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Irreducible complexity is a concept popularized by Lehigh University biochemist and Fellow of the Discovery Institute Michael Behe in his 1996 book Darwin's Black Box. Behe contests some aspects of the generally accepted scientific theory that life evolved through biological evolution alone, and asserts that some other mechanism is required to explain the phenomenon of irreducible complexity. Behe argues that there are biochemical systems that are irreducibly complex because they cannot be broken down into functioning pre-systems. Accordingly, Behe's book supports what is known as intelligent design theory, a form of the argument from design], one of many arguments for the existence of a supernatural deity, usually taken to be God.
Irreducible complexity is not recognised by the wider scientific community, who argue that there is a lack of evidence and that the concepts are incompatible with scientific philosophy and therefore relegate irreducible complexity to pseudoscience.
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The term "irreducible complexity" is defined by Behe as:
Believers in the intelligent design theory use this term to refer to biological systems and organs that could not have come about by a series of small changes. For such mechanisms or organs, anything less than their complete form 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. Proponents of intelligent design argue that while some complex systems and organs can be explained by evolution, organs and biological features which are irreducibly complex cannot be explained by current models, and that an intelligent designer must thus have created or guided life.
The debate on irreducible complexity concerns two questions: whether irreducible complexity be found in nature, and what significance it would have if it did exist in nature.
The argument from irreducible complexity is a descendant of the teleological argument for God (the argument from design or argument from complexity). This states that because certain things in nature are very complicated, they must have been designed, just as the existence of a watch implies the existence of a watchmaker. This argument is a very old one, and can be traced back as far as Cicero's De natura deorum, ii. 34 (see Hallam, Literature of Europe, ii. 385, note).
Charles Darwin's theory of evolution challenges the teleological argument by postulating an alternative explanation to that of an intelligent designer: namely evolution. The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of evolution.
Behe and others, including some evolutionists, have suggested a number of biological features that they believe may be irreducibly complex.
The bombardier beetle (genus Brachinus) is an organism that has become a standard example of irreducible complexity among those who argue for it. These beetles have three chambers in their abdomen, two of which contain liquids that are chemically stable in isolation but react violently when mixed. The third chamber is a reaction chamber into which the two chemicals are squeezed when danger is near, and from which they are expelled explosively towards the perceived danger. Irreducible complexity asserts that, in order for any of the components of the system to function, all components of the system must have been present.
The bird lung is different from other lungs, such as the reptile lung (which evolutionists believe it evolved from). Proponents of irreducible complexity argue that the transition from a reptile lung (bellows lung) to a bird lung (circulatory lung) is unlikely since intermediate stages would be a detriment to the organism.
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 cause 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.
This topic is discussed in the article on the Evolution of flagella.
The biochemistry of light detection requires complex interactions among many different molecules, each performing a very specialized job. Eliminating even one component of the biological pathway can destroy the ability to detect light.
The blood clotting cascade in vertebrates is another complex biological pathway that is given as an example of irreducible complexity.
One evolutionary mechanism that may result in complex biological pathways such as these is "biochemical scaffolding", where a set of biochemical reactions are used to build up a pathway and then are discarded, in much the same way that a building is built from the bottom up even though removing any of the columns would cause the building to collapse.
There is some research to indicate possible evolutionary mechanisms for the development of some purportedly irreducible systems. However, the research is not yet conclusive.
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. Thus they would either be merely very complex, or they would be misunderstood or misrepresented.
The precursors of complex systems, when they are not useful in themselves, may be useful to perform other, unrelated functions. Evolutionary biologists argue that evolution often works in this kind of blind, haphazard manner in which the function of an early form is not necessarily the same as the function of the later form. The mammalian ear (derived from a jawbone) and the panda's thumb (derived from a wrist bone spur) are considered classic examples.
Evolution can act to simplify as well as to complicate. This raises the possibility that apparently irreducibly complex biological features may have been achieved with a period of increasing complexity, followed by a period of simplification. 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.
Behe has been accused of using an argument by lack of imagination, or constructing a "God of the gaps". 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.
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.
Regarding Behe's antibody example, we have the "marker" substance and the "killer" substance, that together hunt and kill marked invaders. Behe is saying that by themselves, the marker and the killer are useless, and thus must have been made at the same time. The killer cannot kill what it cannot find and the marker has no ability to kill even if it can find a target.
However, under gradual replacement, a different marker may have started out as an independent hunter AND killer. After a while, a helper killer joined this army because it had some nice specialties. However, this second killer still depended on the first one to find the target. Thus the first killer served as both a marker and a killer, and the second killer is just a killer, relying on the first to hunt.
Perhaps over time it is more efficient to have the 2nd killer specialize in killing and the first specialize in marking, and so the first killer is replaced by a similar substance that is merely a marker (perhaps a better marker than the first dual-purpose one).
Thus, each step is an advantage, yet the final result is a dependent pair that does not resemble the proto-killer. This example can be laid out as:
All we see today is "MK". Opponents of irreducible complexity state that Behe erroneously assumes that if the structure ended up MK, then it must have started out as M or K by themselves.
Some critics, such as Jerry Coyne (professor of evolutionary biology at the University of Chicago) and Eugenie Scott (pro-evolution activist, Executive Director at the National Center for Science Education) have argued that the concept of irreducible complexity, and more generally, the theory of Intelligent Design is not falsifiable, and therefore, not scientific.
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 flagella 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 Hall on E. coli, which he presents as conclusive evidence that "Behe is wrong".
Behe argues that organs and biological features which are irreducibly complex cannot be wholly explained by current models of evolution. He argues that:
Irreducible complexity is not an argument that evolution does not occur, but rather an argument that it is incomplete. If irreducible complexity is found and it cannot be wholly explained by current models of evolution, then, it is argued, alternative models may be considered such as:
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