Vestigial organ

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The human vermiform appendix is a vestigial structure; it no longer retains its original function.

In evolutionary biology and comparative anatomy, vestigiality is a term which describes homologous biological structures of organisms (such as anatomical structures (vestigial structures), behaviors and biochemical pathways) which have seemingly lost all or most of their original function in a species through evolution. These structures are typically in a degenerate, atrophied, or rudimentary condition, and are often called vestigial organs, although not all of them are actually organs.

Although structures deemed "vestigial" are identified as largely or entirely functionless, a vestigial structure may still retain lesser functions or develop new ones.[1] Thus, a "vestigial wing" is one useless for flight, but may serve some other purpose. However, care must be taken not to apply the label of vestigiality to exaptations, in which a structure originally used for one purpose is modified for a new one. For example, the wings of penguin would not be vestigial, as they have been modified for a substantial new purpose (underwater locomotion), while those of an emu would be, as they have no major purpose anymore (not even for display as in ostriches).

History

The blind mole rat (Spalax typhlus) has tiny eyes completely covered by a layer of skin.

Prior to the theory of Darwinian evolution, the reason for the existence of vestigial structures was long speculated upon, starting in the 4th century B.C.E., with Aristotle's comments, in his History of Animals, on the vestigial eyes of moles, which lack pattern recognition due to a layer of skin that completely covers them. Despite this long-held curiosity, it is only in recent centuries that anatomical vestiges have become a subject of serious study.

In 1798, Étienne Geoffroy Saint-Hilaire hypothesized that vestigial structures exist because nature leaves vestiges of an organ as long as it plays an important role in species of the same family. His colleague, Jean-Baptiste Lamarck, also researched vestigial structures, naming a number of them in his 1809 book Philosophie Zoologique. Lamarck speculated that Olivier's Spalax had lost its eyesight because it rarely came in contact with daylight, thus leaving behind vestiges of the unnecessary organs, and passing on these traits to its offspring. Thus, he advocated the once popular concept of inheritance of acquired characters, or that the individual efforts of an organism during its lifetime were the main mechanism driving species to adaptation.

Several decades later, the inheritance of acquired characters concept was largely diminished by Charles Darwin's theory of natural selection (though Darwin was loathe to abandon the concept), which used such vestigial structures as the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the eye as evidence for his theory. He also made the important distinction in The Origin of Species(1859), that if a structure had lost its primary function, but still retained secondary anatomical roles, it could still be described as vestigial.

In 1893, Robert Wiedersheim published a list of 86 human organs that, he claimed, had lost their original function. He then labeled them vestigial, theorizing that they were vestiges of evolution. Since the publication of his list, the function of some of these structures has been discovered, while other anatomical vestiges have been unearthed, resulting in the use of the list as a record of the knowledge of human anatomy at the time. Later versions of Wiedersheim's list were expanded to as many as 180 human "vestigial organs," which was later referenced in the landmark Scopes Monkey Trial by the zoologist Newman, as evidence of evolution.

Currently, the term vestigial organ is used in the media to refer to a wholly useless organ, while it retains Darwin's functional distinction in evolutionary biology. Darwin's theory of natural selection remains the most widely accepted scientific theory regarding vestigial organs, but is still questioned by creationists.

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Evidence of evolution

Further information: Evidence of evolution

The fact that vestigial structures are often homologous to functioning structures in other species can be considered evidence for evolution, the process by which beneficial, heritable traits arise in populations over an extended period of time. The existence of vestigial organs can be attributed to changes in the environment and behavior patterns of the species in question. As the function of the structure is no longer beneficial for survival, the likelihood that future offspring will inherit the "normal" form of the structure decreases. In some cases the structure becomes detrimental to the organism (for example the eyes of a mole can become infected[6]). In many cases, while the structure may not be directly harmful, all structures require extra energy to build and maintain, providing some selective pressure for the removal of parts that do not contribute to an organism's fitness. A structure that is not harmful is theorized to take longer to be phased out by natural selection than one that is.

The vestigial versions of the structure can be compared to the original version of the structure in other species in order to determine the homology of a vestigial structure. Evolution theory states that homologous structures indicate common ancestry with those organisms that have a functional version of the structure.[8]

Examples

Animals

Letter c in the picture indicates the undeveloped hind legs of a baleen whale.

There are numerous examples of vestigial organs in the animal kingdom. In whales and other cetaceans, one can find small vestigial leg bones deeply buried within the back of the body. The evolutionary explanation is that these bones are the remnants of their land dwelling ancestor's legs. Many whales also have undeveloped, unused, pelvis bones in the anterior part of their torsos, also remains from their land dwelling ancestors.

Several flightless birds, including Dodo birds (now extinct) and penguins, have hollow bones, a feature usually reserved for flying birds (since the weight reduction is crucial to staying in the air). Thus, these hollow bones are explained by evolution to be the remnants of a flying ancestor. The wings of emus, and some other flightless birds, like the dodo, are often identified as vestigial as well, much like the hollow bones, however there is a difference with the penguin. While penguin's wings are not used for flight, they are essential for the penguin to be able to navigate underwater (they essentially act as flippers). Thus, penguin wings are often not identified as vestigial, since they serve an important function, and are thought of as an adaptation instead.

The eyes of certain cavefish, like the blind fish Astyanax Mexicanus, are also vestigial, presumably as a result of living in total darkness. While the fish are developing in their eggs, their newly formed eyes start to degenerate, until the fish is left with the collapsed remnant of an eye, covered by a flap of skin. These are thought to be the remnant of their ancestors' functional eyes.

The modern day duck billed platypus is said to have vestigial teeth, which they quickly lose in early life, resulting in gums that are more suited for scooping crustaceans and worms out of riverbeds. While fossil records of prehistoric platypus ancestors are not extensive, the fossils that have been found show platypus-like creatures with rows of teeth, thus adding to evolutionists' arguments.

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Humans
The muscles connected to the ears of a human do not develop enough to have the same mobility allowed to monkeys.

Although the list of human vestigial structures is still essentially the same, the relative usefulness of certain structures on the list is a subject of debate. The following are some of the structures often included in this list:

The vermiform appendix is a vestige of the cecum, an organ that, according to evolutionists, may have been used to digest cellulose by humans' herbivorous ancestors. Analogous organs in other animals similar to humans continue to perform that function, whereas other meat-eating animals may have similarly diminished appendices. The modern functionality of the appendix is still controversial in the field of human physiology, although it is now believed to act as a storehouse for good bacteria. Thus, when good bacteria that assist in digestion are purged by disease or some other cause, the good bacteria are thought to be replenished by the appendix.

The coccyx, or tailbone, is thought to be the remnant of a lost ancestral tail, selected out as humans began to walk increasingly upright. All mammals have a tail at one point in their development; in humans, it is present for a short time during embryonic development. The tailbone, located at the end of the spine, has lost its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles, which explains why it has not degraded further. In rare cases it can persist after birth and must be surgically removed.

The plica semilunaris is the small, pink fold of tissue on the inside corner of the eye. It is claimed to be the vestigial remnant of the nictitating membrane (or the clear "third eyelid") which is fully present in other animals, and which can be controlled to protect the eye from debris and moisture loss, while still allowing the animal to see. It forms while a human is an embryo, but then declines afterwards until it is only present in the corner of the eye. Whether or not this structure is vestigial is a matter of popular controversy, partially due to the interpretation of the word vestigial to mean useless. This is because the plica semilunaris in humans currently serves the important purpose of cleaning out dust and other particles from the eye, resulting in a common crusted mucus. However, evolutionists claim that it evolved from the nictitating membrane found in other animals, and is thus vestigial.

Wisdom teeth are vestigial third molars that human ancestors used to help in grinding down plant tissue. It has been postulated that the skulls of human ancestors had larger jaws with more teeth, which were possibly used to help chew down foliage to compensate for a lack of ability to efficiently digest the cellulose that makes up a plant cell wall. As human diet changed, it is thought that a smaller jaw was selected by evolution, yet the third molars, or "wisdom teeth," still commonly develop in human mouths.

Goose bumps are an example of a vestigial human reaction to stress.

Evolutionists also state that humans bear some vestigial behaviors and reflexes. For example, the formation of goose bumps in humans under stress is a vestigial reflex; its purpose in human evolutionary ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold is not vestigial in humans, but the reflex to form them under stress is a function that humans lack.

The ears of a Macaque monkey, and most other monkeys, have muscles that are more developed than those of humans and therefore have the capability to move their ears to better hear potential threats. In humans, the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys. A function once provided by one structure is now replaced by another, which evolutionists classify as vestigial.

Delving deeper into vestigiality arguments, there are also thought to be vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is L-gulono-gamma-lactone oxidase, a gene, found functional in most other mammals, which produces a Vitamin C-catalyzing enzyme. In humans, an earlier mutation may have caused it to become disabled (unable to produce the enzyme), and it now remains in the human genome only as a vestigial genetic sequence.


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Plants

Vestigial structures are not only found in animals; plants are also said to have vestigial parts. Dandelions and other asexually reproducing plants retain flowers, and produce pollen, both of which are only necessary for the process of sexual reproduction.

Grass often has small, undeveloped structures which strongly resemble those of flowering plants.

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Controversy

Further information: Creationism

Because vestigial organs are used as supporting evidence for evolution, some creationists oppose the validity of the idea. They question whether these organs are useless, since they believe that gods or God gave each organism its organs for a specific reason and use. Other creationists allow for "micro-evolution" (variations in individual species) but not "macro-evolution" (all species originating from common ancestors). Vestigial structures do not oppose the beliefs of this branch of creationism.

Those who question the existence of vestigial organs usually claim a different definition for vestigial, giving a strict interpretation that an organ must be utterly useless to qualify.[16] This is a definition often used in dictionaries[17] and children's encyclopedias.[18] Biology textbooks[19][20] and scientific encyclopedias[1] usually describe an organ as vestigial if it does not serve the same function in the modern animal as the cognate organ served in an ancestor, even if the modern organ serves a completely different use (preadaptation).

Those who consider the true meaning of vestigial to be "completely without use" tend to claim that the meaning has been changed over time as structures thought to be vestigial were found to have other uses.[21] However, documentation indicates that from the theory's beginnings in the 19th century, vestigial structures have invariably been understood to "sometimes retain their potentiality"[6], becoming either "wholly or in part functionless".[22] It was thought that "not infrequently the degenerating organ can be turned to account in some other way".[23]

An example of the dispute is the gas bladder of many fish, which is thought to be a vestigial lung, "left over" from the occasionally-air-gasping common ancestor of ray-finned fish and land vertebrates.

See also

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References
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  1. 1.0 1.1 Muller, G. B. (2002) "Vestigial Organs and Structures." In M. Pagel, eds., Encyclopedia of Evolution. New York: Oxford University Press. 1131-1133. ISBN: 0195122003 Cite error: Invalid <ref> tag; name "Muller" defined multiple times with different content
  2. Creation Insights: Evolution Hall of Shame
  3. St. Hilaire, Geoffroy (1798). "Observations sur l'aile de l'Autruche, par le citoyen Geoffroy," La Decade Egyptienne, Journal Litteraire et D'Economie Politique 1 (pp. 46–51).
  4. Aristotle."History of Animals" (Book 1, Chapter 9)
  5. Lamarck, Jean-Baptiste (1809). Philosophie zoologique ou exposition des considérations relatives à l'histoire naturelle des animaux.
  6. 6.0 6.1 6.2 Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection. John Murray: London.
  7. Wiedersheim, Robert (1893). The Structure of Man: An Index to His Past History. London: Macmillan and Co.
  8. Reeder, Alex. "Evolution." Bioweb. 29 Dec 1997. 8 Jun 2006 <http://bioweb.cs.earlham.edu/9-12/evolution/HTML/live.html>.
  9. http://www.livescience.com/animals/top10_vestigial_organs-1.html
  10. http://findarticles.com/p/articles/mi_m1511/is_n1_v14/ai_13670058
  11. Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. John Murray: London.
  12. Johnson, Dr. George B.. "Evidence for Evolution (Page 12)." Txtwriter Inc.. 8 Jun 2006 <http://www.txtwriter.com/backgrounders/Evolution/EVpage12.html>.
  13. Prof. A. Macalister, Annals and Magazine of Natural History, vol. vii., 1871, p. 342.
  14. Mr. St. George Mivart, Elementary Anatomy, 1873, p. 396.
  15. Knobloch, I. (1951) Are There Vestigial Structures in Plants? Science New Series, Vol. 113: 465
  16. Bergman, J. and Howe, G. (1990) "Vestigial Organs" Are Fully Functional. Kansas City, MO. Creation Research Society Books.
  17. New Shorter Oxford English Dictionary fourth edition (1993)
  18. World Book Encyclopedia 2000
  19. Futuyma DJ (1998) Evolutionary Biology 3rd edition. Sinauer Associates, Inc (Sunderland, MA)
  20. Freeman S & Herron JC (2004) Evolutionary Analysis 3rd edition. p.30 Pearson Prentice Hall (Upper Saddle River, NJ)
  21. Sarfati J (2002) "AiG misunderstands evolution?" Answers In Genesis Feedback Response. June 3 [1](accessed 8th June 2006)
  22. Wiedersheim R (1893) The Structure of Man: An Index to His Past History Second Edition. Translated by H. and M. Bernard (1895). Macmillan and Co. (London)
  23. Weismann, A. (1886) "IX. Retrogressive Development in Nature." reproduced in Essays upon Heredity and Kindred Biological Problems. Volume II. pp. 5-9 Poulton, E. B. and Shipley, A. E., editors., Clarendon Press: Oxford. 1892.

External links

Basic topics in evolutionary biology (edit)
Processes of evolution: evidence - macroevolution - microevolution - speciation
Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity
Modes: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics

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