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Devolution, de-evolution, or backward evolution is the notion that species can change into more "primitive" forms over time. In modern biology the term is redundant: evolutionary science deals with selection or adaptation that results in populations of organisms genetically different from their ancestral forms. The discipline makes no general distinction between changes leading to populations of forms less complex or more complex than their ancestors, and in such terms the concept of a primitive species cannot be defined consistently. Consequently, within the discipline such a word is rarely useful. Current non-technical application of the concept of "devolution" is based largely on the fallacies that: * in biology there is a preferred hierarchy of structure and function, and that * evolution must mean "progress" to "more advanced" organisms with more complex structure and function. Those errors in turn are related to two misconceptions: that: * evolution is supposed to make species more "advanced", as opposed to "primitive"; and that * modern species that have lost some of the functions or complexity of their ancestors must accordingly be degenerate forms. (Note however that degeneracy in this context has little to do with the current technical use of the term degeneracy in biology). ==Concepts underlying ideas of devolution== The idea of de-evolution is based at least partly on the presumption that "evolution" requires some sort of purposeful direction towards "increasing complexity". Modern evolutionary theory, beginning with Darwin at least, poses no such presumption and the concept of evolutionary change is independent of either any increase in complexity of organisms sharing a gene pool, or any decrease, such as in vestigiality or in loss of genes.〔Michael J. Dougherty. (Is the human race evolving or devolving? ) ''Scientific American'' July 20, 1998.〕 Earlier views that species are subject to "cultural decay", "drives to perfection", or "devolution" are practically meaningless in terms of current (neo-)Darwinian theory. Early scientific theories of transmutation of species such as Lamarckism and orthogenesis perceived species diversity as a result of a purposeful internal drive or tendency to form improved adaptations to the environment. In contrast, Darwinian evolution and its elaboration in the light of subsequent advances in biological research, have shown that adaptation through natural selection comes about when particular heritable attributes in a population happen to give a better chance of successful reproduction in the reigning environment than rival attributes do. By the same process less advantageous attributes are less "successful"; they decrease in frequency or are lost completely. Since Darwin's time it has been shown how these changes in the frequencies of attributes occur according to the mechanisms of genetics and the laws of inheritance originally investigated by Gregor Mendel. Combined with Darwin's original insights, genetic advances led to what has variously been called the modern evolutionary synthesis or neo-Darwinism. In these terms evolutionary adaptation may occur most obviously through the natural selection of particular alleles. Such alleles may be long established, or they may be new mutations. Selection also might arise from more complex epigenetic or other chromosomal changes, but the fundamental requirement is that any adaptive effect must be heritable. The concept of devolution on the other hand, requires that there be a preferred hierarchy of structure and function, and that evolution must mean "progress" to "more advanced" organisms. For example, it could be said that "feet are better than hooves" or "lungs are better than gills", so their development is "evolutionary" whereas change to an inferior or "less advanced" structure would be called "devolution". In reality an evolutionary biologist defines all heritable changes to relative frequencies of the genes or indeed to epigenetic states in the gene pool as evolution. All gene pool changes that lead to increased fitness in terms of appropriate aspects of reproduction are seen as (neo-)Darwinian adaptation because, for the organisms possessing the changed structures, each is a useful adaptation to their circumstances. For example, hooves have advantages for running quickly on plains, which benefits horses, and feet offer advantages in climbing trees, which some ancestors of humans did.〔 The concept of devolution as regress from progress relates to the ancient ideas that either life came into being through special creation or that humans are the ultimate product or goal of evolution. The latter belief is related to anthropocentrism, the idea that human existence is the point of all universal existence. Such thinking can lead on to the idea that species evolve because they "need to" in order to adapt to environmental changes. Biologists refer to this misconception as teleology, the idea of intrinsic finality that things are "supposed" to be and behave a certain way, and naturally tend to act that way to pursue their own good. From a biological viewpoint, in contrast, if species evolve it is not a reaction to necessity, but rather that the population contains variations with traits that favour their natural selection. This view is supported by the fossil record which demonstrates that roughly ninety-nine percent of all species that ever lived are now extinct.〔 People thinking in terms of devolution commonly assume that progress is shown by increasing complexity, but biologists studying the evolution of complexity find evidence of many examples of decreasing complexity in the record of evolution. The lower jaw in fish, reptiles and mammals has seen a decrease in complexity, if measured by the number of bones. Ancestors of modern horses had several toes on each foot; modern horses have a single hooved toe. Modern humans may be evolving towards never having wisdom teeth, and already have lost most of the tail found in many other mammals - not to mention other vestigial structures, such as the vermiform appendix or the nictitating membrane.〔 In some cases, the level of organization of living creatures can also “shift” downwards (e.g., the loss of multicellularity in some groups of protists, animals and fungi).〔Seravin L. N. (2001) The principle of counter-directional morphological evolution and its significance for constructing the megasystem of protists and other eukaryotes. ''Protistology'' 2: 6-14, ().〕 A more rational version of the concept of devolution, a version that does not involve concepts of "primitive" or "advanced" organisms, is based on the observation that if certain genetic changes in a particular combination (sometimes in a particular sequence as well) are precisely reversed, one should get precise reversal of the evolutionary process, yielding an atavism or "throwback", whether more or less complex than the ancestors where the process began.〔Medawar, P. B., Medawar, J. S.; Aristotle to Zoos: A Philosophical Dictionary of Biology; Harvard University Press 1985, ISBN 978-0-674-04537-8〕 At a trivial level, where just one or a few mutations are involved, selection pressure in one direction can have one effect, which can be reversed by new patterns of selection when conditions change. That could be seen as reversed evolution, though the concept is of not much interest because it does not differ in any functional or effective way from any other adaptation to selection pressures.〔Majerus, Michael E. N. (ed:Fellowes, M, et al.); The Peppered Moth: Decline of a Darwinian Disciple; Insect Evolutionary Ecology (Royal Entomological Society) CABI 2005 ISBN 978-0-85199-812-1〕 As the number of genetic changes rises however, one combinatorial effect is that it becomes vanishingly unlikely that the full course of adaptation can be reversed precisely. Also, if one of the original adaptations involved complete loss of a gene, one can neglect any probability of reversal. Accordingly, one might well expect reversal of peppered moth colour changes, but not reversal of the loss of limbs in snakes. 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「Devolution (biology)」の詳細全文を読む スポンサード リンク
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