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Vestigiality refers to genetically determined structures or attributes that have apparently lost most or all of their ancestral function in a given species, but have been retained during the process of evolution. 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. Typical examples of both types occur in the loss of flying capability in island-dwelling species. ==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, which have seemingly lost the entirety of the original function. The issue is controversial and not without dispute; nonetheless, 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 — though vestigial in the sense of retaining no significant ''digestive'' function, the appendix still has immunological roles and is useful in maintaining gut flora. 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 pseudogenes 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. ISBN 0-13-127384-1〕 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 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 judgements 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vestigiality」の詳細全文を読む スポンサード リンク
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