Irreducible complexity (IC) is an argument made by proponents of intelligent design that certain biological systems are too complex to have evolved from simpler, or "less complete" predecessors, through natural selection acting upon a series of advantageous naturally occurring chance mutations. It is one of two main arguments intended to support intelligent design, the other being specified complexity. It is dismissed by the scientific community and intelligent design has been referred to as pseudoscience.
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". 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. In the 2005 Kitzmiller v. Dover Area School District trial, Behe gave testimony on the subject of irreducible complexity. The court found 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." Nonetheless, irreducible complexity continues to be cited as an important argument by creationists, particularly intelligent design proponents.
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 p39 in the 2006 edition)
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)
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 In the late 17th century, Thomas Burnet referred to "a multitude of pieces aptly joyn’d" to argue against the eternity of life. In the early 18th century, Nicolas Malebranche 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. Chapter XV of Paley's Natural Theology discusses at length what he called "relations" of parts of living things as an indication of their design. 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.
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. 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. 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. 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.
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.
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 ...?
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.
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".
An early concept of irreducibly complex systems comes from Ludwig von Bertalanffy, a 20th-century Austrian biologist. 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. However, Systems theory remained popular in the social sciences long after its demise in the physical and biological sciences.
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.
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. (The result of the trial was the ruling that "intelligent design is not science and is essentially religious in nature".)
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.
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 challenged byTalkOrigins. 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...
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. 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.
The 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.
In an often mis-quoted passage from The Origin of Species, Charles Darwin appears to acknowledge the eye's development as a difficulty for his theory. However, the quote in context shows that Darwin actually had a very good understanding of the evolution of the eye. He notes that "to suppose that the eye [...] could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree". Yet this observation was merely a rhetorical device for Darwin. He goes on to explain 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 [...] can hardly be considered real". He then proceeded to roughly map out a likely course for evolution using examples of gradually more complex eyes of various species.
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.
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 so that light is focused, 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. 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. 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.
Mainstream scientists regard this argument as having been largely disproved in the light of fairly recent research. 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 ten elements in common with the flagellum, but it is missing forty 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.
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. 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.
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.
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.
It may be that irreducible complexity does not actually exist in nature, and 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.
Arguments for irreducibility often assume that things started out the same way they ended up—as we see them now. However, that may not necessarily be the case.
Behe argues that the theory that irreducibly complex systems could not have been evolved can be falsified by an experiment where such systems are evolved. For example, he posits taking bacteria with no flagellum and imposing a selective pressure for mobility. If, after a few thousand generations, the bacteria evolved the bacterial flagellum, then Behe believes that this would refute his theory.
Other critics take a different approach, pointing to experimental evidence that they believe falsifies the argument for Intelligent Design from irreducible complexity. For example, Kenneth Miller cites the lab work of Barry G. Hall on E. coli, which he asserts is evidence that "Behe is wrong.
Other evidence that irreducible complexity is not a problem for evolution comes from the field of computer science, where computer analogues of the processes of evolution are routinely used to automatically design complex solutions to problems. The results of such Genetic Algorithms are frequently irreducibly complex since the process, like evolution, both removes non-essential components over time as well as adding new components. The removal of unused components with no essential function, like the natural process where rock underneath a natural arch is removed, can produce irreducibly complex structures without requiring the intervention of a designer. Researchers applying these algorithms are automatically producing human competitive designs--but no human designer is required.
Eugenie Scott, along with Glenn Branch and other critics, has argued that many points raised by intelligent design proponents are arguments from ignorance. Behe has been accused of using an "argument by lack of imagination", and Behe himself acknowledges that simply because scientists cannot currently see how an "irreducibly complex" organism could evolve, it does not prove that there is no possible way for it to have occurred.
Irreducible complexity is at its core an argument against evolution. If truly irreducible systems were found, the argument is that intelligent design is the correct explanation for their existence. However, this conclusion is based on the assumption that current evolutionary theory and intelligent design are the only two valid models to explain life, a false dilemma.
In the final ruling of Kitzmiller v. Dover Area School District, Judge Jones specifically singled out Behe and irreducible complexity:
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