In the modern world, it has become customary to call everything that surrounds us technical terms. The mechanism of reproduction ... That is how scientists “christened” the miracle of the birth of a new life.
A miracle in which any component is so harmonious, diverse and at the same time indispensable that sometimes one can only be surprised. For many millennia, humanity has been puzzling over the question of the primacy of eggs and chicken, and nature has long had an answer to all the questions. Rationalism and diversity of decisions in maintaining the stability of a particular species and at the same time acquiring a variety of characteristics in living nature are unparalleled.
The genetic basis of life
One of these devices is the alternation of generations. The diversity of species of the animal and plant world is achieved by creating various combinations of gene material. Alternation of generations is a special form of conservation of a species under changing environmental conditions, found mainly in many plants and lower invertebrate animals. It represents a change in sexual and asexual reproduction.
What causes the launch of a particular breeding method and what goals do they pursue? To answer this question, we need to understand more deeply what are and what are the differences between sexual and asexual reproduction, what are the advantages for the
biological species and their disadvantages.
Sexual reproduction
The process of sexual reproduction involves the participation in the creation of a new life of two individuals, who, each in themselves, are carriers of their own individual set of chromosomes in a spiral double DNA strand. This unique set of gene material is expressed in the presence of this individual, and only in it, certain characteristics, which it partially transfers to its offspring.
With the participation of two individuals in the process of sexual reproduction, each of which gives the potential successor of the species its own half set of chromosomes, the next generation will have the characteristics of both parental organisms. That is why the alternation of generations is observed in both simple and complex life forms that reproduce due to sexual reproduction.
What is the contribution of sexual reproduction to the gene pool of the species?
Even within a relatively small population, the combination of gene material combinations can be infinitely wide. This type of breeding pursues a policy of introducing diversity into the genetic background of a species population. Diversity can also be achieved through the use of new specimens of a given species within an established population that can penetrate in different ways. Or, as, for example, in plants or some intestinal, due to germ cells "delivered to your home" with the help of wind, water or insects.
An important point during sexual reproduction is to indicate the possibility of participation in it predominantly healthy and strongest individuals. Thus, this type of reproduction makes it possible to implement natural selection, which contributes to the possibility of fixing signs that work for the benefit of this species.
Asexual reproduction as a formula of the multiplier of the number of individuals
Alternation of generations is a system used to increase and preserve a species, in which asexual reproduction plays an important role. Of its advantages, one can safely note the possibility of rapidly increasing the population size upon the occurrence of environmental conditions favorable for a given biological species. Preservation and multiplication of the genetic stock of the population by multiple cloning of existing combinations of genes, which significantly increases the chance of the species participating in these combinations in further sexual reproduction.
Alternation of phenotypes in different kingdoms
The alternation of generations in algae depends on the temperature background, the chemical composition of the water (especially the salt concentration in it), the duration of the daily light period, the light intensity, and the change of season. All these factors regulate the production of various reproductive cells. Some plants produce spores, the basis of asexual reproduction, and are called sporophytes. Plants that produce gametes for reproduction (reproductive cells with a single set of chromosomes in the nucleus) for reproduction are called gametophytes. There are algae producing both types of germ cells (gametes and spores), respectively, they are called gametosporophytes. Algae of all these types can differ from each other both morphologically and biologically. So the red alga Porfira Tenera in the form of a sporophyte looks like a thread, branching in one row, embedded in a substrate, which can serve as calcareous rocks or mollusk shells.
Sporophytes of this species live at great depths, prefer poor lighting. Individuals involved in the production of cells for sexual reproduction (gametophytes) live in the form of plates in the
tidal zone at low depths under intense light.
Red algae, being more highly organized, demonstrate the diverse and most complex development cycles, in which there is a change in the different forms of existence of organisms of the same species during the life cycle - heteromorphic development.
Who is characterized by gametosporophyte reproduction
Gametosporophytes are typical of many species of green, brown, and red algae. Alternation of generations is observed in them in the production of reproductive cells of both types: spores and gametes, occurring at different times and due to changes in the environment. The consistency between the manifestations of characters in the phenotype and the corresponding changes in the environment is the main evolutionary factor that provides a moving form of selection.
Generational alternation in plants and animals: what are two different kingdoms like
The classification, dividing the living world into 4 kingdoms, greatly simplifies the perception of biological science in the early stages of its study. However, with a more in-depth course, it becomes clear that in the existing classification there are many intermediate cases. So, the alternation of generations in the intestinal has a special interesting character. In the life cycle, generations of sexual and asexual reproduction have a different appearance, lead a radically different way of life, live in different places and eat differently. In metagenesis, an alternation of life forms occurs: polyps and jellyfish. Polyps attached to the substrate lead a sedentary lifestyle. Polyps are characterized by asexual reproduction by budding from the maternal organism of new, identical in genetic composition, daughter individuals that spend their lives also in the form of polyps. Nutrition is carried out by filtering masses of water, with the current of which microscopic particles of organics are brought in, which serve as nutrition for the body.
Polyps can organize huge communities. In a similar way, the alternation of generations in intestinal organisms creates for a long time colonial forms of polyps in the form of coral reefs. When certain conditions occur that are individual for each species (change of temperature, season, change of underwater currents, phase of the moon, time of migration, etc.), polyps bud off small jellyfish. Jellyfish are mobile, easily move in the water column, are predators in the way of food. Growing up to the age of sexual readiness, jellyfish continue the cycle of development of the species through sexual reproduction. From the fertilized cells, motile larvae develop that settle to the bottom, attach to the substrate, lose their mobility and grow into a polyp. Alternation of generations is a life cycle undergone by a species that invariably closes, returning to its initial stage, but with a different set of chromosomes, and therefore with other characters.
Mosses also reproduce sexually
Alternation of generations is observed in higher plants, including mosses. A characteristic feature of the life cycle of this department of plants is the fact that the dominant life form is the gametophyte in the form of a perennial green plant with leaf-like outgrowths and rhizoids, which we observe. Alternation of generations in mosses is ensured by the sporophyte, which is an asexual stage of the development cycle, represented by a small box on the stem with spores, connected with the gametophyte by the feet, through which physiological support of the spores takes place. Sporophyte has a short lifespan, cannot root itself. Dries out after maturation and rash of spores.
Why in biology 1 + 1 = 3
Stating the above, we can conclude that both methods of reproduction have their evolutionary significance. Alternation of generations is a process that ensures the fixing of the necessary traits and the rejection of unnecessary, manifested in the phenotype, due to natural selection. Only in the case of asexual reproduction of “natural selection” will spontaneous mutations be “exposed”, and in the case of sexual reproduction, in addition to mutations, the signs of both parental individuals will appear in the phenotype.
Why, in evolutionary biology, when they talk about sexual reproduction, the sum of two units is not equal to two (1 + 1 ≠ 2)? Because, as a result of fertilization, a baby gets a set of genes that is not identical to any parent. The individual will not carry the maternal or paternal gene, but will develop on the basis of the information that came from the parents. She will be the bearer of a third, unique and unique genotype, so biologists solve a mathematical example in a slightly different way. This is what ensures the alternation of generations in plants and mammals, where with each new degeneration of genetic material, it becomes more and more complex, elegant and perfect!