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 father's or the mother's genotype.In some animals, new genotypes can be created as a result of processes of a different kind. new homozygous genotype. Other forms, including some flatworms and molluscs, are hermaphroditic, i.e. have both male (sperm-producing) and female (egg-producing) sex glands. There are hermaphrodite forms capable of self-fertilization.
Not all reproduction is sexual (that is, it creates new genotypes). For example, Paramecia can divide in two with the formation of two new daughter organisms that are genetically identical to the original individual.Hydroid polyps (one of the groups of coelenterates) can produce identical new individuals as a result of the budding process.At the same time, 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
The vast majority of animals, especially forms that have arisen relatively recently, reproduce sexually, that is, through 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 should obviously provide some significant advantages. The following main reasons were put forward for this explanation:
1) an evolutionary advantage for populations that can change faster than others through sexual reproduction;
2) an evolutionary advantage associated with the fact that this mode of reproduction facilitates speciation (the emergence of new species);
3) that individual parents can create diversity in their immediate offspring, making it easier for them to adapt to unpredictable changes in the environment.
During sexual reproduction, as a result of the fusion of gametes, a fertilized egg is formed - a zygote, carrying the hereditary inclinations of both parents, due to which the hereditary variability of the offspring increases sharply. This is the advantage of sexual reproduction over asexual. Those. in the presence of genetic recombination, the parent individuals produce offspring that will differ from them in the most unpredictable way, and among the new random combinations of genes at least half may turn out to be worse than the parental genotype, however, gene shuffling during sexual reproduction contributes to the survival of the species when environmental conditions change. If a parent produces many offspring with a wide variety of gene combinations, there is a better chance that at least one offspring will be well adapted to future life circumstances, whatever they may be.
In the presence of genetic recombination, the parental individuals produce offspring that will differ from them in the most unpredictable way, and among the new random combinations of genes at least half may turn out to be worse than the parental genotype, however, gene shuffling during sexual reproduction contributes to the survival of the species when environmental conditions change. If a parent produces many offspring with a wide variety of gene combinations, there is a better chance that at least one offspring will be well adapted to future life circumstances, whatever they may be.
Many hypotheses have been proposed to explain the benefits of sexual reproduction in the struggle for existence. One of them gives an idea of what the first stages of the evolution of sexual reproduction could be. The course of evolution largely depends on mutations that change existing genes, forming instead of them new alleles (variants) of these genes. Suppose that two individuals in a certain population develop favorable mutations affecting some genetic loci, and hence different functions. In asexual species, each of these individuals will give rise to a clone of mutant offspring, and two new clones will compete until one of them wins. One of the favorable alleles created by mutations will thus spread, while the other will eventually disappear. Now let's imagine that one of the original mutants has a genetically determined feature that allows it from time to time to include genes from other clones in its genome. Under the conditions of the struggle for existence, the acquisition of genes from the cells of a competing clone is tantamount to the creation of a cell that carries all the favorable mutations. Such a cell will have the greatest adaptability, and the advantages obtained by it will ensure the spread in the population of a feature that allows the genes of other cells to be included in its genome. Natural selection will favor this primitive sexual reproduction.
Sperm are one of the main characters in sexual reproduction
Yeast helped scientists show that interpopulation breeding leads to greater ecological adaptability of the species.
The survival of a species is associated with the accumulation of genetic changes that help the body to exist in a particular habitat. It is believed that sexual reproduction, as a result of which increased genetic variability, contributes to the fastest evolution of the species. But in the case of sexual reproduction, the offspring takes on the genes of two different individuals. Imagine that the mother and father came from different populations; the mother's genes allow her to survive under certain conditions, while the father's genes are "sharpened" for others. The offspring in this case will not be adapted to either one or the other: the genes will weaken each other and will not be able to work adequately at all under any conditions. It turns out that sexual reproduction does not contribute to the survival of the species?
Researchers from the University of Auckland (New Zealand) set up an experiment that was supposed to directly answer the question of whether interpopulation crossing helps or slows down evolution. Scientists have used yeast that can reproduce both asexually and sexually. The first culture was grown under the same conditions, the second under different conditions. At some point, the yeast turned on the mechanism of sexual reproduction and allowed fungi from different populations to find each other.
In an article published in the journal Ecology Letters, the authors write that the offspring obtained as a result of sexual reproduction adapted faster to the environment. If the parents were from different populations, then their children felt equally well both in the "maternal" and "paternal" environmental conditions. That is, sexual reproduction not only does not interfere, but also stimulates the evolution of the species, especially when individuals from different populations are found.
In fact, the results of the experiment confirm one alternative, but relatively little-known hypothesis, according to which genes, "sharpened" for some conditions, do not necessarily interfere with living in others. Genes for different habitats do not enter into confrontation, but peacefully coexist in one genome, turning on and off as needed.
Previously, evolutionary biologists had to invent clever tricks that would prevent individuals from different populations from interbreeding with each other and thereby weaken the evolutionary position of a species. And although, as already mentioned, there was an alternative hypothesis, experimental confirmation was needed to raise it above all others. In preparing the article, the information of a computer was used..
1. What is the advantage of sexual reproduction over asexual?
Answer. During sexual reproduction, which is carried out by the fusion of two gamete germ cells, the parents' genetic information is exchanged. As a result, offspring that are diverse in their characteristics appears, which can surpass their parents in vitality, including when environmental conditions change.
2. Give examples of organisms that reproduce mainly asexually.
Answer. With asexual reproduction, a new organism can arise from one or more asexual (somatic) cells of the mother.
Prokaryotic cells multiply by dividing in two. Many protozoa (amoeba, euglena green, etc.), unicellular algae (chlamydomonas) multiply by normal mitotic cell division. Other unicellular and some lower fungi, algae (chlorella), animals (malaria plasmodium) are characterized by sporulation. It consists in the fact that the cell splits into a large number of individuals, equal to the number of nuclei previously formed in the parent cell as a result of multiple division of its nucleus.
Questions after § 32
1. What is the difference between conjugation and copulation?
Answer. If we are talking about methods of sexual reproduction, then:
during copulation, two unicellular individuals merge (the genetic information of both parents is combined, and then they divide by meiosis (an increase in the number of individuals that received genetic information from both parents occurs).
during conjugation, genetic information is exchanged without increasing the number of individuals. There are the following basic forms of conjugation: isogamy, anisogamy and oogamy.
With isogamy, mobile, morphologically identical gametes are formed. Isogamy occurs in many algae.
With heterogamy, mobile gametes that differ morphologically and physiologically are formed. This type of sexual process is characteristic of many algae.
In the case of oogamy, the gametes are very different from each other. The female gamete is a large, immobile egg that contains a large supply of nutrients. Male gametes - sperm - are small, most often motile cells that move with the help of one or more flagella. Oogamy is characteristic of animals, higher plants and many fungi.
2. Where are the eggs formed?
Answer. The ovum is the female gamete of animals, higher plants, as well as many algae and unicellular organisms, which are characterized by oogamy. In animals, eggs are formed in the female gonads - the ovaries, in gymnosperms and angiosperms in the ovules, mosses and ferns in archegonia
3. What is the sperm acrosome for?
Answer. During fertilization, at the moment of contact of the sperm with the egg, the enzymes contained in the acrosome are released and dissolve the egg membranes, allowing the penetration of the sperm into the egg.
Questions at the beginning of the paragraph.
Question 1. Why can a species exist practically indefinitely, while every single individual is mortal?
The individual cannot evolve. It can change, adapting to the conditions of the external environment. 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 the population: mutations occur in individuals, interbreeding occurs between individuals, the struggle for existence and natural selection are active. 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 outside the abdominal cavity, in the skin sac - the scrotum. Full maturation of spermatozoa occurs in the system of the vas deferens, and then they enter the urethra, at the beginning of which the ducts of additional glands - seminal vesicles and the prostate gland, or the prostate - also flow.
The maturation of the oocyte takes place in the graafian vesicle of the ovary. The development of the egg lasts about 28 days. As a result of reduction division, a mature egg, like a sperm cell, remains with a half set of chromosomes. Each egg contains only the X chromosome. Consequently, the sex of the unborn child depends on the father.
Question 3. What determines the sex of the 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 mother and Y from father), a boy will be born.
Questions at the end of the paragraph.
Question 1. What are the advantages of sexual reproduction over asexual?
With the help of sexual reproduction, the gene apparatus of the offspring is renewed, new combinations of genes appear, since the maternal and paternal organisms participate in it, and the variety of individual traits 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 maternal and daughter organisms is the same.
Question 2. Explain the biological significance of the presence of a half set of chromosomes in a sperm and an egg.
In the nuclei of the male and female germ cells, there are half the set of chromosomes characteristic of a given species. When an egg and a sperm merge, their chromosome sets are combined, the chromosome set characteristic of this species is restored, and in the future organism the hereditary characteristics of both parents are combined.
Question 3. Where does fertilization take place? What is the result of this process?
The fusion of the egg and sperm occurs in the fallopian tube. After the penetration of the sperm into the egg, a zygote is formed - an embryonic cell that bears the hereditary characteristics of both parents.
Question 4. Why the embryo can stay in the uterus, but the unfertilized egg cannot?
An unfertilized egg, unlike an embryo, does not have villi, which allow it to stay in the uterus.
Even a cursory review of the methods of reproduction of different groups of organisms shows that in the process of evolution, asexual reproduction gives way to sexual reproduction. Indeed, asexual reproduction is a mandatory attribute of all unicellular organisms, fungi, algae and spore plants, as well as the most primitive multicellular animals. Seed plants already lose their ability to reproduce by spores, and many of them are not even capable of vegetative propagation. Highly organized animals generally reproduce only sexually. What are the benefits of sexual reproduction? Why did the most advanced organisms choose to reproduce with gametes rather than body parts or spores?
Unlike asexual reproduction, which, in fact, is the process of copying the original maternal organism in a series of generations, with the sexual mode of reproduction, each individual is unique. This is due to the fact that during the formation of haploid cells (spores in plants or gametes in animals), the genetic material of the parent individual is recombined.
As a result, despite the fact that gametes or spores are formed in one organism, and some even originate from a common parental cell, they differ from each other, since each carries its own genetic information. In addition, the gametes of different parents, merging with each other, form completely new genetic combinations in the zygote. As a result, it turns out that with the sexual mode of reproduction, each individual acquires "its own face" - its own set of genes, which determines the uniqueness of its structure (Fig. 7) and functioning. This means that each organism has special biological properties. Some individuals tolerate heat better, the second - cold, still others are able to multiply quickly, the fourth are resistant to diseases. As a result, with a sharp warming of the climate, extremely harsh winters or epidemics, there are always resistant to adverse factors. They survive and give rise to new generations. When the conditions are favorable, new leaders emerge, whose value lies in other abilities, for example, in rapid reproduction.
If during asexual reproduction each organism multiplies without any participation of individuals of its own species, then during sexual reproduction all organisms belonging to the same species are potentially linked by "marriage". Hereditary changes that occur in the genetic apparatus and allow the body to have certain advantages over other individuals of its own species, not only increase its chances of survival, but also make it possible to leave more offspring with such useful properties. Material from the site
This key position of modern biology applies to organisms that reproduce both asexually and sexually. However, asexual organisms cannot exchange useful "acquisitions". But it is easily done by creatures that reproduce sexually. Therefore, if the genetic characteristics of one animal organism, allowing it, for example, to endure severe winters, in descendants are combined with others, no less useful, for example, the tolerance of helminthiasis, then it clearly has more chances to survive and leave offspring than ancestors endowed with only one of these positive properties. If we add to this that during sexual reproduction the number of potential offspring is many times greater than during asexual, then such positive changes in a number of generations will accumulate even faster. This means that sexual reproduction provides much more material for evolution than asexual. Hence, it is obvious that in evolutionary terms, sexual reproduction has all the advantages over asexual, and therefore the most highly organized, evolutionarily advanced organisms at a certain stage abandoned asexual reproduction.
Advantages sexual reproduction over asexual are associated with the fact that the offspring receives a variety of properties that significantly increase the resistance of the species to changeable and unfavorable environmental conditions and provides much more source material for evolution.
On this page material on topics:
Biology table of the benefits of sexual reproduction over asexual
What organisms reproduce both asexually and sexually
Why is the offspring diverse during sexual reproduction?
Why can't humans reproduce asexually
Sexually positive and negative
Questions about this material:
The main process is natural selection. He decides which adaptations for a given habitat are favorable and which are not so desirable. If the adaptation is favorable, then organisms that have it encoding this trait will live long enough to reproduce and pass on their genes to the next generation.
For natural selection to work for a population, there must be diversity. To obtain diversity in individuals, different genetics and expressions are needed. It all depends on the breeding method 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 leads to cloning of the parent, that is, the offspring has identical DNA to their ancestor. As a rule, there is no species diversity from generation to generation.
One way to get some species diversity is through DNA mutations. If an error occurs in the process or DNA copying, then this error will be passed on to the offspring, possibly changing its traits. However, some mutations do not change the phenotype, so not all changes during asexual reproduction lead to variations in the offspring.
Sexual reproduction
Sexual reproduction occurs when a female reproductive cell (egg) combines with a male cell (sperm). The offspring is a genetic combination of mother and father, half of its chromosomes come 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 offspring diversity. The process that creates (sex cells) used for reproduction also contributes to increased diversity. It ensures that the resulting gametes are genetically different. Independent recruitment during meiosis and inadvertent fertilization also affects gene mixing and allows offspring to better adapt to their environment.
Reproduction and evolution
As a rule, it is believed that sexual reproduction is more conducive to evolution than asexual, since it has much more
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