Sexual Reproduction

 Sexual reproduction

 In sexual reproduction, the genetic information of the descendants is made up of the genetic contribution of both parents, through the fusion of sexual cells or gametes;[13]​ that is, sexual reproduction is a source of genetic variability.

 Sexual reproduction requires the interaction of a chromosome, generating both male and female gametes or two individuals, being of different sexes, or also hermaphrodites. The offspring produced as a result of this biological process will be the result of the combination of the DNA of both parents and, therefore, will be genetically different from them. This form of reproduction is the most common in complex organisms. Two haploid cells originated by meiosis, the gametes, participate in this type of reproduction, which will join together during fertilization.

Disadvantages

 Normally requires the presence of a second individual. To avoid this problem, parthenogenesis, multiplication from eggs without a partner and therefore without fertilization, in the case of aphids, daphnia, etc., has sometimes been converted. An isolated animal that discovers a new territory will not be able to propagate the species, unless, of course, it is a fertilized female.

 You often have to search for this mate, which can be difficult or dangerous, for example, males of the same species often have to fight each other to win over females.

 Reproduction sometimes involves competition between males or females, therefore competition. This may require additional resources, decreased vigilance against predators, sometimes injury or death.

 In case of mating, there is a risk of exchange of infectious agents: bacteria, viruses, fungi, various parasites, etc.

 Many gametes must be produced and, in the case of males, they are often lost.

 Greater complexity of the genome that must develop two different but compatible versions: one male and one female within each individual (two alleles from both parents). Additionally, the species is forced into a slower evolutionary process to maintain compatibility, where asexual individuals can mutate much faster to adapt to changes like many bacteria and viruses.

Reduction of the diversity of living organisms reduced to a few common species, instead of "asexual individuals" all radically different evolving separately over time in as many evolutionary branches as there are living beings without the limitations linked to the species. Thus, all similar individuals of the same species all have the same "weak points", which can precipitate the end of the entire species in a short time, due to climate change or a new predator.

 The genetic mix resulting from reproduction is an advantage for the species (since one day or another it brings together the best genes) but it is a great disadvantage for the individual. Nothing says that the combination of your genes with those from the other partner (even selected) will not result in deficient individuals. This is the case of all recessive genetic diseases, invisible in the parents.

Human Reproductive System

After fertilization of the egg, called at that time a zygote, a series of mitotic divisions occur, parts of embryonic development, culminating in the formation of the embryo.

 The embryo has three germ layers, called ectoderm, endoderm and mesoderm, from which the different organs of the body will originate.

Animal reproduction

 Four groups are distinguished:

 Ovuliparous: Ovuliparity is a type of oviparity, a process of sexual reproduction by which both the fertilization of the zygote - union of the male and female gametes - and the development of the embryo occurs in the external environment, outside the urogenital apparatus of the female. Females deposit eggs in a medium and males deposit sperm on them (external fertilization). They require an aquatic environment. It occurs in amphibians and bony fish.

 Oviparous: the male introduces the sperm into the female (internal fertilization). Once she is fertilized, she deposits eggs with hard shells that protect the embryo. It occurs in some cartilaginous fish, reptiles, birds and two mammals: the echidna and the platypus.

 Ovoviviparous: fertilization is internal and the embryo is enclosed in an egg inside the mother's body with which it does not exchange substances. When the embryo is developed, the egg hatches and the female gives birth to the offspring, or deposits the egg shortly before the offspring emerges from it. It occurs in sharks and snakes.

 Viviparous: fertilization is internal and the offspring develops inside the mother's body by exchanging substances. It occurs in most mammals, including humans.

Advantages and disadvantages of asexual and sexual reproduction

 Organisms that reproduce through asexual reproduction usually increase their numbers exponentially. However, as the variation is caused by mutations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually produce fewer offspring, but the large amount of variation in their genes makes them less susceptible to disease.

Many organisms reproduce sexually and asexually. We find examples in aphids, slider fungi, sea anemones, some species of starfish (through fragmentation) and many plants. When environmental factors are favorable, asexual reproduction is used to exploit conditions suitable for survival, such as an abundant food supply, adequate shelter, favorable climatic conditions, optimal pH, or an appropriate combination of other lifestyle needs. The populations of these organisms increase exponentially through asexual reproduction strategies to make the most of the abundance of resources.

 When food sources have been depleted, the climate becomes hostile, or individual survival is threatened by some other adverse change in living conditions, these organisms shift toward forms of sexual reproduction. Sexual reproduction guarantees a mixture of the species' genetic pool. The variations found in the offspring of sexually reproducing species allow some individuals to be more prepared for survival and provide them with a mechanism for selective adaptation. Furthermore, sexual reproduction, in general, is useful in a stage of life that requires enduring the conditions that threaten the children of an asexual parent. Therefore, spores, eggs, pupae, cysts, or other “overwintering” stages of sexual reproduction ensure survival when times are not favorable and the organism can.