One of the main mechanisms of evolution along with mutations, migration processes and gene transformations is natural selection. Types of natural selection involve such changes in the genotype that increase the chances of an organism to survive and procreate. Evolution is often seen as a consequence of this process, which may result from differences in species survival, fertility, development rates, mating success, or any other aspect of life.
natural balance
Gene frequencies remain constant from generation to generation, provided that there are no perturbing factors that disturb the natural balance. These include mutations, migrations (or gene flow), random genetic drift, and natural selection. A mutation is a spontaneous change in the frequency of genes in a population that is characterized by a low rate of development. In this case, the individual moves from one population to another and then changes. Random is a change that is passed from one generation to another in a completely random way.
All of these factors alter gene frequencies without regard to increasing or decreasing the likelihood of an organism surviving and reproducing in its natural environment. All of them are random processes. And natural selection, types of natural selection, are mildly disruptive effects of these processes, because they multiply the frequency of beneficial mutations over many generations and eliminate harmful constituents.
What is natural selection?
Natural selection contributes to the conservation of those groups of organisms that are better adapted to the physical and biological conditions of their habitat. He
can act on any heritable phenotypic trait and, through selective pressure, can influence any aspect of the environment, including sexual selection and competition with members of the same or other species.
However, this does not mean that this process is always directed and effective in adaptive evolution. Natural selection, types of natural selection in general, often results in the elimination of less fit variants.
Variations exist within the entire population of organisms. This is partly because random mutations occur in the genome of one organism, and its offspring can inherit such mutations. Throughout life, genomes interact with the environment. Therefore, the population is evolving.
The concept of natural selection
Natural selection is one of the cornerstones of modern biology. It acts on the phenotype, the genetic basis of which gives a reproductive advantage for greater prevalence in the population. Over time, this process can lead to the emergence of new species. In other words, this is an important (though not the only) evolutionary process within a population.
The concept itself was formulated and published in 1858 by Charles Darwin and Alfredo Russell Wallace in a joint presentation of papers regarding
The term has been described as analogue, meaning it is the process by which animals and plants with certain traits are deemed desirable for breeding and reproduction. The concept of "natural selection" was originally developed in the absence of the theory of heredity. At the time of Darwin's writings, science had yet to develop The unification of traditional Darwinian evolution with subsequent discoveries in classical and molecular genetics is called the modern evolutionary synthesis. The 3 kinds of natural selection remain the main explanation for adaptive evolution.
How does natural selection work?
Natural selection is the mechanism by which an animal organism adapts and evolves. At their core, the individual organisms that are best adapted to their environment survive and reproduce most successfully, producing fertile offspring. After numerous breeding cycles, such species are dominant. In this way, nature filters out ill-adapted individuals for the benefit of the entire population.
This is a relatively simple mechanism that causes members of a particular population to change over time. In fact, it can be broken down into five main stages: variability, inheritance, selection, timing, and adaptation.
Darwin on natural selection
According to Darwin, natural selection has four components:
- Variations. Organisms within a population exhibit individual differences in appearance and behavior. These changes may include body size, hair color, muzzle patches, voice quality, or the number of offspring produced. On the other hand, some character traits are not associated with differences between individuals, such as the number of eyes in vertebrates.
- Inheritance. Some traits are passed down sequentially from parent to offspring. Such traits are inherited, while others are strongly influenced by environmental conditions and are weakly inherited.
- high populations. The bulk of animals annually produces offspring in much larger numbers than is necessary for an equal distribution of resources between them. This leads to interspecific competition and premature mortality.
- Differential survival and reproduction. All types of natural selection in populations leave behind those animals that can fight for local resources.
Natural selection: types of natural selection
Darwin's theory of evolution radically changed the direction of future scientific thought. At its center is natural selection, a process that occurs over successive generations and is defined as the differential reproduction of genotypes. Any change in the environment (such as changing the color of a tree trunk) can lead to local adaptation. There are the following types of natural selection (Table No. 1):
Stabilizing selection
Often, the frequency of mutations in DNA in some species is statistically higher than in others. This type of natural selection tends to eliminate any extremes in the phenotypes of the fittest individuals in a population. This reduces diversity within the same species. However, this does not mean that all individuals are exactly the same.
Stabilizing natural selection and its types can be briefly described as averaging or stabilization in which a population becomes more homogeneous. First of all, polygenic traits are affected. This means that the phenotype is controlled by several genes and there is a wide range of possible outcomes. Over time, some of the genes are turned off or masked by others, depending on the favorable adaptation.
Many human characteristics are the result of such selection. Human birth weight is not only a polygenic trait, it is also controlled by environmental factors. Newborns with an average birth weight are more likely to survive than those who are too small or too large.
Directed natural selection
This phenomenon is usually observed in conditions that have changed over time, for example, weather, climate or food supply can lead to directional breeding. Human involvement can also speed up this process. Hunters most often kill large individuals for meat or other large ornamental or useful parts. Consequently, the population will tend to skew towards smaller individuals.
The more predators kill and eat slow individuals in the population, the more the bias will be towards the luckier and faster members of the population. The types of natural selection (example table no. 1) can be more clearly demonstrated using examples from wildlife.
Charles Darwin studied directional selection when he was in the Galapagos Islands. The beak length of native finches has varied over time due to available food sources. In the absence of insects, finches survived with large and long beaks, which helped them eat the seeds. Over time, insects became more numerous, and with the help of directional selection, bird beaks gradually became smaller.
Features of diversification (disruptive) selection
Disruptive selection is a type of natural selection that opposes the averaging of species characteristics within a population. This process is the rarest, if we describe the types of natural selection briefly. Diversification selection can lead to speciation of two or more different forms in places of abrupt environmental change. Like directional selection, this process can also be slowed down due to the destructive influence of the human factor and environmental pollution.
One of the best studied examples of subversive selection is the case of butterflies in London. In rural areas, almost all individuals were light in color. However, these same butterflies were very dark in color in industrial areas. There were also specimens with an average color intensity. This is because dark butterflies have learned to survive and escape predators in industrial areas in urban environments. Light moths in industrial areas were easily found and eaten by predators. The opposite picture was observed in rural areas. Butterflies of medium color intensity were easily visible in both places and therefore very few remained.
Thus, the meaning of subversive selection is the movement of the phenotype to the extreme that is necessary for the survival of the species.
Natural selection and evolution
The main idea of the theory of evolution is that all species diversity gradually developed from simple shapes life that appeared more than three billion years ago (for comparison, the age of the Earth is about 4.5 billion years). Types of natural selection with examples from the first bacteria to the first modern people played a significant role in this evolutionary development.
Organisms that have been poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed on to the next generation. The path to genetic diversity must not be lost, nor is the ability at the cellular level to respond to changing environmental conditions.
Living in natural conditions, there is individual variability, which can manifest itself in three forms - useful, neutral and harmful. Usually, organisms with harmful variability die at various stages of individual development. The neutral variability of organisms does not affect their viability. Individuals with beneficial variability survive by virtue of an advantage in intraspecific, interspecific, or against adverse conditions environment.
driving selection
When environmental conditions change, those individuals of the species survive in which hereditary variability has manifested itself and, in connection with this, signs and properties have developed that correspond to new conditions, and those individuals that did not have such variability die. During his voyage, Darwin discovered that on oceanic islands where strong winds prevail, there are few long-winged insects and many insects with rudimentary wings and wingless insects. As Darwin explains, insects with normal wings could not withstand the strong winds on these islands and died. And insects with rudimentary wings and wingless did not rise at all into the air and hid in the cracks, finding shelter there. This process, which was accompanied by hereditary variability and natural selection and continued for many thousands of years, led to a decrease in the number of long-winged insects on these islands and the appearance of individuals with rudimentary wings and wingless insects. Natural selection, which ensures the emergence and development of new features and properties of organisms, is called motive selection.
Disruptive selection
Disruptive selection- this is a form of natural selection, leading to the formation of a number of polymorphic forms that differ from each other within the same population.
magazine
4.1
Women subconsciously look for certain traits in a man that have always been relevant. Stop wandering around unsuccessfully stone jungle looking for a girlfriend!
For women, the key points in a man are his inner viability, the ability to become her protection and support, to provide food and care for her cubs. And women are almost not mistaken: indeed, a man with a high status is able to give them more. So what is the expression of this damned status, thanks to which every one of us makes eyes at one of us, and the other seems to merge with the landscape and go unnoticed by women?
What a troublesome business - the marriage rituals of living beings. Mother Nature is clearly too clever here. Each representative of the fauna during the courtship period goes crazy in its own way. An Argentine stove-maker bird lures a friend to his house, decorating the entrance with flowers, pebbles and empty Coke cans. Deer, as you know, in the fight for their artiodactyl girlfriend arrange fights, but simply give each other horns. And just the cuckold wins the tender. And the marsupial mouse from Australia, a tiny creature like a shrew, nature has given one single attempt to start a family - and whoever did not have time, he was late. Therefore, these poor souls in their mating season run like mad, in a feverish attempt to find a mate. In the process of matchmaking, they go bald no worse than we do, lose their teeth and a third of their weight. By the end of the first week of the mating season, all the lawns in the Australian forest are littered with emaciated, bald, toothless animals. Of course, not all marsupials will face such a depressing fate. Some of them survive, get married, have children, and even make a tidy bag towards the end of their lives. But only a few, chosen natures, who have learned how to strike the Australian mouse in the very heart, find happiness. So the question is: what makes a female prefer one microscopic hairless rodent to another? And it is even more interesting to know why some males belonging to the species Homo sapiens, have an irresistible attraction in the eyes of a female of the same species, while others unsuccessfully wander through the stone jungle in search of a mate? Both in humans and in shrews, the mechanism of mutual attraction is the same. Individuals of opposite sexes seek to find certain qualities in each other. These qualities are biologically valuable for maintaining population size. So women subconsciously look for certain traits in a man. Moreover, these features are relevant for representatives of any race or nation, because they are laid much deeper than social or historical preferences. Females of all kinds, both egg-laying and talkative, act according to the same instinctive pattern. In biology, the attraction factor is called status. AT wild nature it takes a wide variety of forms. To demonstrate their high status, thrushes seek to get a bigger piece of food for the bride, cats mark their territory, peacocks show off their stunning tail. People are pretty much the same. However, taking the object of your courtship to a restaurant, using the thrush method, or showing her a cool outfit, using the peacock method, is not enough to confirm the status. For women, the key points in a man are his inner viability, the ability to become her protection and support, to provide food and care for her cubs. And women are almost not mistaken: indeed, a man with a high status is able to give them more. So what is the expression of this damned status, thanks to which every one of us makes eyes at one of us, and the other seems to merge with the landscape and go unnoticed by women? Money and power, you say, and you can't go wrong. But not only that - there are also character traits that make it possible to judge that a given individual is able to become good father and reliable protector. And this is also evidence of status, no less significant than a gold chain, a Jeep "Cherokee" or a mandate of a deputy of the State Duma. If the fact that women are so utilitarian in choosing a partner hurts you, don't despair. Many of the "status" qualities you already have, others can be developed. And most importantly, you need to learn how to demonstrate them favorably - and you will see how your natural sex appeal will increase.
ambition
Ambition is the most powerful love spell. And not because they demonstrate your current status, but because they testify to your future conquests. Even if now your income is impressive, but you do not demonstrate the desire to rise higher and higher, your attractiveness in the eyes of a woman will begin to fade. And all because it is not just your achievements that are important to her, but that elusive aroma of success that surrounds a truly purposeful person. Polls show that the majority of women in the first places in the list of male virtues put diligence and ambition. In primitive times, a persistent man always brought an elk from the hunt to feed his family, and then worked on a formidable spear from the horns of this same elk, in which case to protect the family from enemies. So in every possible way to demonstrate your career and other aspirations means to give the right signals.Obey the law of the jungle:
Even if you are satisfied with your official position, continue to be interested in new opportunities and try to open new horizons for your activities. This will certainly increase your shares in the eyes of the women around you.
love of life
Striving, in accordance with the previous recommendation, through thorns to the stars, remember that success requires the ability to relax. Women are looking for a balanced partner, and excessive efficiency makes them nervous. They foresee, and not without reason, that the workaholic will not be able to devote enough time to the family, that sooner or later work will swallow him up and he will deprive himself and his family of simple human joys.Obey the law of the jungle:
Everything is good in moderation. Do not give her a reason to suspect that you are a slave to your business - allow yourself to be sometimes free, independent and contented with life.
Modesty
Decorates a person. Of course, there is no escape from the peacock method of seduction, and it is worth learning how to be spectacular. But any excess - and the whole effect down the drain. Women, as a rule, perfectly distinguish false bravado from the display of true brilliance or self-respect. Exaggerating one's own strength, power, importance, or sexuality is a sure sign of low status.Obey the law of the jungle:
Remove this signet ring immediately. Do not tell her how you slept with all your classmates in your time. Don't try to be what you are not.
Talent
Each of us has our own talents that distinguish us from others. It could be the ability to ski great. Or the ability to make a wonderful birdhouse. Or the talent for showing simple tricks, which was so useful to David Copperfield when he seduced Claudia Schiffer. Having beaten all rivals in any field, you will demonstrate the ability to lead - an important component of status.Obey the law of the jungle:
Try to demonstrate your unique talent in action, and not just in words. Rather than paint your culinary skills, it's better to feed her dinner. (Just don't rush to make a birdhouse in her bedroom.)
Imagine sensitivity
Some vulnerability shown from time to time, not only will not damage your status, but will strengthen it. In an attractive, from the point of view of a woman, male character, there is a so-called androgenicity - an alloy of male and female. This, above all, means the ability to empathize and understand, tenderness and care. Women find intelligent and decent men attractive. However, the above qualities must necessarily be combined with some firmness and reliability in financial matters - what women call "with him as behind a stone wall." That is, sensitivity in no case should mean helplessness.
Obey the law of the jungle:
Admit your mistakes when you're wrong. Feel free to ask for advice. If you are sad or hurt, say so. But don't pretend to be helpless in things you just don't feel like doing. And don't go back on what you promised.
composure
The so-called "African passions" are often associated with heightened sexuality. In vain. Demonstration of unbridled emotions is more likely to frighten a woman than excite. A much more significant quality for your status is stability and self-control. On a subconscious level, these qualities signal that you can not lose your head in extreme situations.Obey the law of the jungle:
There are exercises that allow in a stressful situation not to lose self-control and not to succumb to all sorts of provocations.
slight jealousy
You appear in public with beautiful woman, and all the men immediately begin to dislocate their cervical vertebrae in her direction. Maybe you are flattered by such attention, but you should not show true feelings. It is better to pretend that their undisguised envy annoys you a little. For a woman, light signals of jealousy are. And crazy unreasonable jealousy lowers the status, as it speaks of your self-doubt.Obey the law of the jungle:
Avoid the manifestation of malicious aggressiveness towards imaginary rivals, otherwise they will turn from imaginary into real ones.
Tact
In sex play, there are three laws of tact that must not be violated.Even if a girl passionately kisses you all evening, this does not mean that she is already ready for everything that you can offer her.
Overly persistent harassment of women, as a rule, is annoying.
Even if she perceives the night spent with you as a one-time adventure, it will seem insulting to her if you let her know that you yourself treat her like a.
A woman, even the most free-thinking, regards manifestations of sexual incontinence as a threat to her safety. Deranged males who do not understand that in human language "no" means "no", women do not trust. And as for the "fleeting" adventure, then the game is a game, and at the moment when you are with her, it is better to act as if you intend to remain at her feet for the rest of your life. The requirements that a woman makes for a casual partner are not so different from her requirements for a permanent one. Therefore, it is always flattering for her to think that her one-time lover is not averse to starting a stronger relationship. She just won't let him. Ha ha ha.
Obey the law of the jungle:
Do not rush. Slowly and gently. Move only when you're pretty sure she doesn't mind. This will allow her to relax and increase your status in her eyes. After all, she will not be afraid that all you need from her is to quickly bring the matter to bed and run away.
Humor
Girls need to be laughed at. In fact, women like witty and funny. And all because self-confident people with a high status are capable of joking and feeling relaxed.Obey the law of the jungle:
Someone knows how to joke, someone is not capable of an instant witty reaction. If you do not belong to the latter - it does not matter. Learn to treat yourself without undue seriousness, sometimes sneer at yourself, find amusing in different life situations. People with low status are most often afraid of appearing ridiculous.
Voice
A low velvety confident voice attracts women. Squeaky, shrill and nasal - vice versa.Obey the law of the jungle:
You can get an idea of your own voice if you record it on a tape recorder and listen to it. Listening to your own voice for the first time is usually a shock. However, this is very helpful. Because we can pretty much vary our voice with a little practice. In addition, it's not so much the timbre, but the intonation. And they can always be fixed.
Sympathy for children
Why do all politicians now and then strive in public to grab the first child they come across and kiss him on his chubby cheek? Because half of the electorate are women. And they are ready to forgive a man a lot for his love for children. If she sees how you aunt with a child or is ready to answer his questions without irritation, then she does not just evaluate you as a potential dad. If you care about children, then you are generally a caring person. And that means she can rely on you.Obey the law of the jungle:
Are you convinced that children are squealing, snotty and annoying pesters? Pretend you really think they're cute. Gradually you will get used to this idea. (Our dads used to do that.)
a little glory
The surest way to win a woman is to become famous. Fame is the most visible evidence of your status. As long as you haven't received Nobel Prize, try to become "widely known in narrow circles." The woman who walks next to you is even pleased that the neighbors at home greet you.Obey the law of the jungle:
Don't miss the opportunity to expand your circle of acquaintances. Any person can become, if not famous, then at least irreplaceable for many people.
NATURAL SELECTION, the process of selective survival and differential reproduction of organisms, the main driving factor in their evolution. Ideas about the existence of natural selection have been expressed since the beginning of the 19th century by various English naturalists (including A. Wallace). But only C. Darwin (1842, 1859) assessed it as the main factor in evolution. According to Darwin, natural selection is the result of the struggle for existence; even slight heritable differences between individuals of the same species can provide advantages in this struggle, which is due to the tendency of organisms to a high intensity of reproduction (in geometric progression) and the impossibility of preserving all offspring due to limited natural resources. The death of the overwhelming number of individuals in each generation inevitably leads to natural selection - "survival of the fittest" to given conditions. As a result of the summation of beneficial changes over many generations, new adaptations are formed and, ultimately, new species arise. Darwin built his reasoning about the action of natural selection mainly on the generalization of the experience of domestication of animals and plants by analogy with artificial selection, emphasizing, however, that, unlike human selection, natural selection is determined by the interaction of organisms with environmental conditions and does not have a specific goal.
The systematic study of natural selection, the expansion and improvement of methods for its study began at the end of the 19th century. The use of biometric methods made it possible to establish statistically significant differences between surviving and dead organisms under changing environmental conditions. Thanks to the developments of R. Fisher, J. Haldane, S. Wright and S. S. Chetverikov, who synthesized classical Darwinism and genetics, it became possible to begin an experimental study of the genetic foundations of natural selection. The examined natural populations turned out to be literally saturated with mutations, many of which became useful when conditions of existence changed or when combined with other mutations. It was found that the mutation process and free crossing (panmixia) ensure the genetic heterogeneity of populations and the uniqueness of individuals with different chances of survival; this causes a high intensity and efficiency of natural selection. In addition, it became obvious that natural selection does not deal with single characters, but with whole organisms, and that the genetic essence of natural selection lies in the non-random (differentiated) preservation in a population of certain genotypes that are selectively transmitted to the next generations. Natural selection is probabilistic in nature, acts on the basis of the mutation process and the existing gene pool, affects the frequency of distribution of genes and their combinations, helps to reduce the negative effects of mutations and the formation of protection mechanisms from their harmful effects, thereby determining the pace and direction of evolution. Under the control of natural selection are not only a variety of characteristics, but also the factors of evolution themselves, for example, the intensity and nature of mutability, the apparatus of heredity (hence the concept of "evolution of evolution"). In the absence of natural selection, there is a decrease or loss of fitness of organisms due to the accumulation of undesirable mutations, which is manifested in an increase in genetic burden, including in populations of modern man.
There are more than 30 forms of natural selection; none of them exists in its pure form, but rather characterizes the tendency of selection in a particular ecological situation. Thus, driving selection contributes to the preservation of a certain deviation from the previous norm and leads to the development of new adaptations through a directed restructuring of the entire gene pool of populations, as well as genotypes and phenotypes of individuals. It may lead to the dominance of one (or several) pre-existing forms over others. A classic example of its action was the predominance in industrial areas of dark-colored forms of the birch moth butterfly, invisible to birds on tree trunks polluted with soot (until the middle of the 19th century, only a light form was found that imitated lichen spots on light birch trunks). Rapid adaptation to the poisons of various insect and rodent species, the emergence of antibiotic resistance of microorganisms indicate that the pressure of driving selection in natural populations is sufficient to ensure a rapid adaptive response to abrupt environmental changes. As a rule, selection for one trait entails a number of transformations. For example, long-term selection for the content of protein or oil in corn grains is accompanied by changes in the shape of the grains, the size of the cobs, their location above the soil level, etc.
The result of the action of motive selection in the phylogenesis of large taxa is orthoselection, an example of which is the directed evolution of the limbs of the horse's ancestors (from five-fingered to one-fingered), established by V. O. Kovalevsky, which proceeded for millions of years and ensured an increase in the speed and economy of running.
Disruptive, or tearing, selection favors the preservation of extreme deviations and leads to an increase in polymorphism. It manifests itself in those cases when none of the intraspecific forms with different genotypes receives an absolute advantage in the struggle for existence due to the variety of conditions simultaneously encountered in one territory; in this case, individuals with an average or intermediate character of characters are eliminated first of all. As early as the beginning of the 20th century, the Russian botanist N.V. Tsinger showed that the large rattle (Alectoroleophus major), which blooms and bears fruit in unmowed meadows throughout the summer, forms two races in mowed meadows: early spring, which has time to bring seeds before mowing, and late autumn - low plants, not damaged by mowing, and then quickly blooming and having time to give seeds before the onset of frost. Another example of polymorphism is the difference in the color of the shells of the earth snail (Capacea nemoralis), which is food for birds: in dense beech forests, where the litter of red-brown litter is preserved throughout the year, individuals with brown and pink color are common; yellow-colored snails predominate in meadows with yellow litter. In mixed deciduous forests, where the nature of the background changes with the onset of a new season, snails with brown and pink colors dominate in early spring, and yellow ones in summer. Darwin's finches (Geospizinae) in the Galapagos Islands (a classic example of adaptive radiation) - final result prolonged disruptive selection, which led to the formation of dozens of closely related species.
If these forms of natural selection lead to a change in both the phenotypic and genetic structure of populations, then the stabilizing selection first described by I.I. this norm. It is aimed at maintaining and increasing resistance in a population of an average, previously established phenotype. It is known, for example, that during snow storms birds survive, which in many ways (wing length, beak, body weight, etc.) approach the average norm, and individuals that deviate from this norm die. The size and shape of flowers in plants pollinated by insects are more stable than in plants pollinated by the wind, which is due to the conjugated evolution of plants and their pollinators, the “culling” of deviant forms (for example, a bumblebee cannot penetrate a too narrow corolla of a flower, and the proboscis of the butterfly does not touch the too short stamens in plants with a long corolla). Thanks to stabilizing selection, with an external unchanged phenotype, significant genetic changes can occur, ensuring the independence of the development of adaptations from fluctuating environmental conditions. One of the results of the action of stabilizing selection can be considered the "biochemical universality" of life on Earth.
Destabilizing selection (the name was proposed by D.K. Belyaev, 1970) leads to a sharp disruption of ontogenesis regulation systems, the opening of the mobilization reserve and an increase in phenotypic variability with intensive selection in any particular direction. For example, selection to reduce the aggressiveness of predatory animals in captivity through the restructuring of the neurohumoral system leads to destabilization of the breeding cycle, shifts in the timing of molting, changes in the position of the tail, ears, color, etc.
Genes have been found that can be lethal or reduce the viability of organisms in the homozygous state, while in the heterozygous state, on the contrary, increase ecological plasticity and other indicators. In this case, we can talk about the so-called balanced selection, which ensures the maintenance of genetic diversity with a certain ratio of allele frequencies. An example of its action is the increase in resistance in patients with sickle cell anemia (heterozygous for the hemoglobin S gene) to infection with various strains of malarial plasmodium (see Hemoglobins).
An important step in overcoming the desire to explain all the characteristics of organisms by the action of natural selection was the concept of neutral evolution, according to which some of the changes at the level of proteins and nucleic acids occur through the fixation of adaptively neutral or almost neutral mutations. It is possible to select species that appear in peripheral populations "suddenly" from a geochronological point of view. Even earlier, it was proved that catastrophic selection, in which a small number of individuals and even a single organism survive during a period of abrupt environmental changes, can become the basis for the formation of a new species due to chromosomal rearrangement and a change in ecological niche. Thus, the formation of a xerophytic, endemic species of Clarkia lingulata in the Sierra Nevada mountains in California is explained by a severe drought that caused mass death of plants, which became catastrophic in peripheral populations.
Natural selection, affecting the secondary sexual characteristics of individuals, is called sexual (for example, the bright mating coloration of males in many species of fish and birds, inviting cries, specific smells, highly developed tools for tournament combat in mammals). These traits are useful, as they increase the possibility of their carriers participating in the reproduction of offspring. In sexual selection, males are most active, which is beneficial for the species as a whole, because. females remain safer during the breeding season.
Group selection is also distinguished, which contributes to the preservation of traits that are useful to the family, pack, colony. Its particular case in colonial insects is the selection of relatives, in which sterile castes (workers, soldiers, etc.) ensure (often at the cost of their own lives) the survival of fertile individuals (queens) and larvae and thereby the preservation of the entire colony. The altruistic behavior of parents pretending to be injured in order to lure a predator away from their children threatens the imitator with death, but in general increases the chances of survival of his offspring.
Although the ideas about the leading role of natural selection in evolution have been confirmed in many experiments, they are still being criticized based on the idea that organisms cannot be formed as a result of a random combination of mutations. This ignores the fact that each act of natural selection is performed on the basis of the previous results of its own action, which, in turn, predetermine the forms, intensity and directions of natural selection, and hence the paths and patterns of evolution.
Lit .: Shmalgauzen II Factors of evolution. 2nd ed. M., 1968; Mayr E. Zoological species and evolution. M., 1968; Sheppard F. M. Natural selection and heredity. M., 1970; Levontin R. Genetic foundations of evolution. M., 1978; Wilson D. S. The natural selection of populations and communities. Menlo Park, 1980; Gall Ya. M. Research on natural selection // Development of evolutionary theory in the USSR. L., 1983; Gause G. F. Ecology and some problems of the origin of species // Ecology and evolutionary theory. L., 1984; Ratner V. A. Brief essay theory of molecular evolution. Novosib., 1992; Dawkins R. Selfish General M., 1993; Sober E. The nature of selection: evolutionary theory in philosophical focus. Chi., 1993; Darwin C. The Origin of Species... 2nd ed. SPb., 2001; Coyne J., Orr H. A. Speciation. Sunderland, 2004; Gavrilets S. Fitness landscapes and the origin of species. Princeton, 2004; Yablokov A. V., Yusufov A. G. Evolutionary doctrine. 5th ed. M., 2004; Severtsov A.S. Theory of evolution. M., 2005; Kolchinsky E. I. E. Mayr and modern evolutionary synthesis. M., 2006.
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 certain 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, there is a positive Feedback, 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 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. W. 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. In the same way, favorable new gene combinations that manage to form in some local fraction of a given population are broken apart and eliminated as a result of crossing with individuals of neighboring fractions.
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 the flour beetle (Tribolium castaneum) in order 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 the 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 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 animal 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 the soil Brown color specimens of earth snails 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. In evolution, natural selection has creative role. 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.
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