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Driving selection examples of animals. Driving selection. How natural selection works

Snezhinsky Polytechnic College

Report on biology on the topic:

"Natural selection"

Completed by: 1st year student

F-18D groups

Yakunina Elena

Checked by: Budalova I.B.

Snezhinsk 2009

Natural selection

a) Destabilizing selection

b) Sexual selection

c) Group selection

d) Directed selection (moving)

e) Stabilizing selection

f) Disruptive (dismembering) selection

Conclusion

Bibliography

Natural selection

Natural selection- the result of the struggle for existence; it is based on preferential survival and leaving offspring with the most adapted individuals of each species and the death of less adapted organisms.

The mutation process, population fluctuations, isolation create genetic heterogeneity within a species. But their action is not directed. Evolution, on the other hand, is a directed process associated with the development of adaptations, with a progressive complication of the structure and functions of animals and plants. There is only one directed evolutionary factor - natural selection.

Either certain individuals or entire groups can be subject to selection. As a result of group selection, traits and properties are often accumulated that are unfavorable for an individual, but useful for the population and the whole species (a stinging bee dies, but attacking the enemy, it saves the family). In any case, selection preserves the organisms most adapted to a given environment and operates within populations. Thus, it is populations that are the field of action of selection.

Natural selection should be understood as selective (differential) reproduction of genotypes (or gene complexes). In the process of natural selection, it is not so much the survival or death of individuals that is important, but their differential reproduction. Success in reproduction of different individuals can serve as an objective genetic-evolutionary criterion of natural selection. The biological significance of an individual that has given offspring is determined by the contribution of its genotype to the gene pool of the population. Selection from generation to generation according to phenotypes leads to the selection of genotypes, since not traits, but gene complexes are transmitted to descendants. For evolution, not only genotypes are important, but also phenotypes and phenotypic variability.

During expression, a gene can influence many traits. Therefore, the scope of selection can include not only properties that increase the likelihood of leaving offspring, but also traits that are not directly related to reproduction. They are selected indirectly as a result of correlations.

a) Destabilizing selection

Destabilizing selection- this is the destruction of correlations in the body with intensive selection in each specific direction. An example is the case when selection aimed at reducing aggressiveness leads to destabilization of the breeding cycle.

Stabilizing selection narrows the reaction rate. However, in nature there are cases when the ecological niche of a species may become wider over time. In this case, the selective advantage is obtained by individuals and populations with a wider reaction rate, while maintaining the same average value of the trait. This form of natural selection was first described by the American evolutionist George G. Simpson under the name centrifugal selection. As a result, a process occurs that is the reverse of stabilizing selection: mutations with a wider reaction rate gain an advantage.

Thus, populations of marsh frogs living in ponds with heterogeneous illumination, with alternating areas overgrown with duckweed, reed, cattail, with “windows” of open water, are characterized by a wide range of color variability (the result of a destabilizing form of natural selection). On the contrary, in water bodies with uniform illumination and coloration (ponds completely overgrown with duckweed, or open ponds), the range of variability in frog coloration is narrow (the result of the action of a stabilizing form of natural selection).

Thus, a destabilizing form of selection in goes to the expansion of the reaction rate.

b) sexual selection

sexual selection- natural selection within the same sex, aimed at developing traits that give mainly the opportunity to leave the largest number of descendants.

In males of many species, pronounced secondary sexual characteristics are found that at first glance seem maladaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features make life difficult for their carriers, making them easily visible to predators. It would seem that these signs do not give any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their origin and spread?

We already know that the survival of organisms is an important but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. Charles Darwin called this phenomenon sexual selection. He first mentioned this form of selection in The Origin of Species and later analyzed it in detail in The Descent of Man and Sexual Selection. He believed that "this form of selection is determined not by the struggle for existence in the relationship of organic beings among themselves or with external conditions, but by the rivalry between individuals of the same sex, usually males, for the possession of individuals of the other sex."

Sexual selection is natural selection for success in reproduction. Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition for females arises between males. This competition can be direct, and manifest itself in the form of a struggle for territories or tournament fights. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition is shown in displaying their flamboyant appearance or complex courtship behavior. Females choose those males that they like the most. As a rule, these are the brightest males. But why do females like bright males?

Rice. 7. The bright colors of birds arise in evolution due to sexual selection.

The fitness of the female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability. Imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases, because females choose males not with a certain tail size, but with a larger than average size. In the end, the tail reaches such a length that its harm to the viability of the male is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all that amazing variety of forms, colors and instincts that we observe in the world of wildlife. .

c) Group selection

Group selection is often also called group selection, it is the differential reproduction of different local populations. Wright compares population systems of two types - a large continuous population and a number of small semi-isolated colonies - in relation to the theoretical selection efficiency. It is assumed that the total size of both population systems is the same and the organisms interbreed freely.

In a large contiguous population, selection is relatively inefficient in terms of increasing the frequency of favorable but rare recessive mutations. In addition, any tendency to increase the frequency of any favorable allele in one part of a given large population is counteracted by crossing with neighboring subpopulations in which that allele is rare. Similarly, favorable new gene combinations that manage to form in any local fraction of a given population are broken up into parts and eliminated as a result of crossing with individuals of neighboring shares.

All these difficulties are eliminated to a large extent in a population system that resembles in its structure a series of separate islands. Here, selection, or selection in conjunction with genetic drift, can quickly and effectively increase the frequency of some rare favorable allele in one or more small colonies. New favorable combinations of genes can also easily gain a foothold in one or more small colonies. Isolation protects the gene pools of these colonies from "flooding" as a result of migration from other colonies that do not have such favorable genes, and from crossing with them. Up to this point, only individual selection or - for some colonies - individual selection combined with genetic drift has been included in the model.

Let us now assume that the environment in which this population system is located has changed, as a result of which the adaptability of the former genotypes has decreased. In a new environment, new favorable genes or combinations of genes that are fixed in some colonies have a high potential adaptive value for the population system as a whole. All conditions are now in place for group selection to take effect. The less fit colonies gradually shrink and die out, while the more fit colonies expand and replace them throughout the area occupied by a given population system. Such a subdivided population system acquires a new set of adaptive traits as a result of individual selection within certain colonies, followed by differential reproduction of different colonies. The combination of group and individual selection can lead to results that cannot be achieved through individual selection alone.

It has been established that group selection is a second-order process that complements the main process of individual selection. As a second order process, group selection must be slow, probably much slower than individual selection. Updating populations takes more time than updating individuals.

The concept of group selection has been widely accepted in some circles, but has been rejected by other scientists. They argue that the various possible patterns of individual selection are capable of producing all the effects attributed to group selection. Wade conducted a series of breeding experiments with mealworm (Tribolium castaneum) to ascertain the effectiveness of group selection and found that the beetles responded to this type of selection. In addition, when a trait is simultaneously affected by individual and group selection and, moreover, in the same direction, the rate of change of this trait is higher than in the case of individual selection alone (Even moderate immigration (6 and 12%) does not prevent differentiation populations caused by group selection.

One of the features of the organic world, which is difficult to explain on the basis of individual selection, but can be considered as the result of group selection, is sexual reproduction. Although models have been created in which sexual reproduction is favored by individual selection, they appear to be unrealistic. Sexual reproduction is the process that creates recombination variation in interbreeding populations. It is not the parental genotypes that break up in the process of recombination that benefit from sexual reproduction, but the population of future generations, in which the margin of variability increases. This implies participation as one of the factors of the selective process at the population level.

G)

Rice. 1. Driving form of natural selection

Directional selection (moving) was described by Ch. Darwin, and the modern doctrine of driving selection was developed by J. Simpson.

The essence of this form of selection is that it causes a progressive or unidirectional change in the genetic composition of populations, which manifests itself in a shift in the average values of the selected traits in the direction of their strengthening or weakening. It occurs when a population is in the process of adapting to a new environment, or when there is a gradual change in the environment, followed by a gradual change in the population.

With a long-term change in the external environment, a part of the individuals of the species with some deviations from the average norm may gain an advantage in life and reproduction. This will lead to a change in the genetic structure, the emergence of evolutionarily new adaptations and a restructuring of the organization of the species. The variation curve shifts in the direction of adaptation to new conditions of existence.

Figure 2. Dependence of the frequency of dark forms of the birch moth on the degree of atmospheric pollution

Light-colored forms were invisible on birch trunks covered with lichens. With the intensive development of industry, sulfur dioxide produced by burning coal caused the death of lichens in industrial areas, and as a result, dark bark of trees was discovered. On a dark background, light-colored moths were pecked by robins and thrushes, while melanic forms survived and successfully reproduced, which are less noticeable against a dark background. Over the past 100 years, more than 80 species of butterflies have developed dark forms. This phenomenon is now known under the name of industrial (industrial) melanism. Driving selection leads to the emergence of a new species.

Rice. 3. Industrial melanism. Dark forms of butterflies are invisible on dark trunks, and light ones on light ones.

Insects, lizards and a number of other inhabitants of the grass are green or brown in color, the inhabitants of the desert are the color of sand. The fur of animals living in the forests, such as a leopard, is colored with small spots resembling sun glare, while in a tiger it imitates the color and shadow from the stems of reeds or reeds. This coloring is called patronizing.

In predators, it was fixed due to the fact that its owners could sneak up on prey unnoticed, and in organisms that are prey, due to the fact that the prey remained less noticeable to predators. How did she appear? Numerous mutations gave and give a wide variety of forms that differ in color. In a number of cases, the coloring of the animal turned out to be close to the background of the environment, i.e. hid the animal, played the role of a patron. Those animals in which the protective coloration was weakly expressed were left without food or became victims themselves, and their relatives with the best protective coloration emerged victorious in the interspecific struggle for existence.

Directed selection underlies artificial selection, in which selective breeding of individuals with desirable phenotypic traits increases the frequency of those traits in a population. In a series of experiments, Falconer chose the heaviest individuals from a population of six-week-old mice and let them mate with each other. He did the same with the lightest mice. Such selective crossing on the basis of body weight led to the creation of two populations, in one of which the mass increased, and in the other it decreased.

After the selection was stopped, neither group returned to its original weight (approximately 22 grams). This shows that artificial selection for phenotypic traits has led to some genotypic selection and partial loss of some alleles by both populations.

e) Stabilizing selection

Rice. 4. Stabilizing form of natural selection

Stabilizing selection in relatively constant environmental conditions, natural selection is directed against individuals whose characters deviate from the average norm in one direction or another.

Stabilizing selection preserves the state of the population, which ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value in terms of adaptive characteristics are removed.

Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation.

However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them. As a result, individuals with average fecundity turn out to be the most adapted.

Selection in favor of averages has been found for a variety of traits. In mammals, very low and very high birth weight newborns are more likely to die at birth or in the first weeks of life than middle weight newborns. Accounting for the size of the wings of birds that died after the storm showed that most of them had either too small or too large wings. And in this case, the average individuals turned out to be the most adapted.

What is the reason for the constant appearance of poorly adapted forms in constant conditions of existence? Why is natural selection unable to once and for all clear a population of unwanted evasive forms? The reason is not only and not so much in the constant emergence of more and more new mutations. The reason is that heterozygous genotypes are often the fittest. When crossing, they constantly give splitting and homozygous descendants with reduced fitness appear in their offspring. This phenomenon is called balanced polymorphism.

Fig.5. Map of the distribution of sickle cell anemia in malarial areas. Colors indicate malarial areas. The shaded area shows a high incidence of sickle cell anemia.

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutant hemoglobin allele (Hb S) and leads to their death at an early age. In most human populations, the frequency of this alley is very low and approximately equal to the frequency of its occurrence due to mutations. However, it is quite common in areas of the world where malaria is common. It turned out that heterozygotes for Hb S have a higher resistance to malaria than homozygotes for the normal alley. Due to this, in populations inhabiting malarial areas, heterozygosity is created and stably maintained for this lethal alley in the homozygote.

Stabilizing selection is a mechanism for the accumulation of variability in natural populations. The outstanding scientist I. I. Shmalgauzen was the first to pay attention to this feature of stabilizing selection. He showed that even under stable conditions of existence, neither natural selection nor evolution ceases. Even remaining phenotypically unchanged, the population does not cease to evolve. Its genetic makeup is constantly changing. Stabilizing selection creates such genetic systems that provide the formation of similar optimal phenotypes on the basis of a wide variety of genotypes. Such genetic mechanisms as dominance, epistasis, complementary action of genes, incomplete penetrance, and other means of hiding genetic variability owe their existence to stabilizing selection.

The stabilizing form of natural selection protects the existing genotype from the destructive influence of the mutation process, which explains, for example, the existence of such ancient forms as the tuatara and ginkgo.

Thanks to stabilizing selection, "living fossils" that live in relatively constant environmental conditions have survived to this day:

1. tuatara, bearing the features of reptiles of the Mesozoic era;

2. coelacanth, a descendant of lobe-finned fish, widespread in the Paleozoic era;

3. North American opossum - a marsupial known from the Cretaceous period;

The stabilizing form of selection acts as long as the conditions that led to the formation of a particular trait or property persist.

It is important to note here that the constancy of conditions does not mean their immutability. During the year, environmental conditions change regularly. Stabilizing selection adapts populations to these seasonal changes. Breeding cycles are timed to them, so that the young are born in that season of the year when food resources are maximum. All deviations from this optimal cycle, reproducible from year to year, are eliminated by stabilizing selection. Descendants born too early die from starvation, too late - they do not have time to prepare for winter. How do animals and plants know when winter is coming? On the onset of frost? No, it's not a very reliable pointer. Short-term temperature fluctuations can be very deceptive. If in some year it gets warmer earlier than usual, this does not mean at all that spring has come. Those who react too quickly to this unreliable signal risk being left without offspring. It is better to wait for a more reliable sign of spring - an increase in daylight hours. In most animal species, it is this signal that triggers the mechanisms of seasonal changes in vital functions: cycles of reproduction, molting, migration, etc. I.I. Schmalhausen convincingly showed that these universal adaptations arise as a result of stabilizing selection.

Thus, stabilizing selection, sweeping aside deviations from the norm, actively forms genetic mechanisms that ensure the stable development of organisms and the formation of optimal phenotypes based on various genotypes. It ensures the stable functioning of organisms in a wide range of fluctuations in external conditions familiar to the species.

f) Disruptive (dismembering) selection

Rice. 6. Disruptive form of natural selection

Disruptive (dismembering) selection favors the preservation of extreme types and the elimination of intermediate ones. As a result, it leads to the preservation and strengthening of polymorphism. Disruptive selection operates in a variety of environmental conditions found in the same area, and maintains several phenotypically different forms at the expense of individuals with an average norm. If environmental conditions have changed so much that the bulk of the species loses fitness, then individuals with extreme deviations from the average norm acquire an advantage. Such forms multiply rapidly and on the basis of one group several new ones are formed.

A model of disruptive selection can be the situation of the emergence of dwarf races of predatory fish in a water body with little food. Often, juveniles of the year do not have enough food in the form of fish fry. In this case, the advantage is gained by the fastest growing ones, which very quickly reach a size that allows them to eat their fellows. On the other hand, squints with the maximum delay in growth rate will be in an advantageous position, since their small size allows them to remain planktivorous for a long time. A similar situation through stabilizing selection can lead to the emergence of two races of predatory fish.

An interesting example is given by Darwin regarding insects - inhabitants of small oceanic islands. They fly well or are completely devoid of wings. Apparently, the insects were blown out to sea by sudden gusts of wind; only those that could either resist the wind or not fly at all survived. Selection in this direction has led to the fact that out of 550 species of beetles on the island of Madeira, 200 are flightless.

Another example: in forests where soils are brown, earth snail specimens often have brown and pink shells, in areas with coarse and yellow grass, yellow color prevails, etc.

Populations adapted to ecologically dissimilar habitats may occupy contiguous geographic areas; for example, in coastal areas of California, the Giliaachilleaefolia plant is represented by two races. One race - "sunny" - grows on open grassy southern slopes, while the "shady" race is found in shady oak forests and sequoia groves. These races differ in the size of the petals - a trait determined genetically.

The main result of this selection is the formation of population polymorphism, i.e. the presence of several groups that differ in some way or in the isolation of populations that differ in their properties, which may be the cause of divergence.

Conclusion

Like other elementary evolutionary factors, natural selection causes changes in the ratio of alleles in the gene pools of populations. Natural selection plays a creative role in evolution. By excluding genotypes with low adaptive value from reproduction, while preserving favorable gene combinations of different merits, he transforms the picture of genotypic variability, which is formed initially under the influence of random factors, in a biologically expedient direction.

Bibliography

1) Vlasova Z.A. Biology. Student Handbook - Moscow, 1997

2) Green N. Biology - Moscow, 2003

3) Kamluk L.V. Biology in questions and answers - Minsk, 1994

4) Lemeza N.A. Biology manual - Minsk, 1998

In 1859, the English scientist Charles Darwin published his fundamental work "The Origin of Species by Means of Natural Selection". This book was the first to formulate the modern theory of evolution. Its driving force is natural selection, which, in turn, is divided into several types, including examples of this hypothesis given in the Origin of Species, clearly demonstrated how the mechanism for the development of life on earth works.

The essence of motive selection

The principle of driving selection is that individuals who have received some differences from the general norm adopted by the species are in a privileged position and eventually win the struggle for survival. This is a long and difficult process. Intraspecific variability affects all structures and organs in each species. It concerns both quantitative features (presence or absence of variation) and qualitative features (dimensional, countable).

The history of mammalian development provides researchers with numerous examples of the driving form of selection. Their most variable features are the number of hairs per unit area, the mass of various organs, and the number of red blood cells in the blood. In the course of evolution, the size of the human brain has increased. A huge number of variations lies in the features of attachment of different muscles, the structure of the bronchial tree of the lungs, and the shape of the liver.

Doubtful Species

A multitude of intermediate species forms has given rise to motive selection. Darwin himself cited examples of this group. This is the British red grouse, descended from the Norwegian species, Madeira insects, birds All of them can be described as "dubious species". What are their main features? These are forms that are significantly similar to a species, but are so similar to some other forms or are closely interconnected with them by intermediate steps that biologists do not recognize them as independent species.

Such living beings are links in evolution. Dubious species are actually emerging new ones. They are not yet so well separated from their ancestors, but have already begun the process of separation. These are examples of motive selection in animals. They result from the struggle for life. However slight the accidental changes in the species, if they are in any way useful, they will no doubt be preserved and inherited by posterity.

Avian driving selection

The struggle for existence is primarily a struggle for food. If a species fails to secure its position in the food chain, it is bound to become extinct. Examples of driving are clearly seen in the appetite of animals.

Consider several types of birds. in one day it eats a mass of insects equivalent to the mass of its own body, and brings food to its chicks hundreds of times a day, grabbing 5-6 caterpillars in one serving. The pied flycatcher feeds offspring a kilogram of beetles and worms in two weeks. A kinglet can eat up to 10 million insects a year in a year. The American kestrel needs to catch up to 300 mice and dozens of small birds over the same period. The food delivered by starlings to their chicks can fill three birdhouses.

Each of these cases is an example of a driving form of natural selection at work. Changes in the stomach, intestines and beak gradually changed the birds. Some of them became more enduring and prolific, others became large predators, others died out, left without food and turned into food for their neighbors.

dominant species

Diversity is generated when an animal or plant is widely distributed throughout the world. Darwin also called these species dominant. It is they who are most often distinguished by motive selection. An example - living in different regions of Eurasia It forms several geographical forms, constantly replacing each other. The foxes living in the north are much larger than the foxes living in the south, in the zone of steppes and semi-deserts. The smallest of them live in Central Asia, and especially in Afghanistan.

The wide range of the fox world is the result of evolution by motive selection. The example is obvious: in the north, animals need to be more enduring than in the south. This is due to both climatic conditions and dangerous neighbors. During the migration of foxes to the south, each new generation became smaller as a result of small natural changes. New individuals became more adapted to the steppes and deserts and continued to conquer unfamiliar territories.

Driving selection and food supply

All examples of motive natural selection show that in each individual case, nature maintains a biological balance. Even if a new species gains an advantage and becomes dominant, there is always a limit to its dominance. This principle also manifests itself in the event that a person tries to interfere in natural processes.

In 1911, 25 reindeer were brought to Pribilof Island near Alaska. They took root well in the new place - in 1938 there were already two thousand of them. There were too many individuals, because of which the food supply was undermined and the entire population gradually died out. In 1950, only 8 deer remained on the island. The characteristics of driving selection and examples show that if a species finds itself in too good conditions, it multiplies en masse, destroys the food it needs, and eventually dies itself.

A similar situation has developed on Arizona's Keibab Plateau, where people, trying to restore the number of black-tailed deer, shot all the coyotes and cougars and banned hunting. Exceeding the permissible population density was the starting point for the extinction of the population.

Randomness of Mutations

The motive selection mechanism operates chaotically. Darwin could not understand how the changes that appear in new generations of living organisms are regulated. Scientists of the 20th century came to the conclusion that new traits arise in animals and plants as a result of random mutations. They can appear imperceptibly and disappear imperceptibly, but if such changes turn out to be beneficial for the individual, they are preserved and inherited by offspring.

The Europeans who discovered Australia brought an ordinary bee to the continent, which quickly exterminated the native bees, which had a smaller sting. This case is artificial. It was caused by human activity. But it is precisely on the same principle that natural motive selection operates.

Intraspecific struggle

The struggle for survival is always stubborn, but the struggle for life between individuals and varieties of the same species is doubly stubborn. There is a similarity in habits and the structure of the body.

In Scotland in the 19th century, there was a confrontation between two species of thrushes - an increase in the number of mistle thrushes led to the disappearance of song thrushes. An example of the action of a driving form of natural selection is the fact that in Russia the Asian Prussian cockroaches have everywhere replaced their larger relatives.

Interspecies struggle

Examples of motive selection in plants can also be considered in the context of interspecific struggle. Well-known to everyone, the dandelion has tufted tufts. They carry seeds and are closely related to the dense population of areas in which this plant is present. Such a structure helps not only to survive, but also to multiply in large numbers. Seeds on flyers are able to spread far through the air and fall on soil that has not yet been occupied by anyone.

Expansion

At first glance, the supply of food in the seeds of many plants has nothing to do with other plants. However, in fact, they have a fundamentally important meaning. It lies in the rate of growth of seedlings, forced to fight with the extraneous vegetation surrounding them. In the early stages of existence, young shoots of peas or beans develop rapidly. In the course of their own evolution, their seeds began to receive a large supply of food, which helped them to occupy a significant niche in the organic world. The competing species of peas and beans that did not receive this advantage lost the interspecific struggle and disappeared from the face of the earth.

The above example shows an important pattern. When an animal or plant enters a new country and finds itself among previously unfamiliar competitors, its living conditions change greatly, even if the climate remains the same. In order to gain a foothold in a new territory, a species must necessarily deviate in its development from its ancestors.

Selection slowness

Driving selection operates hourly and daily. It preserves and adds up useful changes, thereby improving the organic being, depending on the conditions of its life. Selection is slow and imperceptible to the human eye, but at the same time relentless. Evolution cannot be seen over several generations. To do this, scientists have to study entire geological epochs and periods lasting thousands and millions of years.

Selection can work at the expense of signs, it would seem, completely insignificant. For example, leaf-eating insects are green, while bark-eating trees are mottled gray. If the color changes, these creatures will become visible and vulnerable to predators. Similarly, for a flock of white sheep, the presence of lambs with even a small black spot is disastrous.

Correlation and Fitting

They change not only as a result of random mutations, but also according to the principle of correlation. What is its essence? When one part of the body changes, it will necessarily lead to changes in other parts. Often such evolutionary turns have the most unexpected properties.

The main function of change is adaptation. They can appear at various stages of life. For example, ostrich chicks develop characteristic horny bumps on the upper part of the beak, which are also called chick teeth. In the very first days after hatching from the egg, they dissolve and disappear. Their only purpose is to help the chick break the shell. This is the so-called embryonic adaptation. It allows the species to increase its birth rate and more effectively fight for survival. Due to such seemingly insignificant features, motive selection functions.

Natural selection increases the chances of survival and continuation of the whole genus, it is on the same level as mutations, migrations and transformations in genes. The main mechanism of evolution works flawlessly, but on the condition that no one interferes with its work.

What is natural selection?

The meaning of this term was given by the English scientist Charles Darwin. He established that natural selection is a process that determines the survival and reproduction of only individuals adapted to environmental conditions. According to Darwin's theory, the most important role in evolution is played by random hereditary changes.

recombination of genotypes;
mutations and their combinations.

Natural selection in humans

In times of underdeveloped medicine and other sciences, only a person with strong immunity and a stable healthy body survived. They did not know how to care for premature newborns, they did not use antibiotics in the treatment, they did not perform operations, and they had to cope with their illnesses on their own. Natural selection in humans has selected the strongest representatives of humanity for further reproduction.

In the civilized world, it is not customary to acquire numerous offspring, and in most families there are no more than two children, who, thanks to modern living conditions and medicine, may well live to a ripe old age. Previously, families had 12 or more children, and no more than four survived under favorable conditions. Natural selection in man has led to the fact that for the most part hardened, exceptionally healthy and strong people survived. Thanks to their gene pool, humanity still lives on earth.

Reasons for natural selection

All life on earth developed gradually, from the simplest organisms to the most complex ones. Representatives of certain forms of life that failed to adapt to the environment did not survive and did not reproduce, their genes were not passed on to subsequent generations. The role of natural selection in evolution has led to the emergence of the ability at the cellular level to adapt to the environment and quickly respond to its changes. The causes of natural selection are influenced by a number of simple factors:

Natural selection works when more offspring are produced than can survive.
In the genes of the body there is hereditary variability.
Genetic differences dictate the survival and ability to reproduce offspring in different conditions.

Signs of natural selection

The evolution of any living organism is the creativity of nature itself and this is not a whim, but a necessity. Acting in various environmental conditions, it is not difficult to guess what features natural selection retains, all of them are aimed at the evolution of the species, increasing its resistance to external influences:

The selection factor plays an important role. If in artificial selection a person chooses which features of the species to preserve and which not (for example, when breeding a new breed of dogs), then in natural selection the strongest wins in the struggle for his existence.
The material for selection is hereditary changes, the signs of which can help in adapting to new living conditions or for specific purposes.
The result is another stage of natural selection, as a result of which new species were formed with traits that are beneficial in certain environmental conditions.
The speed of action - mother nature is not in a hurry, she thinks over her every step, and therefore, natural selection is characterized by a low rate of change, while artificial selection is fast.

What is the result of natural selection?

All organisms have their own degree of adaptability and it is impossible to say with certainty how one or another species will behave in unfamiliar environmental conditions. The struggle for survival and hereditary variability is the essence of natural selection. There are many examples of plants and animals that have been introduced from other continents and have adapted better to new living conditions. The result of natural selection is a whole set of acquired changes.

adaptation - adaptation to new conditions;
variety of forms of organisms - arise from a common ancestor;
evolutionary progress - the complication of species.

How is natural selection different from artificial selection?

It can be said with certainty that almost everything that is eaten by humans sooner or later was subjected to artificial selection. The fundamental difference is that by conducting "his" selection, a person pursues his own benefit. Thanks to selection, he received selected products, brought out new breeds of animals. Natural, natural selection is not focused on the benefit for humanity, it pursues only the interests of this particular organism.

Natural and artificial selection equally affect the lives of all people. They fight for the life of a premature baby, as well as for the life of a healthy one, but at the same time, natural selection kills drunkards frozen on the streets, deadly diseases take the lives of ordinary people, the mentally unbalanced commit suicide, natural disasters fall on the earth.

Types of natural selection

Why are only certain representatives of species able to survive in different environmental conditions? The forms of natural selection are not written rules of nature:

Driving selection occurs when environmental conditions change and species have to adapt, it keeps the genetic heritage in certain directions.
Stabilizing selection is aimed at individuals with deviations from the average statistical norm in favor of average individuals of the same species.
Disruptive selection is when individuals with extreme indicators survive, and not with average ones. As a result of such selection, two new species can be formed at once. More common in plants.
Sexual selection - based on reproduction, when the key role is played not by the ability to survive, but by attractiveness. Females, without thinking about the reasons for their behavior, choose beautiful, bright males.

Why is a person able to weaken the impact of natural selection?

Medical progress has come a long way. People who were supposed to die - survive, develop, have their own children. By passing on their genetics to them, they give rise to a weak race. Natural selection and the struggle for existence clash hourly. Nature comes up with more and more sophisticated ways to control people, and man tries to keep up with her, thereby preventing natural selection. Human humanism leads to the weak appearance of people.

Natural selection favors the survival and increase in the number of individuals in the population, carriers of some genotypes to the detriment of carriers of others. This contributes to the accumulation in the population of traits that have adaptive value.

Under different environmental conditions, natural selection has a different character. There are three main forms of natural selection:

Moving;
stabilizing;
disruptive.

Driving Form (with examples)

The manifestation of driving selection occurs when the resulting changes in the new environment are more useful. The selection will be aimed at their preservation. This will entail gradual changes in the phenotype of individuals in the population, a change in the reaction norm and a change in the average value of the trait.

A classic example of driving selection is the color change of moths in the vicinity of industrial cities in Europe and America. If previously light coloration was typical for them, then as the tree trunks were contaminated with soot and soot, the light variants that became noticeable on the bark of the trees were primarily eaten by birds and the dark variants gained more and more advantage, it was they who were preserved by natural selection. This led to a change in color.

Evolution, the emergence of new adaptations, is associated with driving selection. In recent decades, many species of insects have developed races that are resistant to insecticides (drugs that are poisonous to insects). Insects sensitive to poison died, but in some individuals a new mutation arose, or they previously had a neutral gene for insensitivity to any insecticides. Under changed conditions, the gene ceased to be neutral. Driving selection has preserved the carriers of this gene. They became the ancestors of new races.

Stabilizing form (with examples)

Stabilizing selection occurs under relatively constant conditions. Here, deviations from the average value of the trait may already turn out to be unfavorable and are swept aside. In these cases, selection is aimed at preserving mutations that lead to less variability of the trait.

It has been established that representatives of the population with an average manifestation of the trait are more resistant to extreme changes in conditions, so sparrows with an average wing length survive the winter more easily than long-winged or short-winged ones. Also, a constant body temperature in homoiothermic animals is a consequence of stabilizing selection.

In plants pollinated by certain types of insects, the structure of the corolla of the flower cannot vary, it corresponds in shape and size to the size and shape of the pollinators. Any deviations from the "standard" are immediately swept aside by selection, since they do not leave offspring.

Stabilizing selection occurs most often, is considered the main thing in the development of organisms, when the improvement of average indicators leads to evolutionary progress.

When the conditions of existence change, the driving and stabilizing selection can replace each other.

Disruptive form (with examples)

Disruptive selection can be observed when among all variants of the genotype, there is no dominant one, which is associated with the heterogeneity of the territory they inhabit. Under the action of certain factors, some signs contribute to survival, when conditions change, others.

Disruptive selection is directed against those representatives of the species that have average manifestations of the trait, which leads to the appearance of polymorphism among one population. The disruptive form is also called tearing, because the population is divided into separate parts according to the current trait. Thus, the disruptive form is responsible for the development of extreme phenotypes and is directed against the average forms.

An example of disruptive selection is the color of the snail shell. The color of the shell depends on the environmental conditions in which the snail enters. In the forest zone, where the surface layer of the earth is colored brown, snails with brown shells live. In the steppe region, where the grass is dry and yellow, they have yellow shells. The difference in the color of the shells is adaptive in nature, since it protects the snails from being eaten by birds of prey.

Table of the main types of natural selection

Characteristic	driving form	Stabilizing form	Disruptive form
Action	Occurs under gradually changing living conditions of the individual.	The living conditions of the body do not change for a long time.	With a sharp change in the living conditions of the body.
Orientation	Aimed at the conservation of organisms with characteristics that contribute to the survival of the species.	Maintaining the homogeneity of the population, the destruction of extreme forms.	The action is aimed at the survival of individuals in heterogeneous conditions, through the manifestation of different phenotypes.
Outcome	The appearance of an average norm, which comes to replace the old one, which is not suitable in the new environment.	Saving the average indicators of the norm.	Formation of several average norms necessary for survival.

Other types of natural selection

The main forms of selection are described above, there are also additional ones:

Destabilizing;
sexual;
group.

Destabilizing form in action it is opposite to the stabilizing one, while the reaction rate expands, but the average indicators are also preserved.

So frogs that live in swamps, in an environment with different illumination, differ significantly in the color of their skin - this is a manifestation of destabilizing selection. Frogs inhabiting a territory that is completely shaded or, conversely, with good access to light, have a uniform color - this is a manifestation of stabilizing selection.

Sexual form of natural selection is aimed at the formation of secondary sexual characteristics, which help to choose a pair for crossing. For example, the bright color of feathers and the singing of birds, a loud voice, mating dances or the release of odorous substances to attract the opposite side of insects, and more.

group form aimed at the survival of the population, not individuals. The death of several members of the group for the sake of saving the species will be justified. So, in a herd of wild animals at the genetic level, it is laid that the life of the group is more important than one's own. When danger approaches, the animal will make loud noises to warn its relatives, while it will die, but save the rest.

a) Destabilizing selection

Thus, a destabilizing form of selection leads to an expansion of the reaction rate.

b) sexual selection

sexual selection- natural selection within the same sex, aimed at developing traits that give mainly the opportunity to leave the largest number of descendants.

Rice. 7.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all that amazing variety of shapes, colors and instincts that we observe in the world of wildlife. .

c) Group selection

Group selection is often also called group selection, it is the differential reproduction of different local populations. Wright compares population systems of two types - a large continuous population and a number of small semi-isolated colonies - in relation to the theoretical efficiency of selection. It is assumed that the total size of both population systems is the same and the organisms interbreed freely.

All these difficulties are eliminated to a large extent in a population system that resembles in its structure a series of separate islands. Here, selection, or selection in conjunction with genetic drift, can quickly and effectively increase the frequency of some rare favorable allele in one or more small colonies. New favorable combinations of genes can also easily gain a foothold in one or more small colonies. Isolation protects the gene pools of these colonies from "flooding" as a result of migration from other colonies that do not have such favorable genes, and from crossing with them. Up to this point, only individual selection or, for some colonies, individual selection combined with genetic drift has been included in the model.

G) Directional selection (moving)

Rice. 1.

Directed selection (moving) was described by Ch. Darwin, and the modern doctrine of driving selection was developed by J. Simpson.

Fig 2. The dependence of the frequency of dark forms of the birch moth on the degree of atmospheric pollution

Rice. 3.

e) Stabilizing selection

Rice. 4.

Stabilizing selection in relatively constant environmental conditions, natural selection is directed against individuals whose characters deviate from the average norm in one direction or another.

Fig.5.

Thanks to stabilizing selection, "living fossils" that live in relatively constant environmental conditions have survived to this day:

tuatara, bearing the features of reptiles of the Mesozoic era;

coelacanth, a descendant of lobe-finned fish, widespread in the Paleozoic era;

the North American opossum is a marsupial known from the Cretaceous period;

The stabilizing form of selection acts as long as the conditions that led to the formation of a particular trait or property persist.

f) Disruptive (dismembering) selection

Rice. 6.

Another example: in forests where soils are brown, earth snail specimens often have brown and pink shells, in areas with coarse and yellow grass, yellow color prevails, etc.

Populations adapted to ecologically dissimilar habitats may occupy contiguous geographic areas; for example, in coastal areas of California, the plant Gilia achilleaefolia is represented by two races. One race - "sunny" - grows on open grassy southern slopes, while the "shady" race is found in shady oak forests and sequoia groves. These races differ in the size of the petals - a trait determined genetically.

Conclusion

Bibliography

Vlasova Z.A. Biology. Student Handbook - Moscow, 1997

Green N. Biology - Moscow, 2003

Kamlyuk L.V. Biology in questions and answers - Minsk, 1994

Lemeza N.A. Biology manual - Minsk, 1998