural populations are saturated with a wide variety of mutations. A Russian scientist drew attention to this Sergei Sergeevich Chetverikov(1880–1959), who found that a significant part gene pool variability hidden from view, since the vast majority of mutations that occur are recessive and do not appear externally. Recessive mutations are, as it were, “absorbed by the species in a heterozygous state,” because most organisms are heterozygous for many genes. Such hidden variability can be revealed in experiments with crossing closely related individuals. With such a crossing, some recessive alleles that were in a heterozygous and therefore latent state will become homozygous and will be able to appear. Significant genetic variability in natural populations is also easily detected during artificial selection. In artificial selection, those individuals are selected from a population in which any economically valuable traits are most strongly expressed, and these individuals are crossed with each other. Artificial selection proves effective in almost all cases where it is resorted to. Consequently, in populations there is genetic variability for literally every trait of a given organism.
The forces that cause gene mutations act randomly. The probability of a mutant individual appearing in an environment in which selection will favor it is no greater than in an environment in which it will almost certainly die. S.S. Chetverikov showed that, with rare exceptions, the majority of newly emerged mutations turn out to be harmful and in the homozygous state, as a rule, reduce the viability of individuals. They are preserved in populations only due to selection in favor of heterozygotes. However, mutations that are deleterious in one environment may enhance viability in other conditions. Thus, a mutation that causes underdevelopment or complete absence of wings in insects is certainly harmful under normal conditions, and wingless individuals are quickly replaced by normal ones. But on oceanic islands and mountain passes where strong winds blow, such insects have advantages over individuals with normally developed wings.
Since any population is usually well adapted to its environment, large changes usually reduce this fitness, just as large random changes in the mechanism of a watch (removing a spring or adding a wheel) lead to its malfunction. Populations have large reserves of alleles that do not bring any benefit to it in a given place or at a given time; they remain in the population in a heterozygous state until, as a result of changes in environmental conditions, they suddenly turn out to be useful. Once this happens, their frequency begins to increase under the influence of selection, and ultimately they become the main genetic material. This is where the population’s ability to adapt lies, that is, to adapt to new factors - climate change, the emergence of a new predator or competitor, and even human pollution.
An example of such adaptation is the evolution of insect species that are resistant to insecticides. In all cases, events develop in the same way: when a new insecticide (poison acting on insects) is introduced into practice, a small amount is sufficient to successfully combat an insect pest. Over time, the concentration of the insecticide has to be increased until it is finally ineffective. The first report of insecticide resistance in an insect appeared in 1947 and concerned housefly resistance to DDT. Resistance to one or more insecticides has subsequently been found in at least 225 species of insects and other arthropods. Genes capable of conferring insecticide resistance were apparently present in each of the populations of these species; their action ultimately ensured a decrease in the effectiveness of poisons used to control pests.
Thus, the mutation process creates material for evolutionary transformations, forming a reserve of hereditary variability in the gene pool of each population and the species as a whole. By maintaining a high degree of genetic diversity in populations, it provides the basis for the action of natural selection and microevolution.
Gene pool of the population.
1. What is the gene pool of a population?
2. Why do most mutations not appear externally?
3. What is the ability of a population to adapt (adapt) to new conditions?
4. How can recessive alleles be identified?
§ 56. Changes in the gene pool of populations
1. What is the content of the concept of “population gene pool”?
2. What is the source of changes in the gene pool?
Possessing a specific gene pool under the control of natural selection, populations play a vital role in the evolutionary transformations of the species. All processes leading to changes in a species begin at the level of species populations and are directed processes of transformation of the population gene pool.
Genetic balance in populations. The frequency of occurrence of various alleles in a population is determined by the frequency of mutations, selection pressure, and sometimes the exchange of hereditary information with other populations as a result of migrations of individuals. With relative constancy of conditions and high population size, all these processes lead to a state of relative equilibrium. As a result, the gene pool of such populations becomes balanced, it establishes genetic balance, or the constancy of the frequencies of occurrence of various alleles.
Causes of genetic imbalance. The example given earlier with the action of insecticides suggests that the action of natural selection leads to directed changes in the gene pool of the population– increasing the frequencies of “useful” genes. Microevolutionary changes occur. However, changes in the gene pool can also be non-directional random character. Most often they are associated with fluctuations in the number of natural populations or with the spatial isolation of part of the organisms of a given population.
Undirected, random changes in the gene pool may occur due to various reasons. One of them - migration, i.e., moving part of a population to a new habitat. If a small part of a population of animals or plants settles in a new place, the gene pool of the newly formed population will inevitably be smaller than the gene pool of the parent population. Due to random reasons, allele frequencies in the new population may not coincide with those in the original one. Genes that were previously rare can spread rapidly (through sexual reproduction) among individuals in a new population. And previously widespread genes may be absent if they were not in the genotypes of the founders of the new settlement.
Similar changes can be observed in cases where the population is divided into two unequal parts with natural or artificial barriers. For example, a dam was built on a river, dividing the fish population that lived there into two parts. The gene pool of a small population, originating from a small number of individuals, may, again due to random reasons, differ in composition from the gene pool of the original one. It will carry only those genotypes that were randomly selected among the small number of founders of the new population. Rare alleles may turn out to be common in a new population that arises as a result of its separation from the original population.
The composition of the gene pool may change due to various natural disasters, when only a few organisms remain alive (for example, due to floods, droughts, or fires). In a population that has survived a catastrophe, consisting of individuals who survived by chance, the composition of the gene pool will be formed from randomly selected genotypes. Following the decline in numbers, mass reproduction begins, which begins with a small group. The genetic composition of this group will determine the genetic structure of the entire population during its heyday. In this case, some mutations may completely disappear, while the concentration of others may increase sharply. The set of genes remaining in living individuals may differ slightly from that which existed in the population before the disaster.
Sharp fluctuations in population numbers, no matter what causes them, change the frequency of alleles in the gene pool of populations. When unfavorable conditions are created and the population declines due to the death of individuals, the loss of some genes, especially rare ones, may occur. In general, the smaller the population size, the higher the probability of losing rare genes, and the greater the influence random factors have on the composition of the gene pool. Periodic fluctuations in numbers are characteristic of almost all organisms. These fluctuations change the frequency of genes in populations that arise to replace each other. An example is some insects; only a small number survive the winter. This small proportion gives rise to a new summer population, its gene pool often different from that of the population that existed a year ago.
Thus, the action of random factors impoverishes and changes the gene pool of a small population compared to its initial state. This phenomenon is called genetic drift. As a result of genetic drift, a viable population with a unique gene pool can emerge, largely random, since selection in this case did not play a leading role. As the number of individuals increases, the action of natural selection will again be restored, which will spread to the new gene pool, leading to its directed changes. The combination of all these processes can lead to the isolation of a new species.
Directed changes in the gene pool occur as a result of natural selection. Natural selection leads to a consistent increase in the frequencies of some genes (useful under given conditions) and a decrease in others. As a result of natural selection, useful genes are fixed in the gene pool of populations, i.e., those that favor the survival of individuals in given environmental conditions. Their share is increasing, and the overall composition of the gene pool is changing. Changes in the gene pool under the influence of natural selection should lead to changes in phenotypes, features of the external structure of organisms, their behavior and lifestyle, and ultimately to better adaptability of the population to given environmental conditions.
Genetic balance. Random changes in the composition of the gene pool. Genetic drift. Directed changes in the gene pool.
1. Under what conditions is equilibrium between different alleles of the population gene pool possible?
2. What forces cause directed changes in the gene pool?
3. What factors cause genetic imbalance?
4. What are the reasons for the differences in the gene pools of isolated populations of the same species?
Discuss how human activities change the gene pool of wild and domestic animal and plant species.
In nature, each existing species is a complex complex or even a system of intraspecific groups, which include individuals with specific structural features, physiology and behavior. This intraspecific association of individuals is population.
The word “population” comes from the Latin “populus” - people, population. Hence, population- a collection of individuals of the same species living in a certain territory, i.e. those that only interbreed with each other. The term “population” is currently used in the narrow sense of the word, when talking about a specific intraspecific group inhabiting a certain biogeocenosis, and in a broad, general sense - to designate isolated groups of a species, regardless of what territory it occupies and what genetic information it carries.
Members of the same population have no less impact on each other than physical environmental factors or other species of organisms living together. In populations, all forms of connections characteristic of interspecific relationships are manifested to one degree or another, but most clearly expressed mutualistic(mutually beneficial) and competitive. Populations can be monolithic or consist of subpopulation-level groups - families, clans, herds, packs and so on. The combination of organisms of the same species into a population creates qualitatively new properties. Compared to the lifespan of an individual organism, a population can exist for a very long time.
At the same time, a population is similar to an organism as a biosystem, since it has a certain structure, integrity, a genetic program for self-reproduction, and the ability to reproduce and adapt. The interaction of people with species of organisms found in the environment, in the natural environment or under human economic control, is usually mediated through populations. It is important that many patterns of population ecology also apply to human populations.
Population is the genetic unit of a species, changes in which are carried out by the evolution of the species. As a group of cohabiting individuals of the same species, a population acts as the first supraorganismal biological macrosystem. A population's adaptive capabilities are significantly higher than those of its constituent individuals. A population as a biological unit has certain structure and functions.
Population structure characterized by its constituent individuals and their distribution in space.
Population functions similar to the functions of other biological systems. They are characterized by growth, development, and the ability to maintain existence in constantly changing conditions, i.e. populations have specific genetic and environmental characteristics.
Populations have laws that allow limited environmental resources to be used in this way to ensure the preservation of offspring. Populations of many species have properties that allow them to regulate their numbers. Maintaining optimal numbers under given conditions is called population homeostasis.
Thus, populations, as group associations, have a number of specific properties that are not inherent in each individual individual. Main characteristics of populations: number, density, birth rate, death rate, growth rate.
A population is characterized by a certain organization. The distribution of individuals across the territory, the ratio of groups by sex, age, morphological, physiological, behavioral and genetic characteristics reflect population structure. It is formed, on the one hand, on the basis of the general biological properties of the species, and on the other, under the influence of abiotic environmental factors and populations of other species. The structure of populations therefore has an adaptive character.
The adaptive capabilities of a species as a whole as a system of populations are much broader than the adaptive characteristics of each individual individual.
Population structure of the species
The space or habitat occupied by a population may vary between species and within the same species. The size of a population's range is determined to a large extent by the mobility of individuals or the radius of individual activity. If the radius of individual activity is small, the size of the population range is usually also small. Depending on the size of the occupied territory, we can distinguish three types of populations: elementary, environmental and geographical (Fig. 1).
Rice. 1. Spatial division of populations: 1 - species range; 2-4 - geographical, ecological and elementary populations, respectively
There are sex, age, genetic, spatial and ecological structures of populations.
Sex structure of the population represents the ratio of individuals of different sexes in it.
Age structure of the population- the ratio in the population of individuals of different ages, representing one or different offspring of one or several generations.
Genetic structure of the population is determined by the variability and diversity of genotypes, the frequencies of variations of individual genes - alleles, as well as the division of the population into groups of genetically similar individuals, between which, when crossed, there is a constant exchange of alleles.
Spatial structure of the population - the nature of the placement and distribution of individual members of the population and their groups in the area. The spatial structure of populations differs markedly between sedentary and nomadic or migrating animals.
Ecological population structure represents the division of any population into groups of individuals that interact differently with environmental factors.
Each species, occupying a specific territory ( range), represented on it by a system of populations. The more complex the territory occupied by a species is, the greater the opportunities for the isolation of individual populations. However, to a lesser extent, the population structure of a species is determined by its biological characteristics, such as the mobility of its constituent individuals, the degree of their attachment to the territory, and the ability to overcome natural barriers.
Isolation of populations
If the members of a species are constantly intermingled and intermingled over large areas, the species is characterized by a small number of large populations. With poorly developed ability to move, many small populations are formed within the species, reflecting the mosaic nature of the landscape. In plants and sedentary animals, the number of populations is directly dependent on the degree of heterogeneity of the environment.
The degree of isolation of neighboring populations of the species varies. In some cases, they are sharply separated by territory unsuitable for habitation and are clearly localized in space, for example, populations of perch and tench in lakes isolated from each other.
The opposite option is the complete settlement of vast territories by the species. Within the same species there can be populations with both clearly distinguishable and blurred boundaries, and within the species, populations can be represented by groups of different sizes.
Connections between populations support the species as a whole. Too long and complete isolation of populations can lead to the formation of new species.
Differences between individual populations are expressed to varying degrees. They can affect not only their group characteristics, but also the qualitative features of the physiology, morphology and behavior of individual individuals. These differences are created mainly under the influence of natural selection, which adapts each population to the specific conditions of its existence.
Classification and structure of populations
A mandatory feature of a population is its ability to exist independently in a given territory for an indefinitely long time due to reproduction, and not the influx of individuals from the outside. Temporary settlements of different scales do not belong to the category of populations, but are considered intra-population units. From these positions, the species is represented not by hierarchical subordination, but by a spatial system of neighboring populations of different scales and with varying degrees of connections and isolation between them.
Populations can be classified according to their spatial and age structure, density, kinetics, constancy or change of habitats and other environmental criteria.
The territorial boundaries of populations of different species do not coincide. The diversity of natural populations is also expressed in the variety of types of their internal structure.
The main indicators of population structure are the number, distribution of organisms in space and the ratio of individuals of different qualities.
The individual traits of each organism depend on the characteristics of its hereditary program (genotype) and how this program is implemented during ontogenesis. Each individual has a certain size, sex, distinctive morphological features, behavioral characteristics, its own limits of endurance and adaptability to environmental changes. The distribution of these characteristics in a population also characterizes its structure.
The population structure is not stable. The growth and development of organisms, the birth of new ones, death from various causes, changes in environmental conditions, an increase or decrease in the number of enemies - all this leads to changes in various relationships within the population. The direction of its further changes largely depends on the structure of the population in a given period of time.
Sexual structure of populations
The genetic mechanism for sex determination ensures that the offspring are separated by sex in a 1:1 ratio, the so-called sex ratio. But it does not follow from this that the same ratio is characteristic of the population as a whole. Sex-linked traits often determine significant differences in the physiology, ecology and behavior of females and males. Due to the different viability of male and female organisms, this primary ratio often differs from the secondary and especially from the tertiary - characteristic of adult individuals. Thus, in humans, the secondary sex ratio is 100 girls to 106 boys; by the age of 16-18 this ratio levels out due to increased male mortality and by the age of 50 it is 85 men per 100 women, and by the age of 80 it is 50 men per 100 women.
The sex ratio in a population is established not only according to genetic laws, but also to a certain extent under the influence of the environment.
Age structure of populations
Fertility and mortality, population dynamics are directly related to the age structure of the population. The population consists of individuals of different ages and sexes. Each species, and sometimes each population within a species, has its own age group ratios. In relation to the population it is usually distinguished three ecological ages: pre-reproductive, reproductive and post-reproductive.
With age, an individual's requirements for the environment and resistance to its individual factors naturally and very significantly change. At different stages of ontogenesis, changes in habitats, changes in the type of food, the nature of movement, and the general activity of organisms can occur.
Age differences in a population significantly increase its ecological heterogeneity and, consequently, its resistance to the environment. The likelihood increases that, in the event of strong deviations of conditions from the norm, at least some viable individuals will remain in the population, and it will be able to continue its existence.
The age structure of populations is adaptive in nature. It is formed on the basis of the biological properties of the species, but always also reflects the strength of the influence of environmental factors.
Age structure of plant populations
In plants, the age structure of the cenopopulation, i.e. population of a particular phytocenosis is determined by the ratio of age groups. The absolute, or calendar, age of a plant and its age state are not identical concepts. Plants of the same age can be in different age states. The age-related, or ontogenetic state of an individual is the stage of its ontogenesis, at which it is characterized by certain relationships with the environment.
The age structure of the coenopopulation is largely determined by the biological characteristics of the species: the frequency of fruiting, the number of produced seeds and vegetative rudiments, the ability of vegetative rudiments to rejuvenate, the rate of transition of individuals from one age state to another, the ability to form clones, etc. The manifestation of all these biological characteristics, in turn turn depends on environmental conditions. The course of ontogenesis also changes, which can occur in one species in many ways.
Different plant sizes reflect different vitality individuals within each age group. The vitality of an individual is manifested in the power of its vegetative and generative organs, which corresponds to the amount of accumulated energy, and in resistance to adverse influences, which is determined by the ability to regenerate. The vitality of each individual changes in ontogenesis along a single-peak curve, increasing on the ascending branch of ontogenesis and decreasing on the descending branch.
Many meadow, forest, steppe species, when grown in nurseries or crops, i.e. on the best agrotechnical background, they shorten their ontogeny.
The ability to change the path of ontogenesis ensures adaptation to changing environmental conditions and expands the ecological niche of the species.
Age structure of populations in animals
Depending on the characteristics of reproduction, members of a population may belong to the same generation or to different ones. In the first case, all individuals are close in age and approximately simultaneously go through the next stages of the life cycle. The timing of reproduction and the passage of individual age stages is usually confined to a certain season of the year. The size of such populations is, as a rule, unstable: strong deviations of conditions from the optimum at any stage of the life cycle immediately affect the entire population, causing significant mortality.
In species with single reproduction and short life cycles, several generations occur throughout the year.
When humans exploit natural animal populations, taking into account their age structure is of utmost importance. In species with large annual recruitment, larger portions of the population can be removed without the threat of depleting its numbers. For example, in pink salmon that mature in the second year of life, it is possible to catch up to 50-60% of spawning individuals without the threat of a further decline in population size. For chum salmon, which mature later and have a more complex age structure, removal rates from a mature stock should be lower.
Analysis of the age structure helps to predict the population size over the life of a number of next generations.
The space occupied by a population provides it with the means to live. Each territory can support only a certain number of individuals. Naturally, the complete use of available resources depends not only on the total population size, but also on the distribution of individuals in space. This is clearly manifested in plants, the feeding area of which cannot be less than a certain limiting value.
In nature, an almost uniform, ordered distribution of individuals within an occupied territory is rarely encountered. However, most often the members of a population are distributed unevenly in space.
In each specific case, the type of distribution in the occupied space turns out to be adaptive, i.e. allows optimal use of available resources. Plants in a cenopopulation are most often distributed extremely unevenly. Often the denser center of the aggregation is surrounded by individuals located less densely.
The spatial heterogeneity of the cenopopulation is associated with the nature of the development of clusters over time.
In animals, due to their mobility, the ways of regulating territorial relations are more diverse compared to plants.
In higher animals, intrapopulation distribution is regulated by a system of instincts. They are characterized by special territorial behavior - a reaction to the location of other members of the population. However, a sedentary lifestyle poses the risk of rapid depletion of resources if population densities become too high. The total area occupied by the population is divided into separate individual or group areas, thereby achieving the orderly use of food supplies, natural shelters, breeding sites, etc.
Despite the territorial isolation of members of the population, communication is maintained between them using a system of various signals and direct contacts at the borders of their possessions.
“Securing an area” is achieved in different ways: 1) protecting the boundaries of the occupied space and direct aggression towards a stranger; 2) special ritual behavior demonstrating a threat; 3) a system of special signals and marks indicating the occupancy of the territory.
The usual reaction to territorial marks—avoidance—is inherited in animals. The biological benefit of this type of behavior is obvious. If the mastery of a territory were decided only by the outcome of a physical struggle, the appearance of each stronger alien would threaten the owner with the loss of the site and exclusion from reproduction.
Partial overlapping of individual territories serves as a way to maintain contacts between members of the population. Neighboring individuals often maintain a stable, mutually beneficial system of connections: mutual warning of danger, joint protection from enemies. Normal behavior of animals includes an active search for contacts with members of their own species, which often intensifies during periods of population decline.
Some species form widely wandering groups that are not tied to a specific territory. This is the behavior of many fish species during feeding migrations.
There are no absolute distinctions between different ways of using the territory. The spatial structure of the population is very dynamic. It is subject to seasonal and other adaptive changes in accordance with place and time.
The patterns of animal behavior constitute the subject of a special science - ethology. The system of relationships between members of one population is therefore called the ethological, or behavioral structure of the population.
The behavior of animals in relation to other members of the population depends, first of all, on whether a solitary or group lifestyle is characteristic of the species.
A solitary lifestyle, in which individuals of a population are independent and isolated from each other, is characteristic of many species, but only at certain stages of the life cycle. Completely solitary existence of organisms does not occur in nature, since in this case it would be impossible to carry out their main vital function - reproduction.
With a family lifestyle, the bonds between parents and their offspring also strengthen. The simplest type of such connection is the care of one of the parents for laid eggs: protection of the clutch, incubation, additional aeration, etc. With a family lifestyle, the territorial behavior of animals is most pronounced: various signals, markings, ritual forms of threat and direct aggression ensure ownership of an area sufficient for feeding offspring.
Larger animal associations - flocks, herds And colonies. Their formation is based on the further complication of behavioral connections in populations.
Life in a group, through the nervous and hormonal systems, affects the course of many physiological processes in the animal’s body. In isolated individuals, the level of metabolism changes noticeably, reserve substances are consumed faster, a number of instincts do not manifest themselves, and overall vitality deteriorates.
Positive group effect manifests itself only up to a certain optimal level of population density. If there are too many animals, this threatens everyone with a lack of environmental resources. Then other mechanisms come into play, leading to a decrease in the number of individuals in the group through its division, dispersal, or a drop in the birth rate.