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Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, and its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species.
Examples of vestigial structures (also called degenerate, atrophied, or rudimentary organs) are the loss of functional wings in Island ecology birds; the human vomeronasal organ; and the hindlimbs of the snake and whale.
Vestigiality, biologically speaking, refers to organisms retaining organs that have seemingly lost their original function. Vestigial organs are common evolutionary knowledge. In addition, the term vestigiality is useful in referring to many genetically determined features, either morphological, behavioral, or physiological; in any such context, however, it need not follow that a vestigial feature must be completely useless. A classic example at the level of gross anatomy is the human vermiform appendix, vestigial in the sense of retaining no significant digestive function.
Similar concepts apply at the molecular level—some nucleic acid sequences in eukaryotic genomes have no known biological function; some of them may be "junk DNA", but it is a difficult matter to demonstrate that a particular sequence in a particular region of a given genome is truly nonfunctional. The simple fact that it is noncoding DNA does not establish that it is functionless. Furthermore, even if an extant DNA sequence is functionless, it does not follow that it has descended from an ancestral sequence of functional DNA. Logically such DNA would not be vestigial in the sense of being the vestige of a functional structure. In contrast have lost their protein-coding ability or are otherwise no longer expressed in the cell. Whether they have any extant function or not, they have lost their former function and in that sense, they do fit the definition of vestigiality.
Vestigial structures are often called vestigial organs, although many of them are not actually organs. Such vestigial structures typically are degenerate, atrophied, or rudimentary,Lawrence, Eleanor (2005) Henderson's Dictionary of Biology. Pearson, Prentice Hall. . and tend to be much more variable than homologous non-vestigial parts. Although structures commonly regarded "vestigial" may have lost some or all of the functional roles that they had played in ancestral organisms, such structures may retain lesser functions or may have become adapted to new roles in extant populations.Muller, G. B. (2002) "Vestigial Organs and Structures". in Encyclopedia of Evolution. Mark Pagel, editor in chief, New York: Oxford University Press. pp. 1131–1133
It is important to avoid confusion of the concept of vestigiality with that of exaptation. Both may occur together in the same example, depending on the relevant point of view. In exaptation, a structure originally used for one purpose is modified for a new one. For example, the wings of would be exaptational in the sense of serving a substantial new purpose (underwater locomotion), but might still be regarded as vestigial in the sense of having lost the function of flight. In contrast Darwin argued that the wings of would be definitely vestigial, as they appear to have no major extant function; however, function is a matter of degree, so judgments on what is a "major" function are arbitrary; the emu does seem to use its wings as organs of balance in running. Similarly, the ostrich uses its wings in displays and temperature control, though they are undoubtedly vestigial as structures for flight.
Vestigial characters range from detrimental through neutral to favorable in terms of selection. Some may be of some limited utility to an organism but still degenerate over time if they do not confer a significant enough advantage in terms of fitness to avoid the effects of genetic drift or competing selective pressures. Vestigiality in its various forms presents many examples of evidence for biological evolution. (1980). 9780393300239, W. W. Norton & Company. ISBN 9780393300239
His colleague, Jean-Baptiste Lamarck, named a number of vestigial structures in his 1809 book Philosophie Zoologique. Lamarck noted "Olivier's Spalax, which lives underground like the mole, and is apparently exposed to daylight even less than the mole, has altogether lost the use of sight: so that it shows nothing more than vestiges of this organ."Lamarck, Jean-Baptiste (1809). Philosophie zoologique ou exposition des considérations relatives à l'histoire naturelle des animaux.
Charles Darwin was familiar with the concept of vestigial structures, though the term for them did not yet exist. He listed a number of them in The Descent of Man, including the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the human eye. Darwin also noted, in On the Origin of Species, that a vestigial structure could be useless for its primary function, but still retain secondary anatomical roles: "An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other.... An organ may become rudimentary for its proper purpose, and be used for a distinct object."Charles Darwin (1859). On the Origin of Species by Means of Natural Selection. John Murray: London.
In the first edition of On the Origin of Species, Darwin briefly mentioned inheritance of acquired characters under the heading " Effects of Use and Disuse", expressing little doubt that use "strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited".Darwin, 1859, pp. 134–139. Barrett P. H. et al. 1981, A concordance to Darwin's Origin of Species first edition, Cornell, Ithaca, and London, lists only four mentions of the phrase "use and disuse". In later editions he expanded his thoughts on this,Adrian Desmond & Moore, J. (1991) Darwin Penguin Books p.617 "Darwin was to let go of the notion that a well-used and strengthened organ could be inherited" and in the final chapter of the 6th edition concluded that species have been modified "chiefly through the natural selection of numerous successive, slight, favorable variations; aided in an important manner by the inherited effects of the use and disuse of parts".Darwin (1872) The Origin of Species, 6th Edn., p. 421
In 1893, Robert Wiedersheim published The Structure of Man, a book on human anatomy and its relevance to man's evolutionary history. The Structure of Man contained a list of 86 human organs that Wiedersheim described as, "Organs having become wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly. For the greater part Organs which may be rightly termed Vestigial." Since his time, the function of some of these structures have been discovered, while other anatomical vestiges have been unearthed, making the list primarily of interest 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". This is why the zoologist Horatio Newman said in a written statement read into evidence in the Scopes Trial that "There are, according to Wiedersheim, no less than 180 vestigial structures in the human body, sufficient to make of a man a veritable walking museum of antiquities."Darrow, Clarence and William J. Bryan. (1997). The World's Most Famous Court Trial: The Tennessee Evolution Case Pub. The Lawbook Exchange, Ltd. p. 268
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. Homologous structures indicate common descent with those organisms that have a functional version of the structure. Douglas Futuyma has stated that vestigial structures make no sense without evolution, just as spelling and usage of many modern English language words can only be explained by their Latin or Old Norse antecedents.
Vestigial traits can still be considered . This is because an adaptation is often defined as a trait that has been favored by natural selection. Adaptations, therefore, need not be adaptive, as long as they were at some point.
The wings of , and other are vestigial; they are remnants of their flying ancestors' wings. These birds go through the effort of developing wings, even though most birds are too large to use the wings successfully. Seeing vestigial wings in birds is also common when they no longer need to fly to escape predators, such as birds on the Galapagos Islands. (2025). 9780231190367, Columbia University Press. ISBN 9780231190367 The eyes of certain cavefish and are vestigial, as they no longer allow the organism to see, and are remnants of their ancestors' functional eyes. Animals that reproduce without sex (via asexual reproduction) generally lose their sexual traits, such as the ability to locate/recognize the opposite sex and copulation behavior.CJ van der Kooi & T Schwander 2014. On the fate of sexual traits under asexuality Biological Reviews 89:805-819
Boidae and Pythonidae have vestigial pelvis remnants, which are externally visible as two small on each side of the cloaca. These spurs are sometimes used in copulation, but are not essential, as no colubrid snake (the vast majority of species) possesses these remnants. Furthermore, in most snakes, the left lung is greatly reduced or absent. , which independently evolved limblessness, also retain vestiges of the pelvis as well as the pectoral girdle, and have lost their right lung.
A case of vestigial organs was described in polyopisthocotylean (parasite ). These parasites usually have a posterior attachment organ with several clamps, which are sclerotised organs attaching the worm to the gill of the host fish. These clamps are extremely important for the survival of the parasite. In the family Protomicrocotylidae, species have either normal clamps, simplified clamps, or no clamps at all (in the genus Lethacotyle). After a comparative study of the relative surface of clamps in more than 100 , this has been interpreted as an evolutionary sequence leading to the loss of clamps. Coincidentally, other attachment structures (lateral flaps, transverse striations) have evolved in protomicrocotylids. Therefore, clamps in protomicrocotylids were considered vestigial organs.
In the foregoing examples the vestigiality is generally the (sometimes incidental) result of Adaptation. However, there are many examples of vestigiality as the product of drastic mutation, and such vestigiality is usually harmful or counter-adaptive. One of the earliest documented examples was that of vestigial wings in Drosophila. Many examples in many other contexts have emerged since.Snustad, D. Peter. & Simmons, Michael J. Principles of Genetics. Publisher: Wiley, 2008.
Other structures that are vestigial include the plica semilunaris on the inside corner of the Human eye (a remnant of the nictitating membrane); and (as seen at right) muscles in the ear. Other organic structures (such as the occipitofrontalis muscle) have lost their original functions (to keep the head from falling) but are still useful for other purposes (facial expression).
Humans also bear some vestigial behaviors and reflexes. The formation of goose bumps in humans under stress is a vestigial reflex action;Darwin, Charles. (1872) The Expression of the Emotions in Man and Animals John Murray, London. its function in human ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. The arrector pili (muscle that connects the hair follicle to connective tissue) contracts and creates goosebumps on skin.
There are also vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is a gene that is functional in most other mammals and which produces L-gulonolactone oxidase, an enzyme that can make vitamin C. A documented mutation deactivated the gene in an ancestor of the modern infraorder of Simian, and it now remains in their , including the human genome, as a vestigial sequence called a pseudogene.
The shift in human diet towards soft and processed food over time caused a reduction in the number of powerful grinding teeth, especially the Wisdom tooth (also known as wisdom teeth), which were highly prone to impaction.
For example, Suit often contain a row of buttons at the bottom of the sleeve. These used to serve a purpose, allowing the sleeve to be split and rolled up. The feature has been lost entirely, though most suits still give the impression that it is possible, complete with fake button holes. There is also an example of exaptation to be found in the business suit: it was previously possible to button a jacket up all the way to the top. As it became the fashion to fold the lapel over, the top half of buttons and their accompanying buttonholes disappeared, save for a single hole at the top; it has since found a new use as a place to fasten pins, badges, or boutonnières.
As a final example, soldiers in ceremonial or parade uniform can sometimes be seen wearing a gorget: a small decorative piece of metal suspended around the neck with a chain. The gorget serves no protection to the wearer, yet there exists an unbroken lineage from the gorget to the full Plate armour of the middle ages. With the introduction of gunpowder weapons, armour increasingly lost its usefulness on the battlefield. At the same time, military men were keen to retain the status it provided them. The result: a breastplate that "shrank" away over time, but never disappeared completely.
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