不可化約的複雜性(Irreducible complexity、IC)是一種主張,認為生物系統太過複雜,以致於無法由較為簡單或較不複雜的祖先演化而成,並且無法經由自然發生的突變機會產生。最早將不可化約的複雜性系統定義的人,是一位生物化學教授麥可·貝希(Michael Behe)。他形容不可化約的複雜性是指一種「由和諧的、互相作用的零件組合而成,並形成基本作用,移除其中任何一個部分將導致系統的作用停止。」
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Irreducible complexity (IC) is an argument made by intelligent design proponents that certain biological systems are too complex to have evolved from simpler, or "less complete" predecessors, through naturally occurring chance mutations. It is one of two main arguments intended to support intelligent design, the other being specified complexity.[1]
In 2005 in the Kitzmiller v. Dover Area School District trial in which Behe testified in support of his claims about irreducible complexity the United States federal court ruled that "Professor Behe's claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large."[2]
Biochemistry professor Michael Behe, the originator of the argument of irreducible complexity, defines an irreducibly complex system as one "composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning".[3] These examples are said to demonstrate that modern biological forms could not have evolved naturally. Critics consider that most, or all, of the examples were based on misunderstandings of the workings of the biological systems in question, and consider the low quality of these examples excellent evidence for the argument from ignorance. Irreducible complexity is generally dismissed by the scientific community;[4] it is often referred to as pseudoscience.[5]
Despite being discredited in the Dover trial where the court found in its ruling that Behe's claims for irreducible complexity have been refuted in peer-reviewed research and rejected by the scientific community at large",[2] irreducible complexity has nevertheless remained a popular argument among advocates of intelligent design and other creationists.
- posted on 02/11/2008
Definitions
The term "irreducible complexity" was originally defined by Behe as:
A single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning". (Darwin's Black Box p9)
Supporters of intelligent design use this term to refer to biological systems and organs that they believe could not have come about by any series of small changes. They argue that anything less than the complete form of such a system or organ would not work at all, or would in fact be a detriment to the organism, and would therefore never survive the process of natural selection. Although they accept that some complex systems and organs can be explained by evolution, they claim that organs and biological features which are irreducibly complex cannot be explained by current models, and that an intelligent designer must have created life or guided its evolution. Accordingly, the debate on irreducible complexity concerns two questions: whether irreducible complexity can be found in nature, and what significance it would have if it did exist in nature.
A second definition given by Behe (his "evolutionary definition") is as follows:
An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway.
Intelligent design advocate William Dembski gives this definition:
A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, nonarbitrarily individuated parts such that each part in the set is indispensable to maintaining the system's basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system. (No Free Lunch, 285) - posted on 02/11/2008
History
Forerunners
The argument from irreducible complexity is a descendant of the teleological argument for God (the argument from design or from complexity). This states that because certain things in nature are very complicated, they must have been designed. William Paley famously argued, in his 1802 watchmaker analogy, that complexity in nature implies a God for the same reason that the existence of a watch implies the existence of a watchmaker. This argument has a long history, and can be traced back at least as far as Cicero's De natura deorum ii.34. [6] [7]
The idea that the interrelationship between parts of living things would have implications for their origins was raised by writers starting with Pierre Gassendi in the mid 17th century[8] In the late 17th century, Thomas Burnet referred to "a multitude of pieces aptly joyn’d" to argue against the eternity of life.[9] In the early 18th century, Nicolas Malebranche[10] used this idea to argue in favor of preformation (see homunculus), rather than full development (see epigenesis), of the individual embryo; and a similar argument about the origins of the individual was made by other 18th century students of natural history. In a different application, in the early 19th century Georges Cuvier used the concept of "correlation of parts" in establishing the anatomy of animals from fragmentary remains.[11][12]
While he did not originate the term, Charles Darwin identified the argument as a possible way to falsify a prediction of the theory of evolution at the outset. In The Origin of Species, he wrote, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case."[13] Darwin's theory of evolution challenges the teleological argument by postulating an alternative explanation to that of an intelligent designer—namely, evolution by natural selection.[clarify] The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of Darwinian evolution.
Hermann Muller, in the early 20th century, discussed a concept similar to irreducible complexity. However, far from seeing this as a problem for evolution, he described the "interlocking" of biological features as a consequence to be expected of evolution, which would lead to irreversibility of some evolutionary changes.[14] He wrote, "Being thus finally woven, as it were, into the most intimate fabric of the organism, the once novel character can no longer be withdrawn with impunity, and may have become vitally necessary."[15]
In 1974, Young Earth Creationist Henry M. Morris introduced a similar concept in his book "Scientific Creationism" in which he wrote; "This issue can actually be attacked quantitatively, using simple principles of mathematical probability. The problem is simply whether a complex system, in which many components function unitedly together, and in which each component is uniquely necessary to the efficient functioning of the whole, could ever arise by random processes."[16]
In 1981, Ariel Roth, in defense of the creation science position in the trial McLean v. Arkansas, said of "complex integrated structures" that "This system would not be functional until all the parts were there ... How did these parts survive during evolution ...?"[17]
In 1985 Cairns-Smith wrote of "interlocking", "How can a complex collaboration between components evolve in small steps?" and used the analogy of the scaffolding in building an arch: "Surely there was 'scaffolding'. Before the multitudinous components of present biochemistry could come to lean together they had to lean on something else."[18]
An essay in support of creationism published in 1994 referred to bacterial flagella as showing "multiple, integrated components", where "nothing about them works unless every one of their complexly fashioned and integrated components are in place" and asked the reader to "imagine the effects of natural selection on those organisms that fortuitously evolved the flagella ... without the concommitant [sic] control mechanisms".[19][20]
An early concept of irreducibly complex systems comes from Ludwig von Bertalanffy, a 20th-century Austrian biologist.[21] He believed that complex systems must be examined as complete, irreducible systems in order to fully understand how they work. He extended his work on biological complexity into a general theory of systems in a book titled General Systems Theory. After James Watson and Francis Crick published the structure of DNA in the early 1950s, General Systems Theory lost many of its adherents in the physical and biological sciences.[22] However, Systems theory remained popular in the social sciences long after its demise in the physical and biological sciences.
- posted on 02/11/2008
Origins
Michael Behe's controversial book Darwin's Black Box popularized the concept of irreducible complexity.Michael Behe developed his ideas on the concept around 1992, in the early days of the 'wedge movement', and first presented his ideas about "irreducible complexity" in June 1993 when the "Johnson-Behe cadre of scholars" met at Pajaro Dunes in California.[23] He set out his ideas in the second edition of Of Pandas and People published in 1993, extensively revising Chapter 6 Biochemical Similarities with new sections on the complex mechanism of blood clotting and on the origin of proteins.[24]
He first used the term "irreducible complexity" in his 1996 book Darwin's Black Box, to refer to certain complex biochemical cellular systems. He posits that evolutionary mechanisms cannot explain the development of such "irreducibly complex" systems. Notably, Behe credits philosopher William Paley for the original concept, not von Bertalanffy, and suggests that his application of the concept to biological systems is entirely original. Intelligent design advocates argue that irreducibly complex systems must have been deliberately engineered by some form of intelligence.
In 2001, Michael Behe wrote: "[T]here is an asymmetry between my current definition of irreducible complexity and the task facing natural selection. I hope to repair this defect in future work." Behe specifically explained that the "current definition puts the focus on removing a part from an already functioning system", but the "difficult task facing Darwinian evolution, however, would not be to remove parts from sophisticated pre-existing systems; it would be to bring together components to make a new system in the first place". In the 2005 Kitzmiller v. Dover Area School District trial, Behe testified under oath that he "did not judge [the asymmetry] serious enough to [have revised the book] yet." [25]
Behe additionally testified that the presence of irreducible complexity in organisms would not rule out the involvement of evolutionary mechanisms in the development of organic life. He further testified that he knew of no earlier "peer reviewed articles in scientific journals discussing the intelligent design of the blood clotting cascade," but that there were "probably a large number of peer reviewed articles in science journals that demonstrate that the blood clotting system is indeed a purposeful arrangement of parts of great complexity and sophistication." [26] (The result of the trial was the ruling that "intelligent design is not science and is essentially religious in nature".)[27]
According to the theory of evolution, genetic variations occur without specific design or intent. The environment "selects" the variants that have the highest fitness, which are then passed on to the next generation of organisms. Change occurs by the gradual operation of natural forces over time, perhaps slowly, perhaps more quickly (see punctuated equilibrium). This process is able to adapt complex structures from simpler beginnings, or convert complex structures from one function to another (see spandrel). Most intelligent design advocates accept that evolution occurs through mutation and natural selection at the "micro level", such as changing the relative frequency of various beak lengths in finches, but assert that it cannot account for irreducible complexity, because none of the parts of an irreducible system would be functional or advantageous until the entire system is in place.
Behe uses the mousetrap as an illustrative example of this concept. A mousetrap consists of several interacting pieces—the base, the catch, the spring, the hammer. Behe contends that all of these must be in place for the mousetrap to work, and that the removal of any one piece destroys the function of the mousetrap. Likewise, biological systems require multiple parts working together in order to function. Intelligent design advocates claim that natural selection could not create from scratch those systems for which science is currently unable to find a viable evolutionary pathway of successive, slight modifications, because the selectable function is only present when all parts are assembled. Behe's original examples of irreducibly complex mechanisms included the bacterial flagellum of E. coli, the blood clotting cascade, cilia, and the adaptive immune system.
Michael Behe believes that many aspects of life show evidence of design, using the mousetrap in an analogy which others dispute..[28]Behe argues that organs and biological features which are irreducibly complex cannot be wholly explained by current models of evolution. He argues that:
An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.
Irreducible complexity is not an argument that evolution does not occur, but rather an argument that it is "incomplete". In the last chapter of Darwin's Black Box, Behe goes on to explain his view that irreducible complexity is evidence for intelligent design. Mainstream critics, however, argue that irreducible complexity, as defined by Behe, can be generated by known evolutionary mechanisms. Behe's claim that no scientific literature adequately modeled the origins of biochemical systems through evolutionary mechanisms has been shown by TalkOrigins to be false.[29][30] The judge in the Dover trial wrote "By defining irreducible complexity in the way that he has, Professor Behe attempts to exclude the phenomenon of exaptation by definitional fiat, ignoring as he does so abundant evidence which refutes his argument. Notably, the NAS has rejected Professor Behe’s claim for irreducible complexity..."[31]
- Re: 不可化约的复杂性---Irreducible complexityposted on 02/11/2008
interesting topic (and controversial too). I will sleep on this before I make any comment. - posted on 02/11/2008
Stated examples
Behe and others have suggested a number of biological features that they believe may be irreducibly complex.
Blood clotting cascade
Main article: Coagulation
The blood clotting cascade in vertebrates is a complex biological pathway that is given as an example of apparent irreducible complexity.[32]
The irreducible complexity argument assumes that the necessary parts of a system have always been necessary, and therefore could not have been added sequentially. However, in evolution, something which is at first merely advantageous can later become necessary. For example, one of the clotting factors that Behe listed as a part of the clotting cascade was later found to be absent in whales, demonstrating that it is not essential for a clotting system.[33] Many purportedly irreducible structures can be found in other organisms as much simpler systems that utilize fewer parts. These systems, in turn, may have had even simpler precursors that are now extinct. The "improbability argument" also misrepresents natural selection. It is correct to say that a set of simultaneous mutations that form a complex protein structure is so unlikely as to be unfeasible, but that is not what Darwin advocated. His explanation is based on small accumulated changes that take place without a final goal. Each step must be advantageous in its own right, although biologists may not yet understand the reason behind all of them -- for example, jawless fish accomplish blood clotting with just six proteins instead of the full 10.[34].
Eye
Main article: Evolution of the eye
Often used as an example of irreducible complexity.
(a) A pigment spot
(b) A simple pigment cup
(c) The simple optic cup found in abalone
(d) The complex lensed eye of the marine snail and the octopusThe eye is a famous example of a supposedly irreducibly complex structure, due to its many elaborate and interlocking parts, seemingly all dependent upon one another. It is frequently cited by intelligent design and creationism advocates as an example of irreducible complexity. Behe used the "development of the eye problem" as evidence for intelligent design in Darwin's Black Box. Although Behe acknowledged that the evolution of the larger anatomical features of the eye have been well-explained, he claimed that the complexity of the minute biochemical reactions required at a molecular level for light sensitivity still defies explanation. Creationist Jonathan Sarfati has described the eye as evolutionary biologists' "greatest challenge as an example of superb 'irreducible complexity' in God's creation", specifically pointing to the supposed "vast complexity" required for transparency.[35]
In an oft-quoted passage from The Origin of Species, Charles Darwin himself acknowledged the eye's development as a difficulty for his theory, noting that "to suppose that the eye... could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree". However, he went on to note that, if gradual evolution of the eye could be shown to be possible, "the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real", and proceeded to roughly map a possible course for evolution through examples from gradually more complex eyes of various species.[36]
The eyes of vertebrates (left) and invertebrates such as the octopus (right) developed independently: vertebrates evolved an inverted retina with a blind spot over their optic disc, whereas octopuses avoided this with a non-inverted retina.Since Darwin's day, the eye's ancestry has become much better understood. Although learning about the construction of ancient eyes through fossil evidence is problematic due to the soft tissues leaving no imprint or remains, genetic and comparative anatomical evidence has increasingly supported the idea of a common ancestry for all eyes.[37][38][39]
As Behe admits, current evidence does suggest possible evolutionary lineages for the origins of the anatomical features of the eye, for example, that eyes originated as simple patches of photoreceptor cells that could detect the presence or absence of light, but not its direction. By developing a small depression for the photosensitive cells, the organisms obtained a better sense of the light's source, and by continuing to deepen the depression into a pit so that light would strike certain cells depending on its angle, increasingly precise visible information was possible. The aperture of the eye was then shrunk in order to focus the light, turning the eye into a pinhole camera and allowing the organism to dimly make out shapes—the nautilus is a modern example of an animal with such an eye. Finally, the protective layer of transparent cells over the aperture was differentiated into a crude lens, and the interior of the eye was filled with humours to assist in focusing images.[40][41][42] In this way, eyes are recognized by modern biologists as actually a relatively unambiguous and simple structure to evolve, and many of the major developments of the eye's evolution are believed to have taken place over only a few million years, during the Cambrian explosion.[43] However, according to Behe, the complexity of light sensitivity at the molecular level and the minute biochemical reactions required for those first "simple patches of photoreceptor[s]" still defies explanation.
Flagella
Main article: Evolution of flagella
The flagella of certain bacteria constitute a molecular motor requiring the interaction of about 40 complex protein parts, and the absence of any one of these proteins causes the flagella to fail to function. Behe holds that the flagellum "engine" is irreducibly complex because if we try to reduce its complexity by positing an earlier and simpler stage of its evolutionary development, we get an organism which functions improperly.
Mainstream scientists regard this argument as having been largely disproved in the light of fairly recent research.[44] They point out that the basal body of the flagella has been found to be similar to the Type III secretory system (TTSS), a needle-like structure that pathogenic germs such as Salmonella and Yersinia pestis use to inject toxins into living eucaryote cells. The needle's base has many elements in common with the flagellum, but it is missing most of the proteins that make a flagellum work. Thus, this system seems to negate the claim that taking away any of the flagellum's parts would render it useless. This has caused Kenneth Miller to note that, "The parts of this supposedly irreducibly complex system actually have functions of their own." [45]
- posted on 02/11/2008
Response of the scientific community
Like intelligent design, the concept it seeks to support, irreducible complexity has failed to gain any notable acceptance within the scientific community. One science writer called it a "full-blown intellectual surrender strategy."[46]
Reducibility of "irreducible" systems
Potentially viable evolutionary pathways have been proposed for allegedly irreducibly complex systems such as blood clotting, the immune system[47] and the flagellum,[48][49] which were the three examples Behe used. Even his example of a mousetrap was shown to be reducible by John H. McDonald.[28] If irreducible complexity is an insurmountable obstacle to evolution, it should not be possible to conceive of such pathways—Behe has remarked that such plausible pathways would defeat his argument.
Niall Shanks and Karl H. Joplin, both of East Tennessee State University, have shown that systems satisfying Behe's characterization of irreducible biochemical complexity can arise naturally and spontaneously as the result of self-organizing chemical processes.[50][51] They also assert that what evolved biochemical and molecular systems actually exhibit is "redundant complexity"—a kind of complexity that is the product of an evolved biochemical process. They claim that Behe overestimated the significance of irreducible complexity because of his simple, linear view of biochemical reactions, resulting in his taking snapshots of selective features of biological systems, structures and processes, while ignoring the redundant complexity of the context in which those features are naturally embedded. They also criticized his over-reliance of overly simplistic metaphors, such as his mousetrap. In addition, research published in the peer-reviewed journal Nature has shown that computer simulations of evolution demonstrate that it is possible for irreducible complexity to evolve naturally.[52]
It is illustrative to compare a mousetrap with a cat, in this context. Both normally function so as to control the mouse population. The cat has many parts that can be removed leaving it still functional; for example, its tail can be bobbed, or it can lose an ear in a fight. Comparing the cat and the mousetrap, then, one sees that the mousetrap (which is not alive) offers better evidence, in terms of irreducible complexity, for intelligent design than the cat. Even looking at the mousetrap analogy, several critics have described ways in which the parts of the mousetrap could have independent uses or could develop in stages, demonstrating that it is not irreducibly complex.[28]
Moreover, even cases where removing a certain component in an organic system will cause the system to fail do not demonstrate that the system couldn't have been formed in a step-by-step, evolutionary process. By analogy, stone arches are irreducibly complex—if you remove any stone the arch will collapse—yet we build them easily enough, one stone at a time, by building over scaffolding that is removed afterward. Similarly, naturally occurring arches of stone are formed by weathering away bits of stone from a large concretion that has formed previously.
Evolution can act to simplify as well as to complicate. This raises the possibility that seemingly irreducibly complex biological features may have been achieved with a period of increasing complexity, followed by a period of simplification.
In April 2006 a team led by Joe Thornton, assistant professor of biology at the University of Oregon's Center for Ecology and Evolutionary Biology, using techniques for resurrecting ancient genes, scientists for the first time reconstructed the evolution of an apparently irreducibly complex molecular system. The research was published in the April 7 issue of Science.[53]
It may be that irreducible complexity does not actually exist in nature, that the examples given by Behe and others are not in fact irreducibly complex, but can be explained in terms of simpler precursors. There has also been a theory that challenges irreducible complexity called facilitated variation. The theory has been presented in 2005 by Marc W. Kirschner, a professor and chair of Department of Systems Biology at Harvard Medical School, and John C. Gerhart, a professor in Molecular and Cell Biology, University of California, Berkeley. In their theory, they describe how certain mutation and changes can cause apparent irreducible complexity. Thus, seemingly irreducibly complex structures are merely "very complex", or they are simply misunderstood or misrepresented.
- Re: 不可化约的复杂性---Irreducible complexityposted on 02/11/2008
Behe 的 Darwin's Black Box 在我的reading list上好久了,还没有看到。IC 理论听上去怪有道理的,但是我不懂生化,不能瞎说,只从读到的一点点,我觉着它并不排除更复杂系统的进化,和达尔文本身的观察和理论也不矛盾,不知是不是误读。 - posted on 02/11/2008
我的看法是如何解释进化论,因为在一定范围内的进化论是个不争的事实。我对这个理论感兴趣是想知道,这进化是从何时开始,要进化到何时为止?如果说进化,是否进化到某个完美的状态就停止了?人有进化了许多年,也没比古人聪明。
达尔文自己也有很多东西回答不了,比如,眼睛如何进化。。。
浮生 wrote:
Behe 的 Darwin's Black Box 在我的reading list上好久了,还没有看到。IC 理论听上去怪有道理的,但是我不懂生化,不能瞎说,只从读到的一点点,我觉着它并不排除更复杂系统的进化,和达尔文本身的观察和理论也不矛盾,不知是不是误读。 - posted on 02/11/2008
July wrote:
我的看法是如何解释进化论,因为在一定范围内的进化论是个不争的事实。我对这个理论感兴趣是想知道,这进化是从何时开始,要进化到何时为止?如果说进化,是否进化到某个完美的状态就停止了?人有进化了许多年,也没比古人聪明。
就是因为一切之初谁也解释不了,本也不是进化论的范围,IC 这样的理论就有些意思,虽然也不comforting:)只要有生物在,进化就不会停,因为复制总是会出错的。更聪明倒不是目的。
达尔文自己也有很多东西回答不了,比如,眼睛如何进化。。。
这个Richard Dawkins有解释,他的书:The Blind Watchmaker
- posted on 02/11/2008
对阿,我就不明白为什麽一定要用进化论来证明什麽,根本就不在其范围内吗。IC 里关于眼睛的疑问,和达尔文一样啊。我的意思是说,即使IC是正确的,也不能否认进化论。因为范围不同吗。
浮生 wrote:
July wrote:就是因为一切之初谁也解释不了,本也不是进化论的范围,IC 这样的理论就有些意思,虽然也不comforting:)只要有生物在,进化就不会停,因为复制总是会出错的。更聪明倒不是目的。
我的看法是如何解释进化论,因为在一定范围内的进化论是个不争的事实。我对这个理论感兴趣是想知道,这进化是从何时开始,要进化到何时为止?如果说进化,是否进化到某个完美的状态就停止了?人有进化了许多年,也没比古人聪明。
达尔文自己也有很多东西回答不了,比如,眼睛如何进化。。。这个Richard Dawkins有解释,他的书:The Blind Watchmaker
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