(Redirected from Biological reproduction)
'Reproduction' is the
biological process by which new individual
organisms 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 and
asexual.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a
bacterial cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to
single-celled organisms. Most
plants 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.
Production of new individuals along a leaf margin of the air plant, ''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 articles: Asexual reproduction
Asexual reproduction is the biological 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;
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 do not have different sexes, and they are capable of "splitting" themselves into two or more individuals. Some 'asexual' species, like
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
conjugation. Other ways of asexual reproduction include
parthogenesis,
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
plants,
invertebrates (e.g.
water fleas,
aphids, some
bees and
parasitic wasps), and
vertebrates (e.g. some
reptiles,
[1]
fish,
and, very rarely,
birds
[2] and
sharks
[3]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.
Sexual reproduction
Main articles: Sexual reproduction
Sexual reproduction is a
biological process by which organisms create descendants that have a combination of
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
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, 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
animals (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
expressed, allows deleterious alleles to be masked, an advantage believed to have led to the
evolutionary development of
diploidy (Otto and Goldstein).
Allogamy
Main articles: 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
hermaphroditic organisms where the two
gametes fused in fertilization come from the same individual. They are binded 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.
'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.
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
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 normally enough individuals 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 plants. Often, they die shortly after reproduction. This is a characteristic of
K-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).
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. 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
lowest common ancestor to all present life on earth lived about
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
[4]. 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
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 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
[5]. 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.
References
1. Reptiles & Amphibians, , Tim R., Halliday, Torstar Books, , ISBN 0-920269-81-8
2. A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs
3. "Female Sharks Can Reproduce Alone, Researchers Find", Washington Post, Wednesday, May 23, 2007; Page A02
4. Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template
Scientists Create Artificial Virus
5. Williams G C. 1975. Sex and Evolution. Princeton (NJ): Princeton University Press.
★ S. P. Otto and D. B. Goldstein. "Recombination and the Evolution of Diploidy".
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.
★
★
External links
★
Asexual Reproduction
★
Human reproductive system webpage
★
Journal of Biology of Reproduction
★
Journal of Andrology
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