In the modern world, it has become customary to call everything that surrounds us in technical terms. The mechanism of reproduction … This 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 irreplaceable that sometimes one can only be surprised. For many millennia, mankind has been puzzling over the question of the primacy of eggs and chicken, and nature has long had an answer to all questions. Rationalism and diversity of solutions in maintaining the stability of an individual species and at the same time acquiring a variety of traits in wildlife are unparalleled.
The genetic basis of life
One of these devices is the alternation of generations. The diversity of animal and plant species is achieved through the creation of various combinations of genetic material. Alternation of generations is a special form of species conservation in changing environmental conditions, found mainly in many plants and lower invertebrates. It represents the change of sexual and asexual reproduction.
What causes the launch of one or another method of reproduction and what goals do they pursue? To answer this question, it is necessary to understand more deeply what sexual and asexual reproduction are and how they differ, what advantages and disadvantages they bring for a biological species.
Sexual reproduction
The process of sexual reproduction involves the participation in the creation of a new life of two individuals, which, each in itself, are carriers of their individual set of chromosomes in a helical double strand of DNA. This unique set of genetic material is expressed in the presence in this individual, and only in her, of certain traits, which she partially passes on to her offspring.
When two individuals participate in the process of sexual reproduction, each of which gives the potential successor of the species its 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 through sexual reproduction.
What contribution does sexual reproduction make to the gene pool of a species
Even within a relatively small population, the set of combinations of genetic material can be infinitely wide. This type of reproduction pursues a policy of introducing diversity into the genetic background of the species population. Diversity can also be achieved through the use within an established population of new specimens of a given species, which are different.ways can penetrate from the outside. Or, as, for example, in plants or some coelenterates, at the expense of germ cells "with home delivery" using wind, water or insects.
An important point in sexual reproduction is to indicate the possibility of participation in it of predominantly he althy and strongest individuals. Thus, this type of reproduction allows the implementation of natural selection, which contributes to the possibility of fixing traits that work for the benefit of this species.
Asexual reproduction as a formula for the multiplier of the number of individuals
Alternation of generations is a system used to increase and maintain a species, in which asexual reproduction plays an important role. Of its advantages, one can safely note the ability to rapidly increase the population size when environmental conditions favorable for a given biological species occur. Preservation and enhancement of the population's genetic fund through multiple cloning of already existing gene combinations, which significantly increases the chance of a species for the participation of 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 s alt concentration in it), the duration of the daily light period, the intensity of illumination, and the change of seasons. All these factors regulate the production of certain reproductive cells. Some plants produce spores, the basis of asexualreproduction, and are called sporophytes. Plants that produce gametes for sexual reproduction (sex cells with a single set of chromosomes in the nucleus) for reproduction are called gametophytes. There are algae that produce both types of germ cells (gametes and spores), and they are accordingly called gametosporophytes. Algae of all these types can differ from each other both morphologically and biologically. So the red algae Porphyra Tenera in the form of a sporophyte looks like threads branching in one row, penetrating into the substrate, which can be calcareous rocks or mollusk shells.
Sporophytes of this species live at great depths, prefer low light. Individuals involved in the production of cells for sexual reproduction (gametophytes) live in the form of plates in the ebb and flow zone at shallow depths under intense illumination. Red algae, being more highly organized, demonstrate the most diverse and most complex development cycles, in which there is a change in different forms of existence of organisms of the same species during the life cycle - heteromorphic development.
Who is characterized by reproduction through gametosporophytes
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 environmental conditions. Consistency between manifestations of traits in the phenotype and the correspondingenvironmental changes - the main evolutionary factor that provides a driving form of selection.
Alternation of generations in plants and animals: what are the similarities of two different kingdoms
Classification, which divides 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. Thus, the alternation of generations in coelenterates is of a particularly interesting nature. In the life cycle, generations of sexual and asexual reproduction have a different appearance, lead a radically different lifestyle, live in different places and eat differently. In metagenesis, there is an alternation of life forms: polyps and jellyfish. Polyps attached to the substrate lead a sedentary lifestyle. Polyps are characterized by asexual reproduction by budding from the mother's organism new daughter individuals identical in genetic composition, which also spend their lives in the form of polyps. Nutrition is carried out by filtering masses of water, with the current of which microscopic organic particles are brought, which serve as food for the body.
Polyps can organize huge communities. Similarly, the alternation of generations in coelenterates creates for a long time colonial forms of polyps in the form of coral reefs. When certain conditions occur, which are individual for each species (change in temperature, timeyears, changes in underwater currents, the phase of the moon, the time of migrations, etc.), polyps bud small jellyfish. Jellyfish are mobile, easily move in the water column, and are predators in the way they feed. Growing up to the age of sexual readiness, jellyfish continue the cycle of development of the species through sexual reproduction. Motile larvae develop from fertilized cells, which 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 original stage, but with a different set of chromosomes, and therefore with different characters.
Mosses also reproduce sexually
Alternation of generations is observed in higher plants, including mosses. A characteristic feature of the life cycle of this plant division is the fact that the dominant life form is the gametophyte in the form of a green perennial plant with leaf-like outgrowths and rhizoids, which we observe. The alternation of generations in mosses is provided by the sporophyte, which is an asexual stage of the development cycle, represented by a small box on a stalk with spores, connected with the gametophyte by the feet, through which the physiological supply of spores occurs. The sporophyte has a short lifespan and cannot take root on its own. 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 own evolutionary significance. The alternation of generations is a processensuring the consolidation of the necessary traits and the rejection of unnecessary ones, manifested in the phenotype, due to natural selection. Only in the case of asexual reproduction, spontaneous mutations will be “submitted to the judgment” of natural selection, 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 talking about sexual reproduction, the sum of two units is not equal to two (1+1≠2)? Because as a result of fertilization, the child receives a set of genes that is not identical to any parent. An individual will not carry a maternal or paternal gene, but will develop based on the information that came from the parents. She will be the carrier of the third, unique and inimitable genotype, so biologists solve the mathematical example a little differently. This is what ensures the alternation of generations in plants and mammals, where with each new rebirth of the genetic material it becomes more complex, elegant and perfect!
