Questions at the beginning of the paragraph.

Question 1. Why can a species exist for an almost unlimited time, while each individual is mortal?

An individual cannot evolve. It can change, adapting to environmental conditions. But these changes are not evolutionary, since they are not inherited. The species is usually heterogeneous and consists of a number of populations. The population is relatively independent and can exist for a long time without connection with other populations of the species. All evolutionary processes take place in a population: mutations occur in individuals, crossing occurs between individuals, the struggle for existence and natural selection operate. As a result, the gene pool of the population changes over time and it becomes the ancestor of a new species. That is why the elementary unit of evolution is a population, not a species.

Question 2. How do sperm and eggs mature?

Male reproductive cells - sperm are formed in the testes (testes). Sperm maturation occurs at a temperature of about 35 "C. This is lower than the temperature of the abdominal cavity of the body. Therefore, the testes are located outside the abdominal cavity, in the skin sac - the scrotum. Full maturation of sperm occurs in the system of the vas deferens, and then they enter the urethra, at the beginning of which The ducts of additional glands - the seminal vesicles and the prostate gland, or prostate - also flow into it.

The maturation of the egg occurs in the Graafian vesicle of the ovary. The development of the egg lasts about 28 days. As a result of reduction division, the mature egg, like the sperm, remains with half the set of chromosomes. Each egg contains only an X chromosome. Consequently, the sex of the unborn child depends on the father.

Question 3. What determines the sex of a child?

The sex of the offspring depends on the sex chromosomes.

If there are two X chromosomes in the germ cell (zygote) (X from the mother and X from the father), a girl will be born. If there are X and Y chromosomes (X from the mother and Y from the father), a boy will be born.

Questions at the end of the paragraph.

Question 1. What are the advantages of sexual reproduction over asexual reproduction?

With the help of sexual reproduction, the genetic apparatus of the offspring is updated, new combinations of genes appear, since the maternal and paternal organism participates in it, and the diversity of individual characteristics is beneficial for the survival of the species in changing environmental conditions. In asexual reproduction, in which only one individual participates, the set of genes in the mother and daughter organisms is the same.

Question 2. Explain the biological significance of the presence of a half set of chromosomes in the sperm and egg.

The nuclei of male and female germ cells contain half the set of chromosomes characteristic of a given species. When an egg and sperm merge, their chromosome sets are combined, the chromosome set characteristic of a given species is restored, and the future organism combines the hereditary characteristics of both parents.

Question 3. Where does fertilization occur? What is formed as a result of this process?

The fusion of the egg and sperm occurs in the fallopian tube. After the sperm penetrates the egg, a zygote is formed - a germ cell that carries the hereditary characteristics of both parents.

Question 4. Why can an embryo stay in the uterus, but an unfertilized egg cannot?

An unfertilized egg, unlike an embryo, does not have villi, which allow it to remain in the uterus.

The essence of sexual reproduction is the creation of new genetic combinations. In the most typical cases, a male and a female mate and produce individuals whose genotypes are not identical to either the genotype of the father or the genotype of the mother. In some animals, new genotypes can be created as a result of processes of a different kind. In protozoa such as paramecia, autogamy occurs, in which one individual creates new homozygous genotype Other forms, including some flatworms and mollusks, are hermaphroditic, that is, they have both male (sperm-producing) and female (egg-producing) gonads. There are hermaphroditic forms that are capable of self-fertilization (Bermant, Davidson , 1974)

Not all reproduction is sexual (that is, it creates new genotypes). For example, paramecia are able to divide in two to form two new daughter organisms that are genetically identical to the original individual. Hydroid polyps (one of the groups of coelenterates) can produce new individuals identical to themselves as a result of the budding process. In this case Several new organisms can form in one budding zone. Other animals, including many insects and some fish, are capable of parthenogenetic reproduction, in which offspring develop from unfertilized eggs (Bermant and Davidson, 1974)

The vast majority of animals, especially forms that have arisen relatively recently, reproduce sexually, that is, by the fusion of male and female gametes. Theorists disagree about the reasons for this predominance of the sexual process. Since sexual reproduction requires certain costs, it obviously must provide some significant advantages. The following main reasons have been put forward for explanation:

1) an evolutionary advantage for populations that are able to change faster than others due to sexual reproduction; 2) an evolutionary advantage due to the fact that this method of reproduction facilitates speciation (the emergence of new species); 3) the fact that individual parent individuals can create diversity in their immediate offspring, facilitating their adaptation to unpredictable changes in the environment (Stanley, 1975; Williams, 1966; Wilson, 1975).

BEHAVIORS ASSOCIATED WITH SEXUAL REPRODUCTION

One of the main features of the breeding cycle of many vldas is its seasonality, that is, it is confined to certain times of the year. The complete reproductive cycle includes courtship, mating, mating consequences, and care of eggs and young.

Seasonality

Many species, especially those found in temperate zones, breed only at certain times of the year. The spring peak of reproductive activity in birds has served as a source of inspiration for many writers and poets. Other animals, however, breed at other times of the year. Many species of deer and moose breed in the fall, wolves and coyotes breed in midwinter, and some seals and sea lions breed in late spring and early summer. Within a given species, breeding seasons often vary depending on latitude. The common denominator among all these different breeding seasons is the time of birth of the young. Although gestation periods vary among animals, most give birth to their young in late spring and early summer. Apparently, an equal advantage of seasonality is that it allows the birth of offspring to coincide with good weather and the greatest abundance of food resources. Mating and gestation seasons appear to be coordinated so that in most species young are born in late spring and early summer (see Sadleir, 1969).

The factors that are directly related to the onset of reproductive activity vary from species to species. Conditions such as temperature, precipitation, vegetation development, and day length vary with the seasons, all of which can influence reproductive activity in particular animals. Apparently, the onset of reproduction in many species is determined by the length of the day, “since in many habitats this factor is most reliably correlated with the change of seasons. For example, if the Far Eastern quail is kept under short-day conditions (8 hours of light and 16 hours of darkness) , then in both males and females the reproductive organs are reduced and sexual reactions disappear. An appropriate change in the light regime (16 hours of light and 8 hours of darkness) leads to the restoration of both morphology and behavior (Sachs, 1969; Adkins, 1973).

Courtship

The function of courtship is to bring two animals of different sexes together under conditions that provide a greater likelihood of successful mating. First of all, it is necessary that the animal can find a potential marriage partner. The importance of mating with a conspecific and the role of mating behavior in reproductive isolation are discussed in Chapters 2 and 13. Courtship is often a complex sequence of interacting signals (such as those shown in Fig. 5.1) that should lead to the mating of a given individual with a suitable partner. Timing is of great importance for successful reproduction; When mating, both male and female must be in proper physiological condition. This is achieved by synchronization of cycles, which is ensured by the interaction of external stimuli and the behavior of the partners themselves. Representatives of classical ethology pointed out that many forms of courtship contain elements of conflict, often for the reason that the initial reaction of an individual to another individual appearing nearby can be aggressive.

Mating systems vary greatly among species. A number of species, such as some swans and geese, are true monogamists and choose a partner for their entire lives. Many migratory birds form pairs for only one season. Some primates exhibit sequential polygamy, i.e., the formation of pairs with several partners at different times, over a certain period with each of them. With simultaneous polygamy, one individual is connected by marriage simultaneously with several individuals of the other sex. Many species of mammals seem to be characterized by completely promiscuous sexual relationships, that is, copulation with many different partners in the complete absence of pairs (see Brown, 1975).

Forms of courtship among different species are very diverse. Below are some examples.

Fig. 51. Diagram of courtship behavior of butterflies, illustrating the chain of sequential stimuli and reactions in the relationship between the female (A) and the male (B). (Brower, Brower, Cranston, 1965.)

Arthropods. The initial detection of an individual of the opposite sex often occurs through the sense of smell. For example, male silkworms (Bombyx mori) are unusually sensitive to bombi-col, a sexual attractant secreted by the female. This attractant attracts males over very long distances; one of its molecules is enough to cause a nerve impulse in a receptor cell (Schneider, 1974). Crickets make a variety of sounds that attract mates and also play a role in other aspects of courtship behavior.

Visual signals are also common. Male alluring crabs (Uca) make species-specific ritual movements with their enlarged claws during courtship. When courting, jumping spiders also perform characteristic visual displays.

The most spectacular visual displays include the mating signals of fireflies (Fig. 5.2). Flying over fields or forests, males produce flashes of light that are species-specific in nature. Females respond to signals from the male of their species with a short flash. Both the latent period and the signal itself are characteristic of each species and also depend on temperature. The male responds to the female's signal by moving closer and closer to her, and the pair continues to exchange signals until the male descends and mates with the female (Lloyd, 1966). An experienced observer can attract a male by using a miniature flashlight to imitate the female's reaction to the male's outbursts.

Much work has been devoted to courtship in fruit flies of the genus Drosophila (see Spieth, 1974). A number of behavioral elements have been identified (wing vibration, leg trembling, wing signaling, circling and licking), which in various combinations constitute the courtship ritual in different species.

Fish. Different types of fish exhibit different forms of courtship. The most detailed description of the behavior of one of the tropical aquarium fish is Tilapia melanotheron, in which four “purely mating” forms of behavior have been identified (which do not occur under any other circumstances): rapid bends down and forward, a special kind of head shaking, biting the substrate and a motionless posture. at the nest (Barlow and Green, 1970). The most expressive display in the courtship ceremony of guppies is the “sigmoid”, or S-shaped, pose adopted by the male. In swordtails, the male approaches the female sideways, and can also approach her, backing away, shaking his whole body, or waving his gonopodium (Clark, Aronson, Gordon, 1954).

Amphibians and reptiles. Male bullfrogs occupy certain territories from where their loud choruses can be heard. Females are obviously attracted to such choruses. Many species of crocodiles make loud roars.

The courtship ritual of Anolis carolinensis lizards is well known to many Americans who have a plot at home. The male hops in a rhythmic manner, displaying a bright red gular pouch (a fold of skin hanging under the chin) (Grews, 1975). In most species of snakes, the main role in the courtship procedure is played by tactile stimulation of the female and olfactory stimulation of the male. So-called “mating dances,” in which two snakes are closely coiled together, have been shown to involve two males, and the interaction between them is likely to be aggressive (Porter, 1972).

Birds. Some of the most striking examples of mating behavior are known in birds. The complex courtship ceremonies of grebes, gulls, ducks, herons and other birds have been favorite subjects of study for ethologists. Bird singing and related legends also received significant attention.

Lorenz studied various forms of courtship behavior in drakes (Fig. 5.3); they can be observed on many duck ponds. The drake tilts its beak towards the water and arches its body upward; at the same time, he waves his beak and emits a loud whistle, accompanied by a sound reminiscent of a grunt. The “head up, tail down” pose is accompanied by a loud whistle. In the up-and-down display, the chest plunges into the water and the beak makes a sharp upward and forward sweep, raising a fountain of spray (see McKinney, 1969).

In Fig. Figure 2.1 shows four types of displays by the green night heron. Males occupy individual areas. The male who has occupied the site drives other males away from it, resorting to “full speed ahead” behavior. The calls of males attract females. Females are initially put off by the "full speed ahead" behavior but do not leave the area and eventually courtship behavior in the form of "flicking" and "neck craning" begins. Having formed a pair, the male and female fly around the territory, sometimes displaying intense flapping flight. Subsequent behavior leads to increasing proximity until copulation occurs (Meyerriecks, 1960).

In many birds, singing serves to drive other males away from an occupied area and attract females. The nature of the song in some species is as specific as their external characteristics.

Mammals. Ewer (1968) reviewed some forms of mating behavior in mammals. The sense of smell plays an important role in regulating this behavior in many species. Shirok”, such actions as examining the anogenital area and sniffing urinary marks are common. In many species, males, upon sensing the scent of a female, exhibit a Flehmen reaction, expressed by stretching the neck and raising the upper lip. This reaction appears to facilitate the perception of the odor rather than to serve as a display.

Female mammals often encourage the male to mount, sometimes approaching him, sniffing him and licking him, and often running away from him. In many cases, the female's flight from the male appears to serve to attract him rather than to actually escape.

In bottlenose dolphins, courtship rituals include vocalizations, biting, sniffing their genitals, rubbing against each other, stroking their fins, exposing their white underparts, jumping, chasing each other, and butting (see Puente and Dewsbury, 1976 ).

Rice. 5.3. Ten postures observed in mallards and other ducks that forage on the surface of the water. (Lorenz, 1958.)

/ – initial swaying of the beak; 2 – head toss; 3 – tail wagging; 4 – whistling and rumbling; 5 – “head up – tail up”; 6 – turn towards the female; 7 – bows while swimming; 8 – turn of the head with the back of the head towards the female; 9 – protrusion of the chest; 10 – “up and down”.

Pairing

Mating behavior ends with the act of copulation, which ensures the fertilization of eggs. The forms of copulation are almost as varied as the forms of courtship, and fertilization can be either internal or external. Several different forms of mating will be described here by way of example.

Arthropods. In many species, sperm is packaged in spermatophores, a type of pouch or sac that contains sperm. The highest degree of “impersonality” is achieved in some species of mites, pseudoscorpions, millipedes and springtails; in these forms, the male leaves the spermatophore on the substrate, and later the female comes and takes it; in this case, the male and female may never meet. In many aquatic forms, fertilization is external (for example, the male horseshoe crab Limulus attaches itself to the female from behind and remains there until she lays eggs in the sand, which he inseminates). Most terrestrial species experience some form of internal fertilization, in which the spermatophore is introduced directly into the female's body by means of some kind of appendage (see Alexander, 1964).

Fish. Fertilization in fish can be either external or internal. Representatives of the family Anabantidae, such as gourami or cockerels, build a nest on the surface of the water from air bubbles and a special secretion they secrete, and spawning usually takes place under this nest. Eggs and sperm are spawned simultaneously, and the eggs are fertilized as they float to the nest. In other fish, such as guppies, swordtails and mosquitofish, sperm is introduced into the female's body using a gonopodium (a modified anal fin). The male nurse shark, grasping the female by the posterior edge of one of her pectoral fins, turns her onto her back and inserts specialized copulatory organs, pterygopodia, into her (Budker, 1971).

Amphibians and reptiles. In most tailed amphibians, internal fertilization occurs after the female captures the spermatophore deposited by the male; In some species external fertilization and parthenogenesis occur. Most frogs and toads are characterized by external fertilization, which occurs as eggs are laid. The exceptions are tailed frogs, which have a copulatory organ, and African viviparous toads, whose cloaca touch during mating. As a rule, a sexually mature female frog goes to the male, who makes calling sounds; the male grasps the female in a special way (the so-called “amplexus”). The eggs come out and are fertilized in several stages. If a male grabs another male or a female that has already laid eggs, then such an erroneously chosen partner makes a special sound, after which he is released (Rabb, 1973).

In reptiles, internal fertilization takes place using copulatory organs. In Anolis carolinensis, the male grabs the female by the neck, twists his tail around hers, and inserts one of his paired copulatory organs into her.

Birds. In contrast to the varied mating behavior of birds, the process of copulation occurs in them more or less in the same way. Most species do not have a copulatory organ, so sperm is transferred from the male's cloaca to the female's cloaca. During copulation, both cloaca come into contact. For example, in the Far Eastern quail, the male grabs the feathers on the female’s neck with his beak, climbs onto her back and tries to bring his cloaca into contact with the female’s cloaca (Sachs, 1969).

Mammals. Most of the work on copulatory behavior has been carried out on laboratory strains of the gray rat (Rattus noruegicus). Therefore, copulation in rats will be described here in some detail. If a female in a state of readiness for mating is placed in a cage in which a male is located, then further events are quite predictable. After the initial courtship ritual (sniffing, running around, provocation from the female), the male begins to chase the fleeing female. When he mounts the female from behind, she assumes a stereotypical position called lordosis. At the same time, its head and back of the body are raised, its back is bent down, and its tail is pulled to the side. Two types of copulations are usually observed. One type of copulation, called "intromission," involves the male inserting his penis into the female's vagina for about a quarter of a second and then quickly dismounting the female. Before inserting the penis, the male can make several thrusts with the pelvis, but intravaginal. There are no penile thrusts. After about a dozen of these short intromissions, following each other at minute intervals, the male proceeds to the second type of behavior, called “ejaculation.” When ejaculating, the male makes convulsive thrusts with his penis and grasps the female for a few seconds before dismounting from her, after which he assumes a stereotypical vertical position. The complete set of intromissions that culminate in ejaculation is called a “series.” Ejaculation and, accordingly, the introduction of sperm never occur without preliminary intromissions. After ejaculation, sexual activity temporarily stops; after about 5 minutes it resumes and the second episode begins. Typically, rats complete about seven such series before satisfaction sets in, a sign of which is the absence of attempts at intromission for half an hour (Beach and Jordan, 1956).

Rice. 5.4. Classification scheme for types of copulatory behavior in male mammals. (Dewsbury. 1972.)

The type of behavior characteristic of each species can be classified depending on whether 1) mating occurs, 2) whether the male makes intravaginal thrusts, 3) whether multiple intromissions are necessary before ejaculation, and 4) whether multiple intromissions occur during one mounting. There are 24, i.e. 16 possible types of copulatory behavior.

A male or female can develop a randomly selected operant response, using as reinforcement the opportunity to copulate with an individual of the other sex. If a satisfied male is given another female instead of his previous partner, he can resume sexual activity.

Not all mammals copulate in the same way as in laboratory rats. A possible classification scheme for the copulatory behavior of mammals is presented in Fig. 5.4 (Dewsbury, 1972). This scheme is based on four criteria: 1) whether “gluing” occurs during copulation, that is, whether the penis is held mechanically in the vagina; 2) whether intravaginal thrusts of the penis are produced; 3) are preliminary multiple intromissions necessary for ejaculation; 4) whether repeated ejaculations occur in the same episode. This system allows for 24, i.e. 16 types of copulation and allows any species to be characterized in this regard. In rats, neither mating nor intravaginal thrusts occur, but ejaculation is preceded by multiple intromissions, and ejaculation also occurs multiple times. Thus, rats are type 13.

Dogs behave very differently when copulating than rats (Hart, 1967). Their most characteristic feature is the mating of a male with a female: the male’s penis, while in the vagina, becomes engorged with blood, after which it is very difficult for the partners to separate, even if they strive for this. Usually the male gets off the female when copulation has not yet ended, after which the mated partners stand with their backs to each other. Copulation in dogs often lasts about 20 minutes, in contrast to the very short intromissions in laboratory rats. In dogs, grinding and intravaginal thrusting occur, but in them ejaculation is not preceded by multiple intromissions and there are several ejaculations, so their behavior belongs to type 3.

It should be remembered that the types of copulation differ in many respects and the scheme proposed here represents only one possible method of classification. Different species belonging to the same type in terms of copulatory behavior can differ greatly in their copulation postures and quantitative aspects of activity.

Advantages of sexual and asexual reproduction

Give your last 17 POINTS ONLY begs Explain the evolutionary advantage of sexual reproduction over asexual reproduction 2 What is the biological role of asexual reproduction in the evolution of life 3 Replace the term with highlighted words The union of two adjacent cells lying next to each other is a method of fertilization in many primitive organisms Mobile sex cells of males develop in most animals and plants and the sex cells are still male only in the testes

I WILL GIVE MY LAST 17 POINTS ONLY I BEG
1 Explain the evolutionary advantage of sexual reproduction over asexual reproduction
2 What is the biological role of asexual reproduction in the evolution of living things?
3 Replace the highlighted words with the term:
The fusion of two adjacent, adjacent cells is a method of fertilization in many primitive organisms
Motile male sex cells develop in most animals and plants, while immobile male sex cells develop only in seed plants.

Evolution(from Latin evolutio - “unfolding”) - the process of development of all living organisms, which is accompanied genetic changes, adaptations, modifications and extinction of individual populations and species, resulting in changes in ecosystems And biosphere generally.

Scheme of the evolution of living organisms on Earth.

Today there are several main theories of evolution. The most common is synthetic theory of evolution(STE) is synthesis Darwin's theory of evolution and population genetics.

ADVANTAGES OF SEXUAL REPRODUCTION COMPARED TO Asexual Reproduction

STE explains the connection between way of evolution (genetic mutations) And mechanism of evolution (natural selection according to Darwin). STE defines evolution as a process during which the frequency of gene alleles changes over a period of time that significantly exceeds the lifespan of one member of the population.

The essence of the theory of evolution of Charles Darwin, who formulated it in his work "Origin of Species"(1859), is that the main “engine” of evolution is natural selection, a process consisting of three factors:

1) More offspring are born in populations than can survive, taking into account environmental conditions (amount of food, presence of living creatures that feed on a given species, etc.);

2) Different organisms have different traits that affect their ability to survive and procreate;

3) The above-mentioned traits are inherited.

These three factors explain the emergence of intraspecific competition and selective extinction (elimination) of those individuals who are least adapted to survival. Thus, only the strongest leave offspring, which leads to the gradual evolution of all living things.

Natural selection is the only factor explaining the adaptation of all living things, but it is not the only cause of evolution. Other equally important reasons are mutations, gene flow and genetic drift.

The main process is natural selection. It decides which adaptations for a given habitat are favorable and which are not so desirable. If the adaptation is favorable, then organisms that have , encoding that trait, will live long enough to reproduce and pass on their genes to the next generation.

In order for natural selection to work on a population, there must be diversity. To obtain diversity in individuals, different genetics and expressions are required. All this depends on the method of reproduction of a particular species.

Asexual reproduction

Asexual reproduction is the production of offspring from one parent, which is not accompanied by mating or mixing of genes. Asexual reproduction results in cloning of the parent, meaning the offspring has identical DNA to its ancestor. As a rule, there is a lack of species diversity from generation to generation.

One way to gain some species diversity is through mutations at the DNA level. If an error occurs in the process or copying of DNA, then this error will be passed on to the offspring, possibly changing their traits. However, some mutations do not change the phenotype, so not all changes in asexual reproduction result in variation in the offspring.

Sexual reproduction

Sexual reproduction occurs when a female reproductive cell (egg) combines with a male cell (sperm). An offspring is a genetic combination of mother and father, with half of its chromosomes coming from one parent and the other half from the other. This ensures that the offspring are genetically different from their parents and even siblings.

Mutations can also occur in sexually reproducing species to further increase the diversity of the offspring. The process that creates the (sex cells) used for reproduction also helps increase diversity. It ensures that the resulting gametes are genetically different. Independent recruitment during meiosis and random fertilization also influences gene mixing and allows offspring to better adapt to their environment.

Reproduction and evolution

As a rule, it is believed that sexual reproduction contributes more to evolution than asexual reproduction, since it has a much greater

Reproduction- the most important property of all living things. A species that reproduces only asexually can thrive for quite a long time if it lives in relatively constant conditions. If changes occur in its habitat that cause the death of individual individuals, it is very likely that all individuals will die, because they are very similar genetically.

At sexually maternal and paternal organisms produce specialized germ cells -. Female non-motile gametes are called eggs, male non-motile gametes are called sperm, and mobile gametes are called sperm. These germ cells fuse to form a zygote, i.e. fertilization occurs. Sex cells, as a rule, have a half set of chromosomes (), so that when they merge, a double (diploid) set is restored, and a new individual develops from the zygote. In sexual reproduction, offspring are formed by the fusion of haploid nuclei. Haploid nuclei are formed as a result of meiotic division.

Meiosis leads to a reduction in genetic material by half, due to which the amount of genetic material in individuals of a given species remains constant over a number of generations. During meiosis, several important processes occur: random segregation of chromosomes (independent segregation), exchange of material between homologous chromosomes (crossing over). As a result of these processes, new combinations of genes arise. Since the zygote nucleus contains the genetic material of the two parents after fertilization, this increases genetic diversity within the species. If the essence and biological significance of the sexual process is the same for all organisms, then its forms are very diverse and depend on the level of development, habitat, lifestyle and some other features.

Sexual reproduction All groups of plants have it. Mosses grow in sods. Male and female plants are side by side. Rainwater helps sperm get to the tops of female plants, where they fuse with eggs, forming a zygote, from which a spore box sitting on a long stalk develops. In germ cells, germ cells develop on a germ formed as a result of spore germination. On the underside of the prothallus, the female organs are archegonia, and the male organs are antheridia. In a humid environment, the sex cells fuse, the zygote gives rise to an embryo from which a young one grows. In flowering plants, the most complex sexual reproduction is double fertilization. Pollen (male reproductive cells) lands on the stigma (female reproductive organ) and germinates. The sperm move along the pollen tube towards. Sperm penetrate the embryo sac. One fuses with the egg and gives rise to an embryo, the second sperm fuses with the central cell and gives rise to the endosperm - a supply of nutrients.

Sexual reproduction has very great advantages compared to asexual. The essence of sexual reproduction is the combination in the hereditary material of a descendant of genetic information from two different sources - the parents. Fertilization in animals can be external or internal. Fusion produces a zygote with a double set of chromosomes.

In the nucleus of the zygote, all chromosomes become paired: in each pair, one of the chromosomes is paternal, the other is maternal. The daughter organism that develops from such a zygote is equally equipped with hereditary information from both parents.

Biological meaning of sexual reproduction is that emerging organisms can combine useful characteristics of the father and mother. Such organisms are more viable. Sexual reproduction plays an important role in the evolution of organisms.

Sexual reproduction is the leading form of reproduction in the organic world. Sexual reproduction involves two parents. It is preceded by the formation in the parents’ bodies as a result of meiosis of specialized germ cells - gametes, each of which carries a single (haploid) set of chromosomes. Reproduction itself consists of fertilization - the fusion of gametes into a zygote. meiosis

Male and female reproductive cells in animals are usually formed in the gonads (testes and ovaries). They can be in different individuals or in one; in the latter case, individuals are called hermaphrodites. Hermaphroditism is the most primitive form of reproduction, characteristic of many lower animals (including tapeworms, earthworms, snails) and flowering plants. tapeworms earthworms snails flowering plants

Hermaphroditism: Self-fertilization is POSSIBLE (important for sedentary species or individuals leading a solitary existence) BUT self-fertilization prevents the exchange of genetic material between individuals; THEREFORE, many organisms have adaptations that prevent self-fertilization (genetic incompatibility of germ cells from one organism, the formation of male and female gametes at different times, a special flower structure that favors cross-pollination).

The essence of meiosis Meiosis is an important element of sexual reproduction. During meiosis, cells are formed containing only one set of chromosomes, which makes possible the subsequent fusion of sex cells (gametes) of two parents. sexual reproduction In fact, meiosis is a type of mitosis. It involves two successive cell divisions, but the chromosomes are duplicated only in the first of these divisions. The biological essence of meiosis is to reduce the number of chromosomes by half and form haploid gametes (that is, gametes with one set of chromosomes).
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