Reproduction
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 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 a characteristic of 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. 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 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[5]. 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[6]. 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
References
- ^ Halliday, Tim R.; Kraig Adler (eds.) (1986). Reptiles & Amphibians. Torstar Books, p. 101. ISBN 0-920269-81-8.
- ^ Savage, Thomas F. (September 12, 2005). A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs. Oregon State University. Retrieved on 2006-10-11.
- ^ "Female Sharks Can Reproduce Alone, Researchers Find", Washington Post, Wednesday, May 23, 2007; Page A02
- ^ [1] (timeline of research in the area of same-sex reproduction)
- ^ Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template
Scientists Create Artificial Virus - ^ 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.
- "allogamy, cross-fertilization, cross-pollination, hybridization". GardenWeb Glossary of Botanical Terms (2.1). (2002).
- "allogamy". Stedman's Online Medical Dictionary (27). (2004).
External links
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