Reproduction
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{{pp-semi-vandalism|small=yes}}
{{otheruses|Reproduction (disambiguation)}}
'''Reproduction''' is the [[biological process]] by which new individual [[organism]]s are produced. Reproduction is a fundamental feature of all known [[life]]; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: [[Sexual reproduction|sexual]] and [[asexual reproduction|asexual]]. [[Human reproduction]] belongs to sexual reproduction.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a [[bacteria]]l cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to [[unicellular organism|single-celled organisms]]. Most [[plant]]s have the ability to reproduce asexually.
Sexual reproduction requires the involvement of two individuals, typically one of each [[sex]]. Normal human reproduction is a common example of sexual reproduction.
[[Image:Kalanchoe veg.jpg|thumb|350px|right|Production of new individuals along a leaf margin of the air plant, ''[[Kalanchoe pinnata|Kalanchoë pinnata]]''. The small plant in front is about 1 cm tall. The concept of "individual" is obviously stretched by this asexual reproductive process.]]
== Asexual reproduction ==
{{main|Asexual reproduction}}
Asexual reproduction is the process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. [[Bacteria]] divide asexually via [[binary fission]]; [[virus]]es take control of host cells to produce more viruses; [[Hydra (genus)|Hydras]] ([[invertebrate]]s of the [[Order (biology)|order]] ''Hydroidea'') and [[yeast]]s are able to reproduce by [[budding]]. These organisms do not have different sexes, and they are capable of "splitting" themselves into two or more individuals. Some 'asexual' species, like [[hydra (genus)|hydra]] 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 [[bacterial conjugation|conjugation]]. Other ways of asexual reproduction include [[parthogenesis]], [[Fragmentation (reproduction)|fragmentation]] and [[spore formation]] that involves only [[mitosis]]. [[Parthenogenesis]] (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is the growth and development of [[embryo]] or [[seed]] without [[fertilization]] by a [[male]]. Parthenogenesis occurs naturally in some species, including lower [[plant]]s, [[invertebrate]]s (e.g. [[water flea]]s, [[aphid]]s, some [[bee]]s and [[parasitic wasp]]s), and [[vertebrate]]s (e.g. some
[[reptile]]s,<ref name="reptiles">{{cite book
| last = Halliday
| first = Tim R.
| coauthors = Kraig Adler (eds.)
| title = Reptiles & Amphibians
| publisher = Torstar Books
|date= 1986
| pages = p. 101
| id = ISBN 0-920269-81-8 }}</ref>
[[fish]],
and, very rarely, [[bird]]s<ref>{{cite web|last = Savage
| first = Thomas F.
| title = A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs
| work = Oregon State University
|date= September 12, 2005
| url=http://oregonstate.edu/Dept/animal-sciences/poultry/index.html
| accessdate = 2006-10-11 }}</ref> and [[shark]]s<ref>[http://www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html "Female Sharks Can Reproduce Alone, Researchers Find"], Washington Post, Wednesday, May 23, 2007; Page A02</ref>). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.
== Sexual reproduction ==
{{main|Sexual reproduction}}
{{See|Human reproduction}}
[[Image:Hoverflies mating midair.jpg|thumb|250px|[[Hoverfly|Hoverflies]] mating in midair flight]]
Sexual reproduction is a [[biological process]] by which organisms create descendants that have a combination of [[genetics|genetic]] material contributed from two (usually) different members of the species. Each of two parent organisms contributes half of the offspring's genetic makeup by creating [[haploid]] [[gametes]]. Most [[organism]]s 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, but may have separable properties and then may be given other different names. For example, in the green alga, ''Chlamydomonas reinhardtii'', there are so-called "plus" and "minus" gametes. A few types of organisms, such as [[ciliates]], have more than two kinds of gametes.
Most [[animal]]s (including humans) and [[plants]] reproduce sexually. Sexually reproducing organisms have two sets of genes for every trait (called [[alleles]]). Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is [[gene expression|expressed]], allows deleterious alleles to be masked, an advantage believed to have led to the [[evolution]]ary development of [[diploidy]] (Otto and Goldstein).
===Allogamy===
{{main|Allogamy}}
Allogamy is a term used in the field of biological reproduction describing the [[fertilization]] of an [[ovum]] from one individual with the [[spermatozoa]] of another.
===Autogamy===
Self-fertilization (also known as autogamy) occurs in [[hermaphrodite|hermaphroditic]] organisms where the two [[gamete]]s fused in fertilization come from the same individual. They are bound and all the cells merge to form one new gamete.
=== Mitosis and meiosis ===
[[Mitosis]] and [[meiosis]] are an integral part 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 daughter cells is the same as that of the parent cell.
<br clear="all"><!-- Unsourced image removed: [[Image:mitosis.jpg|center]] -->
'''Meiosis'''
The resultant number of cells is four times the number of original cells. This results in cells with half the number of [[chromosomes]] present in the parent cell. A [[diploid]] cell duplicates itself, then undergoes two divisions (tetroid to diploid to haploid), in the process forming four [[haploid]] cells. This process occurs in two phases, meiosis I and meiosis II.
<br clear="all"><!-- Unsourced image removed: [[Image:meiosis.jpg|center]] -->
== Same-sex reproduction ==
In recent decades, developmental biologists have been researching and developing techniques to facilitate same-sex reproduction <ref> [http://www.samesexprocreation.com/timeline.htm] (timeline of research in the area of same-sex reproduction)</ref>. The obvious approaches, subject to a growing amount of activity, are [[female sperm]] and [[male egg]]s, with female sperm closer to being a reality for humans, given that Japanese scientists have already created female sperm for chickens. More recently, by altering the function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice.
== Reproductive strategies ==
There is a wide range of reproductive strategies employed by different species. Some animals, such as the [[human]] and [[Northern Gannet]], do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to [[adult]]hood. For example, a [[rabbit]] (mature after 8 months) can produce 10–30 offspring per year, and a [[Drosophila melanogster|fruit fly]] (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as [[K-selection]] (few offspring) and [[r-selection]] (many offspring). Which strategy is favoured by [[evolution]] depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population.
=== Other types of reproductive strategies ===
'''Polycyclic animals''' reproduce intermittently throughout their lives.
'''Semelparous organisms''' reproduce only once in their lifetime, such as [[annual plant]]s. Often, they die shortly after reproduction. This is a characteristic of [[R/K selection theory|r-strategists]].
'''Iteroparous organisms''' produce offspring in successive (e.g. annual or seasonal) cycles, such as [[perennial plant]]s. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is a characteristic of [[R/K selection theory|K-strategists]].
== Asexual vs. sexual reproduction ==
Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.
Many organisms can reproduce sexually as well as asexually. [[Aphid]]s, [[slime mold]]s, [[sea anemone]]s, some species of [[starfish]] (by [[fragmentation (biology)|fragmentation]]), and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources.
When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.
== Life without reproduction ==
The existence of life without reproduction is the subject of some speculation. The biological study of how the [[origin of life]] led from non-reproducing elements to reproducing organisms is called [[abiogenesis]]. Whether or not there were several independent abiogenetic events, biologists believe that the [[last universal ancestor]] to all present life on earth lived about [[Timeline of evolution|3.5 billion years ago]].
Today, some scientists have speculated about the possibility of creating life non-reproductively in the laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12114528&dopt=Abstract Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template]<BR>[http://atheism.about.com/b/a/042809.htm Scientists Create Artificial Virus]</ref>. The [[virus]] is often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no [[metabolism]] and can only [[self-replication|replicate]] with the assistance of a hijacked [[cell (biology)|cell]]'s metabolic machinery.
The production of a truly living organism (e.g. a simple bacterium) with no ancestors would be a much more complex task, but may well be possible according to current biological knowledge.
== Lottery principle ==
Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction, and there is some argument about why so many species use it.
[[George C. Williams]] used [[lottery]] tickets as an [[analogy]] in one explanation for the widespread use of sexual reproduction<ref>[[George C. Williams|Williams G C.]] 1975. Sex and Evolution. Princeton (NJ): Princeton University Press.</ref>. He argued that asexual reproduction, which produces little or no genetic variety in offspring, was like buying many tickets that all have the same number, limiting the chance of "winning" - that is, surviving. Sexual reproduction, he argued, was like purchasing fewer tickets but with a greater variety of numbers and therefore a greater chance of success.
The point of this analogy is that since asexual reproduction does not produce genetic variations, there is little ability to quickly adapt to a changing environment. The lottery principle is less accepted these days because of evidence that asexual reproduction is more prevalent in unstable environments, the opposite of what it predicts.
==See also==
* [[Breeding season]]
* [[Mating system]]
* [[Reproductive system]]
* [[Digital reproduction]]
== References ==
<references/>
* S. P. Otto and D. B. Goldstein. "Recombination and the Evolution of Diploidy". [[Genetics (journal)|Genetics]]. Vol 131 (1992): 745-751.
* Tobler, M. & Schlupp,I. (2005) Parasites in sexual and asexual mollies (Poecilia, Poeciliidae, Teleostei): a case for the Red Queen? Biol. Lett. 1 (2): 166-168.
* Zimmer, Carl. "Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures", New York: Touchstone, 2001.
* {{cite encyclopedia
|ency=GardenWeb Glossary of Botanical Terms
|edition=2.1
|year=2002
|article=allogamy, cross-fertilization, cross-pollination, hybridization
}}
* {{cite encyclopedia
|ency=Stedman's Online Medical Dictionary
|edition=27
|year=2004
|article=allogamy
}}
== External links ==
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Asexual Reproduction]
* [http://www.biolreprod.org/ Journal of Biology of Reproduction]
* [http://www.andrologyjournal.org/ Journal of Andrology]
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