Reproduction (or procreation or breeding) is the biological process by which new individual organisms – offspring – are produced from their parent or parents. There are two forms of reproduction: asexual and sexual. In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms. The cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically similar or identical copy of itself. The evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce, and organisms only pass on 50% of their genes. Sexual reproduction typically requires the sexual interaction of two specialized reproductive cells, called gametes, which contain half the number of chromosomes of somatic cells and are created by meiosis, with typically a sperm cell fertilizing an egg cell of the same species to create a fertilized zygote. This produces offspring organisms whose genetic characteristics are derived from both parental organisms.
Methods
Asexual
Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order Hydroidea) and yeasts are able to reproduce by budding. These organisms often do not possess different sexes, and they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce entirely by asexual means. Some species that are capable of reproducing asexually, like hydra, yeast (See Mating of yeasts) and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction – reproduction without seeds or spores – but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthenogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis is the growth and development of embryo or seed without fertilization. Parthenogenesis occurs naturally in some species, including lower plants (where it is called apomixis), invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles, some fish, and very rarely, domestic birds).
Sexual
Sexual reproduction is a biological process that creates a new organism by combining the genetic material of two organisms in a process that starts with meiosis, a specialized type of cell division. Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as male (producing sperm or microspores) and female (producing ova or megaspores). In isogamous species, the gametes are similar or identical in form (isogametes), but may have separable properties and then may be given other different names (see isogamy). Because both gametes look alike, they generally cannot be classified as male or female. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and the ciliate Paramecium aurelia, have more than two "sexes", called mating types. Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait (called alleles). Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes. It is believed that "the masking of deleterious alleles favors the evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely. Bryophytes reproduce sexually, but the larger and commonly-seen organisms are haploid and produce gametes. The gametes fuse to form a zygote which develops into a sporangium, which in turn produces haploid spores. The diploid stage is relatively small and short-lived compared to the haploid stage, i.e. haploid dominance. The advantage of diploidy, heterosis, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis. This may be an indication that the sexual reproduction has advantages other than heterosis, such as genetic recombination between members of the species, allowing the expression of a wider range of traits and thus making the population more able to survive environmental variation.
Allogamy
Allogamy is the fertilization of flowers through cross-pollination, this occurs when a flower's ovum is fertilized by spermatozoa from the pollen of a different plant's flower. Pollen may be transferred through pollen vectors or abiotic carriers such as wind. Fertilization begins when the pollen is brought to a female gamete through the pollen tube. Allogamy is also known as cross fertilization, in contrast to autogamy or geitonogamy which are methods of self-fertilization.
Autogamy
Self-fertilization, also known as autogamy, occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual, e.g., many vascular plants, some foraminiferans, some ciliates. The term "autogamy" is sometimes substituted for autogamous pollination (not necessarily leading to successful fertilization) and describes self-pollination within the same flower, distinguished from geitonogamous pollination, transfer of pollen to a different flower on the same flowering plant, or within a single monoecious gymnosperm plant.
Mitosis and meiosis Mitosis and meiosis are types of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes. Mitosis The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the offspring cells is the same as that of the parent cell.
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