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Sexual reproduction

Sexual reproduction involves two individuals, one male and one female, which produce specialized cells, called sex cells or gametes , which, when united in fertilization , give rise to a new individual.

Gametes (eggs and spermare produced in the gonads (ovaries and testes), and although they contain genetic material from the individual, they are all different. Furthermore, when uniting with another gamete, the resulting being will not be identical to either of its parents. Thus, the greater the variety among individuals, the easier it will be for some to survive in very adverse conditions.

The mechanism of cell reproduction is by meiosis, in which a cell (diploid, with a normal number of chromosomes) gives rise to four cells with half the number of chromosomes (haploid). When the two gametes, which are haploid, unite, they will give rise to an egg cell or zygote (diploid, with a normal number of chromosomes).

Two individuals are involved in sexual reproduction, although two situations can occur:

  • There are species in which the same individual has both sexes, they are hermaphrodite species. The same individual produces male and female gametes, although it cannot self-fertilize. This ensures genetic variability. Some annelids are hermaphrodites.
  • There are species in which individuals differ in two sexes. Male individuals produce sperm and female individuals produce eggs.

Sexual reproduction has advantages and disadvantages:

  • Advantages of sexual reproduction. The offspring have paternal and maternal characteristics, so the diversity is greater. If the environmental conditions are adverse, probably one is better adapted to those conditions and can survive and maintain the species.
  • Disadvantages of sexual reproduction. Since two individuals are required for sexual reproduction to occur, it may not always be easy for an encounter to occur.

For sexual reproduction to occur, fertilization must occur when two gametes (an egg and a sperm) unite.

Fertilization can be:

  • External fertilization. Gamete union occurs in water, outside the female's body. For example, it occurs in many groups of fish. The female expels the unfertilized ovules to the outside, and the male expels the sperm, fertilizing the ovules outside the mother's body.
  • internal fertilization. Fertilization occurs inside the female's body. Copulatory organs develop that allow gametes to meet. For example, in mammals.

The development of the embryo can also vary from one animal to another. Several types of embryonic development are distinguished :

  • Oviparous. The new being develops inside an egg, outside the mother's body. For example, in all groups of animals except mammals.

Las tortugas son ovíparas

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  • Viviparous. The new being develops within the mother's body. For example, in mammals.
  • Ovoviviparous. The new being develops inside an egg, but inside the mother's body until the embryo is fully developed. The hatching of the egg occurs immediately before parturition or immediately after laying. For example, many sharks, fish, and some reptiles.

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