Sex. Unlike physical and behavioral distinguishes individual organisms, according to the functions performed in the processes of reproduction . Through this difference, by which there are males and females, a species can constantly combine genetic information and give rise to offspring with different genes . Some of these descendants become better adapted to possible variations in the environment.
Sex is present at all levels of biological organization, except in viruses . Already at the simplest levels, bacteria exchange a single long chromosome that passes from the male (by analogy), or donor cell , to the female, or recipient cell. In more advanced groups, multicellular beings have specialized organs (gonads), which produce sex cells ( gametes ). At the time of fertilization, genetic information is transferred from small, mobile sperm (male gametes) to larger eggs (female gametes)
Many organisms, including most plants , many Prorozooz and Invertebrates, and some fish, have both male and female gonads and are called hermaphrodites . However, self-fertilization is rare in hermaphroditic organisms. The male and female reproductive organs usually mature at different times, which coincide with the maturation of other individuals, which makes cross-fertilization possible. It is frequent in the world of fish the succession of sexes in the same individual but in a complete way, that is, the fish is totally Male or totally Female according to the moment of life .
Adaptive advantages of sex
Many organisms also carry out a sexual reproduction (in which the parents multiply without a previous sexual union). This is the case of bacteria and protozoa that divide by mitosis into separate individuals. The plants and hydras reproduce by budding. Other organisms, including plants, Daphnia water fleas, and some wasps , reproduce by parthenogenesis; in it, the unfertilized eggs develop into adults. Asexual reproduction has the advantage of generating large populations of a species in a short time. Indeed, both Daphnia water fleas and certain wasps change their sexual reproductionby parthenogenesis during the brief warm season to quickly populate ponds and lay their nests. However, these populations are made up of genetic replicas of the parents and, should an adverse change occur in their environment, the entire population or the species would be in danger of extinction.
Although sexual reproduction is slower and more complicated, it has the great advantage of producing a wide diversity of individuals, each with small differences in their genetic makeup. During the formation of sex cells, or meiosis, the double set of chromosomes (diploid), as it appears in each of the adult cells, is distributed randomly, forming a single set of chromosomes (haploid) in each of the gametes. When this simple group joins another that comes from a different gamete, the genes are mixed again; this makes it possible that the offspring are not an exact copy of the parents. If the environment in which these offspring lives undergoes little or no change, the offspring that most closely resemble their parents will be the most capable of adapting and procreating. If more drastic changes occur in the habitat,some of the most disparate descendants with respect to their parents could be favored by the new situation. The role of sex, by always rearranging parental genes, is a fundamental mechanism of natural selection and probably existed long before the first multicellular organisms appeared.
Sex determination, type XX-XY In humans, the sex of the newborn depends on the type of sperm that performs fertilization. If the sperm that fertilizes the egg carries the X chromosome, the resulting zygote will give rise to a girl (XX) and if the sperm that fertilizes the egg carries the chromosome and the zygote will give rise to a boy (XY). The probability of a boy or girl being born is exactly the same.
In animals, the sex of individuals is usually determined at the time the sperm performs fertilization. Sex chromosomes are what determine sex and sex-linked characters; the rest of the chromosomes are called autosomes. In the human species and in many other animals, the male or heterogametic sex has different sex chromosomes (XY), while the female or homogametic sex has two identical chromosomes (XX). The male XY pair is secreted at meiosis so that half of the sperm carry the X chromosome and the other half carry the Y chromosome.
On the contrary, all ovules carry the X chromosome. In this way, sex is determined by the kind of sperm that fertilizes the ovum; if the sperm carries a chromosome and the offspring will be a male, while if the sperm carries an X chromosome the offspring will be a female. However, this is not the only type of sex determination; in other animals, like many insects, the Y chromosome is missing so that females have 2 X chromosomes (XX) and males only have 1 X chromosome (XO). When the sperm that fertilizes the egg has the X chromosome, the offspring will be a female and if the sperm that fertilizes the egg lacks an accessory chromosome, the offspring will be a male. In the birds and butterfliesday and night the sex chromosomes are called Z and W: the males are ZZ and the females are ZW. In this case there are two kinds of ovules and only one of sperm. In other cases, sex determination may be related to different environmental conditions.
The term primary sexual characteristics refers to the gonads or organs that produce gametes: the ovaries produce ovules or eggs in females and the testes produce sperm in males. The term secondary sexual characteristics indicates all other sexual differences that play an indirect role in the union of the sperm with the egg. Secondary sexual characteristics range from the specialized structures of both the male and female genitalia, to the bright plumage of the males of certain birds, or the facial hair in humans. So are certain facets of behavior such as courtship.
In general, the more advanced a species is in evolution, the more elaborate its secondary sexual characteristics. For example, by the time the starfish egg matures, the male only has to release large amounts of sperm into the water and a tiny but sufficient number of these male sex cells find and fertilize distant eggs. Frogs and toads attract mates through calls and lay their eggs in the water. The male and female make their cloaca coincide and the sperm is released simultaneously with the exit of the eggs. Terrestrial animals, particularly mammals, do not have an aquatic environment that facilitates the diffusion of their sperm. For this reason, they depend on the herds and groups in which they live,of the courtship ceremonies they perform, of the competition between males, as well as of more specialized genitalia, such as erectile penises, or fallopian tubes and auterus , in the latter two, the eggs are fertilized and developed , respectively.
It is believed that in the first eukaryotes that produced gametes, these were undifferentiated in size and shape, without differentiating a male and a female gamete. This condition is called isogamy and we have a current example in the filamentous alga Spirogyra. However, most of the beings with sexual reproduction present heterogamy, which consists of the differentiation of two types of gametes (male and female). Sexual reproduction has evolved in species that lived in water .
Fertilization gives rise to an egg that must be self-sufficient until embryonic development is complete, which implies that the gametes must carry enough nutritional substances. This fertilization took place in free water, so that an excessive size of the gametes would make them not very mobile and would make it difficult for the two gametes necessary for it to meet. In this way, the differentiation of functions, in which a gamete becomes ‘lazy’ and ‘fat’ and passively awaits the arrival of another mobile and active gamete, arises as an adaptation so that an egg loaded with food from the female gamete, without reducing the chances of fertilization since the male gamete, light, can seek the female thanks to its great autonomy.
In mammals , the hormones that influence sexual differentiation and development are androgens (especially testosterone). In embryos whose sex is not yet differentiated, testosterone stimulates the development of the Wolffian duct system, precursors of the male reproductive system. Later, testosterone, together with gonadotropins secreted by the pituitary gland , stimulates spermatogenesis. The Müllerian duct system, the embryonic precursors of the female genital tract, is believed to differentiate spontaneously, without the intervention of a hormonal stimulus. When the sex of the females is already defined, estrogen, which is produced in the ovaries and in thePlacenta , plays a leading role in the development and functioning of the female reproductive system.