
Reproduction (or procreation or breeding) is the
biological process by which new individual
organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known
life; each individual organism exists as the result of reproduction. 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.
[John Maynard Smith ''The Evolution of Sex'' 1978.]
Sexual reproduction typically requires the sexual interaction of two specialized organisms, called
gametes, which contain half the number of
chromosomes of normal cells and are created by
meiosis, with typically a male
fertilizing a female of the same
species to create a fertilized
zygote. This produces
offspring organisms whose genetic characteristics are derived from those of the two parental organisms.
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 by a
male. 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,
fish, and, very rarely,
birds and
sharks). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.
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). 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
syngens.
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 the combination of gametes from two parents, generally the
ovum from one individual with the
spermatozoa of another. (In isogamous species, the two gametes will not be defined as either sperm or ovum.)
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.
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 (
tetraploid to diploid to haploid), in the process forming four
haploid cells. This process occurs in two phases, meiosis I and meiosis II.
Same-sex
In recent decades, developmental biologists have been researching and developing techniques to facilitate same-sex reproduction. The obvious approaches, subject to a growing amount of activity, are
female sperm and
male eggs, with female sperm closer to being a reality for humans, given that Japanese scientists have already created female sperm for chickens. "However, the ratio of produced W chromosome-bearing (W-bearing) spermatozoa fell substantially below expectations. It is therefore concluded that most of the W-bearing PGC could not differentiate into spermatozoa because of restricted spermatogenesis."
In 2004, by altering the function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice and in 2018 Chinese scientists created 29 female mice from two female mice mothers but were unable to produce viable offspring from two father mice.
Strategies
There are 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
adulthood. For example, a
rabbit (mature after 8 months) can produce 10–30 offspring per year, and a
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. Some organisms such as honey bees and fruit flies retain sperm in a process called
sperm storage thereby increasing the duration of their fertility.
Other types
* Polycyclic animals reproduce intermittently throughout their lives.
* Semelparous organisms reproduce only once in their lifetime, such as
annual plants (including all grain crops), and certain species of salmon, spider, bamboo and century plant. Often, they die shortly after reproduction. This is often associated with
r-strategists.
* Iteroparous organisms produce offspring in successive (e.g. annual or seasonal) cycles, such as
perennial plants. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is more associated with
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.
Aphids,
slime molds,
sea anemones, some species of
starfish (by
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. The meiosis stage of the sexual cycle also allows especially effective repair of DNA damages (see
Meiosis). 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
The existence of life without reproduction is the subject of some speculation. The biological study of how the
origin of life produced reproducing organisms from non-reproducing elements 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
3.5 billion years ago.
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. However, viruses are 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
replicate with the assistance of a hijacked
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 to some degree according to current biological knowledge. A
synthetic genome has been transferred into an existing bacterium where it replaced the native DNA, resulting in the artificial production of a new ''
M. mycoides'' organism.
There is some debate within the scientific community over whether this cell can be considered completely synthetic
on the grounds that the chemically synthesized genome was an almost 1:1 copy of a naturally occurring genome and, the recipient cell was a naturally occurring bacterium. The Craig Venter Institute maintains the term "synthetic bacterial cell" but they also clarify "...we do not consider this to be "creating life from scratch" but rather we are creating new life out of already existing life using synthetic DNA". Venter plans to patent his experimental cells, stating that "they are pretty clearly human inventions".
Its creators suggests that building 'synthetic life' would allow researchers to learn about life by building it, rather than by tearing it apart. They also propose to stretch the boundaries between life and machines until the two overlap to yield "truly programmable organisms". Researchers involved stated that the creation of "true synthetic biochemical life" is relatively close in reach with current technology and cheap compared to the effort needed to place man on the Moon.
Lottery principle
Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction and diverts the organisms from other pursuits, 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.
[Williams G. C. 1975. Sex and Evolution. Princeton (NJ): Princeton University Press.] 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, producing surviving offspring. 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
*
Masting
*
Mating system
*
Modes of reproduction
*
Plant reproduction
*
Reproductive system
Notes
References
* 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.
*
*
Further reading
* Judson, Olivia (2003). ''
Dr.Tatiana's Sex Advice to All Creation: Definitive Guide to the Evolutionary Biology of Sex.''
* Richard E. Michod and Bruce E. Levin, editors (1987). ''The Evolution of Sex: An Examination of Current Ideas''. Sinauer Associates Inc., Publishers, Sunderland, MA
* Michod, R.E. (1994). ''Eros and Evolution: A natural philosophy of sex''. Addison-Wesley Publishing Company, Reading, MA
External links
Asexual ReproductionJournal of Biology of ReproductionJournal of Andrology*
"Replication and Reproduction." ''Stanford Encyclopedia of Philosophy''
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Category:Fertility