Populations. Disturbances in the equilibrium state of populations: mutations, natural selection, migration, isolation The ability of a population to adapt to new factors

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 who are more adapted to given environmental conditions and have 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, shape, pattern) - the totality of the body's genes, in a broader sense - the totality of all hereditary factors of the body, as nuclear , and non-nuclear. The combination of unique genomes (sets) obtained from each of the parents creates the genotype that underlies the genetic personality. The concepts of genotype and phenotype are very important in biology. As mentioned above, the totality of all the genes of an organism makes up its genotype. The totality of all signs of an organism (morphological, anatomical, functional, etc.) is the phenotype. Throughout the life of an organism, its phenotype can change, but the genotype remains unchanged. This is due to 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 set up, the word genotype denotes a combination of alleles that control a given trait (for example, organisms can have the genotype AA, Aa or aa).

Thus, the genotype is: - the totality of genetic (genomic) characteristics characteristic of a given individual and the characteristics of certain pairs of alleles that the individual has in the region of the genome under study.

Questions after § 55

1. What is the gene pool of a population?

Answer. Each population is characterized by a certain gene pool, that is, the total amount of genetic material that is made up of the genotypes of individual individuals.

The necessary prerequisites for the evolutionary process are the occurrence of elementary changes in the apparatus of heredity - mutations, their distribution and fixation in the gene pools of populations of organisms. Directed changes in the gene pools of populations under the influence of various factors are elementary evolutionary changes.

As already noted, natural populations in different parts of the range of a species are usually more or less different. Within each population there is free interbreeding of individuals. As a result, each population is characterized by its own gene pool with ratios of various alleles inherent only to this population.

2. Why do most mutations not show up externally?

Answer. Natural populations are saturated with a wide variety of mutations. This was noticed by the Russian scientist Sergey Sergeevich Chetverikov (1880–1959), who found that a significant part of the variability of the gene pool is hidden from view, since the vast majority of emerging mutations are recessive and do not appear outwardly. Recessive mutations seem to be “absorbed by a species in a heterozygous state”, because most organisms are heterozygous for many genes. Such latent variability can be revealed in experiments with crossing closely related individuals. With such a cross, some recessive alleles that were in a heterozygous and therefore latent state will go into a homozygous state and will be able to appear. Significant genetic variability of natural populations is easily detected in the course of artificial selection. In artificial selection, those individuals are selected from the population in which any economically valuable traits are most pronounced, and these individuals are crossed with each other. Artificial selection is 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, major changes usually reduce this fitness, just as large accidental changes in the mechanism of a clock (removal of some spring or addition of a wheel) lead to its failure. Populations have large stocks of such alleles that do not bring it any benefit in a given place or in given time; they remain in the population in a heterozygous state until, as a result of a change in environmental conditions, they suddenly turn out to be useful. As soon as this happens, their frequency under the influence of selection begins to increase, and eventually they become the main genetic material. This is where the ability of a population to adapt lies, i.e., to adapt to new factors - climate change, the emergence of a new predator or competitor, and even to human pollution.

An example of such adaptation is the evolution of insecticide-resistant insect species. Events in all cases develop in the same way: when a new insecticide (a poison that acts on insects) is introduced into practice, a small amount of it is enough to successfully control an insect pest. Over time, the concentration of the insecticide has to be increased until, finally, it is ineffective. The first report of insecticide resistance in insects appeared in 1947 and related to housefly resistance to DDT. Subsequently, resistance to one or more insecticides has been found in at least 225 species of insects and other arthropods. Genes capable of conferring insecticide resistance appear to have been present in each of the populations of these species; their action and ultimately ensured a decrease in the effectiveness of poisons used for pest control

4. How can recessive alleles be identified?

Answer. Recessive allele (recessive allele, from Latin recessus - retreat) - an allele whose phenotype does not appear in heterozygotes, but manifests itself in a homozygous or hemizygous genotype for this allele. If recessive alleles are in the homozygous state, then they will appear in the phenotype. If you need to find out if they are present in the genotype of an organism with a dominant phenotype, then analyze crosses are used. To do this, the tested organism is crossed with a carrier of a recessive phenotype. If there are recessive individuals in the offspring, then the tested organism is the carrier of the recessive gene.

Among the factors of the genetic dynamics of a population that violate its equilibrium state are: the 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 mutational pressure. The magnitude of the 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, and on the presence of mutagenic factors in the environment. In addition, the magnitude of the mutational pressure is affected by the extent to which the mutation affects the viability and fertility of the individual.

Studies show that natural populations are saturated with mutant genes, which are mostly in the heterozygous state. The mutation process creates the primary genetic variability of the population, with which further action must be taken. 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 selection efficiency depends on whether the mutant trait is dominant or recessive. Purification of the population from individuals with a harmful dominant mutation can be achieved in one generation if its carrier does not leave behind 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 rate, which increases the adaptive potential of their owners. With the preservation and reproduction of heterozygotes, 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 b-chain of hemoglobin, 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 in early age. The erythrocytes of such people are sickle-shaped. Heterozygosity for this mutation does not lead to anemia. Erythrocytes in heterozygotes have a normal shape, but contain 60% normal and 40% altered hemoglobin. This suggests that both alleles function in heterozygotes - normal and mutant. 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 with a heterozygous genotype are less likely to be infected with dengue fever, which causes high mortality in these areas, compared to the norm. In this regard, selection preserves both genotypes: normal (dominant homozygous) 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 an adaptive value.

There are other forms of natural selection. Stabilizing selection preserves the norm as the variant of the genotype that best meets the prevailing conditions, eliminating the deviations from it that arise. This form of selection usually operates when the population is under relatively stable conditions of existence for a long time. In contrast, motive selection retains a new trait if the resulting mutation is beneficial and gives its bearers some advantage. Selection disruptive(tearing) acts simultaneously in two directions, preserving the extreme variants of the development of the trait. Ch. Darwin gave a typical example of this form of selection. 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 is reduced to an increase in the number of individuals with traits, in the direction of which selection proceeds. At the same time, signs linked to them and signs that are in a correlative relationship with the first are also selected. For genes that control traits that are 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 the organism. It is aimed at preserving the traits that are beneficial for the organism, which increase its adaptability and give it an advantage over other organisms. In contrast, the action of artificial selection that takes place in populations cultivated plants and domestic animals, is narrower and most often affects signs that are useful for humans, and not for their carriers.

genetic drift

Random causes have a great influence on the genotypic structure of populations. These include: fluctuations in population size, the age and sex composition of populations, the quality and quantity of food resources, the presence or absence of competition, the random nature of the sample that gives rise to the next generation, etc. Change in gene frequencies in a population for random reasons American geneticist S. Wright named genetic drift, and N.P. Dubinin is a genetic-automatic process. Sharp fluctuations in population size have a particularly noticeable effect on the genetic structure of populations - population waves, or waves of life. It has been established that in small populations, dynamic processes proceed much more intensively, and the role of chance in the accumulation of individual genotypes increases. When a population is reduced, some mutant genes can be accidentally preserved in it, while others can also be randomly eliminated. With a subsequent increase in the population size, the number of these preserved genes can increase rapidly. The drift rate is inversely proportional to the size of the population. At the time of the decrease in numbers, the drift is especially intense. With a very sharp reduction in the population, there may be a threat of its extinction. This is the so-called situation bottleneck". If the population manages to survive, then as a result of genetic drift, their frequencies will change, which will affect the structure of the new generation.

Genetic-automatic processes are especially clear in isolates, when a group of individuals is isolated from a large population and forms a new settlement. There are many such examples in the genetics of human populations. So, in the state of Pennsylvania (USA) lives a sect of Mennonites, numbering several thousand people. Marriages here are allowed only between members of the sect. The isolate began with 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 the pleiotropic gene, which in the homozygous state causes a special form of dwarfism with polydactyly. About 13% of the members of this sect are heterozygous for this rare mutation. It is likely that the “ancestor effect” took place 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. In other groups of Mennonites scattered throughout the United States, such a disease has not been found.

Migrations

Another reason for changing the frequencies of genes in a population is migration. During the movement of groups of individuals and their crossing with members of another population, genes are transferred from one population to another. The effect of migration depends on the size of the group of migrants and the difference in gene frequencies between exchanging populations. If the initial frequencies of genes in populations are very different, then a significant shift in frequencies can occur. As the migration progresses, the genetic differences between populations level off. The end result of the pressure of migrations is the establishment of a certain average concentration for each mutation throughout the system of populations between which there is an exchange of individuals.

An example of the role of migrations is the distribution of genes that determine human blood groups of the system AB0. Europe is characterized by the predominance of the group BUT, for Asia - groups AT. The reason for the differences, according to geneticists, lies in the large migrations of the population that took place from East to West in the period from 500 to 1500 years. ad.

Insulation

If individuals of one population do not fully 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 at the interpopulation level.

There are two main types of insulation: spatial, or mechanical, isolation 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 water bodies; volcanic eruption, etc.), or as a result of human activity (plowing land, draining swamps, forest plantations, 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, common frog, sedge, and common loach.

biological isolation subdivided into morpho-physiological, ecological, ethological and genetic. All these types of isolation are characterized by the appearance 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 copulatory organs, which is especially true for insects and some rodents. In plants, such features as the size of the pollen grain, the length of the pollen tube, and the coincidence of the maturation periods of pollen and stigmas play an important role.

At ethological isolation in animals, differences in the behavior of individuals during the reproductive period serve as an obstacle, for example, unsuccessful courtship of a male for a female is observed.

Environmental isolation can manifest itself in different forms: in the preference for a certain reproductive territory, in different periods of maturation of germ cells, reproduction rates, etc. For example, in marine fish migrating to breed in rivers, a special population develops in each river. Representatives of these populations may differ in size, color, time of onset of puberty, and other features related to the reproductive process.

genetic isolation includes different mechanisms. Most often, it occurs due to violations of the normal course of meiosis and the formation of non-viable gametes. The causes of violations can be polyploidy, chromosomal rearrangements, nuclear-plasma incompatibility. Each of these phenomena can lead to limited panmixia and infertility of hybrids, and, consequently, to limiting the process of free combination of genes.

Isolation is rarely created by any one mechanism. Usually several different forms of isolation take place at the same time. They can act both at the stage preceding fertilization and after it. In the latter case, the insulation system is less economical, since a significant amount of energy resources is wasted, for example, on the production of sterile offspring.

The listed factors of the genetic dynamics of populations can act singly and jointly. In the latter case, either a cumulative effect can be observed (for example, a 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 gene drift).

The study of dynamic processes in populations allowed S.S. Chetverikov (1928) to 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 free crossing of individuals, under the very conditions of which, according to Chetverikov, an apparatus for stabilizing the numerical ratios of alleles is laid.

The genetic processes that 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 similar in many respects, only the scale of the changes taking place is different.

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Biology Lesson Plan

Topic: Genetic composition of populations

genetics mutational hereditary population

Type of lesson: a lesson that reveals the content of the topic.

The purpose of the lesson: continue to deepen and expand knowledge about populations, to characterize the concept of the gene pool of populations.

Tasks:

Educational. To form the concept of population genetics; characterize the gene pool of a population; find out that the mutation process is a constant source of hereditary variability.

Developing. Continue to form the ability to observe and note the main thing when listening to messages, working with textbook material.

Educational. Continue to form a scientific outlook, love for nature, work culture based on keeping records in a notebook.

Equipment

Tables, textbook.

During the classes

1. Organizational moment 1-2 min. Interview homework: 1) What is a population? 2) Why species exist in the form of populations? 5-7 min.

2. Learning new material. 25 min.

3. Consolidation of the studied material. Grading.

4. Homework.

2. Learning new material

Consolidation of the studied material

4. Homework

population genetics. At the time of Darwin, the science of genetics did not yet exist. It began to develop at the beginning of the 20th century. It became known that the carriers of hereditary variability are genes.

Representations of genetics have introduced additional in-depth explanations into the theory of natural selection by Charles Darwin. The synthesis of genetics and classical Darwinism led to the birth of a special area of research - population genetics, which made it possible to explain from new positions the processes of changing 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.

The mutation process is a constant source of hereditary variability. In a population consisting of several million individuals, several mutations of literally every gene present in this population can occur in each generation. Due to combinative variability, mutations spread in a population.

Natural populations are saturated with a wide variety of mutations. This was noticed by the Russian scientist Sergey Sergeevich Chetverikov (1880-1959), who found that a significant part of the variability of the gene pool is hidden from view, since the vast majority of the resulting mutations are recessive and do not appear outwardly. Recessive mutations seem to be “absorbed by a species in a heterozygous state”, because most organisms are heterozygous for many genes. Such latent variability can be revealed in experiments with crossing closely related individuals. With such a cross, some recessive alleles that were in a heterozygous and therefore latent state will go into a homozygous state and will be able to appear.

Significant genetic variability of natural populations is easily detected in the course of artificial selection. In artificial selection, those individuals are selected from a population in which any economically valuable traits are most pronounced, and these individuals are crossed with each other. Artificial selection is effective in almost all cases when 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 operate 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 perish. S.S. Chetverikov showed that, with rare exceptions, most of the newly emerging mutations are harmful and in the homozygous state, as a rule, reduce the viability of individuals. They persist in populations only through selection in favor of heterozygotes. However, mutations that are detrimental in some conditions may increase viability in other conditions. Thus, a mutation that causes the 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, major changes usually reduce this fitness, just as large accidental changes in the mechanism of a clock (removal of some spring or addition of a wheel) lead to its failure. There are large stocks of such alleles in populations that do not bring it any benefit at a given place or at a given time; they remain in the population in a heterozygous state until, as a result of a change in environmental conditions, they suddenly turn out to be useful. As soon as this happens, their frequency under the influence of selection begins to increase, and eventually they become the main genetic material. This is where the population's ability to adapt lies, i.e. 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 insecticide-resistant insect species. Events in all cases develop in the same way: when a new insecticide (a poison that acts on insects) is introduced into practice, a small amount of it is enough to successfully control an insect pest. Over time, the concentration of the insecticide has to be increased until, finally, it is ineffective. The first report of insecticide resistance in insects appeared in 1947 and related to housefly resistance to DDT. Subsequently, resistance to one or more insecticides has been found in at least 225 species of insects and other arthropods. Genes capable of conferring insecticide resistance appear to have been present in each of the populations of these species; their action and ensured the ultimate reduction in the effectiveness of poisons used for pest control.

Thus, the mutation process creates material for evolutionary transformations, forming a reserve of hereditary variability in the gene pool of each population and species as a whole. By maintaining a high degree of genetic diversity in populations, it provides the basis for the operation of natural selection and microevolution.

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