Vestigiality
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.[1] 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-dwelling birds; the human vomeronasal organ; and the hindlimbs of the snake and whale.
Overview
Vestigial features may take various forms; for example, they may be patterns of behavior, anatomical structures, or biochemical processes. Like most other physical features, however functional, vestigial features in a given species may successively appear, develop, and persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood.
Vestigiality, biologically speaking, refers to organisms retaining organs that have seemingly lost their original function. Vestigial organs are common evolutionary knowledge.
Similar concepts apply at the molecular level—some
Vestigial structures are often called vestigial organs, although many of them are not actually
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 penguins 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 emus 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.[5]
History
Vestigial structures have been noticed since ancient times, and the reason for their existence was long speculated upon before
Whereas useless in this circumstance, these rudiments... have not been eliminated, because Nature never works by rapid jumps, and She always leaves vestiges of an organ, even though it is completely superfluous, if that organ plays an important role in the other species of the same family.[7]
His colleague,
In the first edition of On the Origin of Species, Darwin briefly mentioned
In 1893,
Common descent and evolutionary theory
Vestigial structures are often homologous to structures that are functioning normally in other species. Therefore, vestigial structures 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 traits can be attributed to changes in the environment and behavior patterns of the organism in question. Through an examination of these various traits, it is clear that evolution had a hard role in the development of organisms. Every anatomical structure or behavior response has origins in which they were, at one time, useful. As time progressed, the ancient common ancestor organisms did as well. Evolving with time, natural selection played a huge role. More advantageous structures were selected, while others were not. With this expansion, some traits were left to the wayside. As the function of the trait is no longer beneficial for survival, the likelihood that future offspring will inherit the "normal" form of it decreases. In some cases, the structure becomes detrimental to the organism (for example the eyes of a mole can become infected[9]). In many cases the structure is of no direct harm, yet all structures require extra energy in terms of development, maintenance, and weight, and are also a risk in terms of disease (e.g., infection, cancer), providing some selective pressure for the removal of parts that do not contribute to an organism's fitness. A structure that is not harmful will take longer to be 'phased out' than one that is. However, some vestigial structures may persist due to limitations in development, such that complete loss of the structure could not occur without major alterations of the organism's developmental pattern, and such alterations would likely produce numerous negative side-effects. The toes of many animals such as horses, which stand on a single toe, are still evident in a vestigial form and may become evident, although rarely, from time to time in individuals.
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
Vestigial traits can still be considered adaptations. 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.[17]
Examples
Non-human animals
Vestigial characters are present throughout the animal kingdom, and an almost endless list could be given. Darwin said that "it would be impossible to name one of the higher animals in which some part or other is not in a rudimentary condition."[9]
The wings of ostriches, emus and other flightless birds 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.[18] The eyes of certain cavefish and salamanders 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.[19]
Boas and pythons have vestigial pelvis remnants, which are externally visible as two small pelvic spurs 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. Amphisbaenians, which independently evolved limblessness, also retain vestiges of the pelvis as well as the pectoral girdle, and have lost their right lung.[citation needed]
A case of vestigial organs was described in
In the foregoing examples the vestigiality is generally the (sometimes incidental) result of adaptive evolution. 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.[21] Many examples in many other contexts have emerged since.[22]
Humans
Human vestigiality is related to
Other structures that are vestigial include the
Humans also bear some vestigial behaviors and reflexes. The formation of
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 monkeys, and apes, and it now remains in their genomes, including the human genome, as a vestigial sequence called a pseudogene.[33]
The shift in human diet towards soft and processed food over time caused a reduction in the number of powerful grinding teeth, especially the
Plants and fungi
Plants also have vestigial parts, including functionless
Objects
Many objects in daily use contain vestigial structures. While not the result of natural selection through random mutation, much of the process is the same. Product design, like evolution, is iterative; it builds on features and processes that already exist, with limited resources available to make tweaks. To spend resources on completely weeding out a form that serves no purpose (if at the same time it is not an obstruction either) is not economically astute. These vestigial structures differ from the concept of skeuomorphism in that a skeuomorph is a design feature that has been specifically implemented as a reference to the past, enabling users to acclimatise quicker. A vestigial feature doesn’t exist intentionally, or even usefully.
For example, men’s business suits 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.[38]
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. Needless to say the gorget serves no protection to the wearer, yet there exists an unbroken lineage from the gorget to the full suits of 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.[39]
See also
- Atavism
- Dewclaw
- Exaptation
- Human vestigiality
- Maladaptation
- Plantaris muscle
- Recessive refuge
- Spandrel (biology)
- Vestigial response
References
- ISBN 978-87-7675-700-7.
- ^ Steen Hyldgaard Christensen; Bernard Delahousse; Martin Meganck, eds. (2009). Engineering in Context. Academica.
- ISBN 0-13-127384-1.
- ^ 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
- ISBN 978-0-393-30023-9.
- ^ Aristotle."History of Animals" (Book 1, Chapter 9)
- ^ 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).
- Philosophie zoologique ou exposition des considérations relatives à l'histoire naturelle des animaux.[page needed]
- ^ a b c Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection. John Murray: London.
- ^ 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".
- ^ Desmond A. & Moore, J. (1991) Darwin Penguin Books p.617 "Darwin was loathe [sic?] to let go of the notion that a well-used and strengthened organ could be inherited"
- ^ Darwin (1872) The Origin of Species, 6th Edn., p. 421
- OL 7171834M.
- ^ 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
- ^ Reeder, Alex (29 December 1997). "Evolution: Evidence from Living Organisms". Bioweb. Retrieved 2008-10-16.
- ISBN 978-0-87893-184-2.
- ^ Sober, E. (1993). Philosophy of Biology. Boulder: Westview Press. p. 84.
- ISBN 978-0-231-19036-7.
- ^ CJ van der Kooi & T Schwander 2014. On the fate of sexual traits under asexuality Biological Reviews 89:805-819
- PMID 24278118.
- ^ Morgan, Thomas Hunt & Bridges, Calvin B. (1916). Sex-linked Inheritance in Drosophila. Carnegie Institution of Washington.
- ISBN 978-0-470-38825-9
- ^ Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. John Murray: London.
- ^ "Purpose of appendix believed found". CNN/AP. 2007-10-05. Archived from the original on 2008-06-26. Retrieved 2008-10-16.
- PMID 17936308.
- PMID 8059973.
- ^ Foye, Patrick (2014). "Coccyx". Medscape.
- ISBN 978-0-8493-8811-8.
- ISBN 978-81-7041-819-1.
- ^ Saladin, Kenneth S. (2003). Anatomy & Physiology: The Unity of Form and Function (3rd ed.). McGraw-Hill. pp. 286–287.
- ^ Darwin, Charles. (1872) The Expression of the Emotions in Man and Animals John Murray, London.
- PMID 25210331.
- PMID 8175804.
- ^ "Do Any Vestigial Organs Exist in Humans? - Explainry.com". explainry.com. Archived from the original on 16 October 2018. Retrieved 14 March 2022.
- ^ Knobloch, I. (1951) "Are There Vestigial Structures in Plants?" Science New Series, Vol. 113: 465
- ^ R Ornduff (1969) Reproductive Biology in Relation to Systematics Taxon 18:121-133
- ^ CG Eckert (2002) The loss of sex in clonal plants Evolutionary Ecology 45:501-520
- ^ "Why Do Suits Have a Random Buttonhole on the Lapel? We Found Out". Gear Patrol. 2020-12-30. Retrieved 2024-03-23.
- ^ corporateName=National Museum of Australia; address=Lawson Crescent, Acton Peninsula. "National Museum of Australia - Military gorgets". www.nma.gov.au. Retrieved 2024-03-23.
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