For other uses, see Reproduction (disambiguation)
Biological 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 by an antecedent. The known methods of reproduction are broadly grouped into two main types: sexual and asexual reproduction.
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. In general, more-complex organisms reproduce sexually while simpler, usually unicellular, organisms reproduce asexually.
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 as well reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include fragmentation and spore formation that involves only mitosis.
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.
Humans, most animals, 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).
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 cell 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 forms two 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 (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 a large number of offspring. On the other hand, animals with many offspring may devote less resources to each individual offspring; for these types of animals it is common for a large number of offspring to die soon after birth, but normally enough individuals survive to maintain the population.
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 amount 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 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.
The Red Queen hypothesis
Sexual reproduction is best known for providing means of adaptation to an ever-changing environment by adding phenotypic variance to the population. The variation produced by sexual reproduction is not only outward (such as the advent of horns or fusion of digits) but also physiological. One hypothesis termed the Red Queen Hypothesis states that sexual reproduction leading to variation is as much an evolutionary arms race against parasites as it is ammunition against environmental factors. Variation that arises from sexual reproduction makes it harder for parasites to "get accustomed" to their hosts, ultimately making them less efficient. However, the idea of the hypothesis is that parasites continue to evolve as well, making both species continuously change but stay in the same place relative to each other. Organisms that facultatively reproduce asexually or sexually usually enter their sexual reproduction phase when pathogens become prevalent in the population.3
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 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. One group of scientists has succeeded in producing a simple virus from entirely non-living materials. The production of a truly living organism, such as a simple bacterium, with no ancestors would be a much more complex task, but may well be possible according to current understanding of biology.
Finally, life without reproduction is a feature of many religious Creation myths. The biblical Adam, for example, was created by God and had no ancestors.
Self-replication
A major goal in the field of robots is the self-replication of machines. Since all robots (at least in modern times) have a fair number of the same features, a self-replicating robot (or possibly a hive of robots) would need to do the following:
- Obtain construction materials
- Manufacture new parts
- Provide a consistent power source
- Program the new members
To date, this has not been done.
On a nanotechnical scale, nanomachines might also be designed to reproduce under their own power. This, in turn, has given rise to the "gray goo" theory of Armaggedon, as featured in such science fiction novels as Bloom and Prey.
For a detailed article on mechanical reproduction as it relates to the industrial age see mass production.
See also
- Lottery principle -- The idea that sexual reproduction is adaptive because it produces greater diversity.
- Parthenogenesis -- (from the Greek παρθενος, "virgin", + γενεσις, "birth") means the growth and development of an embryo or seed without fertilization by a male.
References
- S. P. Otto and D. B. Goldstein. "Recombination and the Evolution of Diploidy". Genetics. Vol 131 (1992): 745-751.
- Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2003.
- Zimmer, Carl. "Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures", New York: Touchstone, 2001.