Irreducible complexity

From Academic Kids

Template:Creationism2 Irreducible complexity is a concept which considers that the generally accepted scientific theory that life evolved through biological evolution by natural selection alone is incomplete or flawed, and that some additional mechanism is required to explain the origins of life.

The concept was popularized by Lehigh University biochemist and Fellow of the Discovery Institute Michael Behe in his 1996 book Darwin's Black Box, where Behe argued that there are biochemical systems which are "irreducibly complex" because he saw no way in which these systems could be broken down into smaller functioning systems. With this argument the book in effect supports what is known as intelligent design, a form of the argument from design, one of the arguments for the existence of a supernatural deity.

Irreducible complexity is rejected by the majority of the scientific community. The main concerns with the concept is that it utilises an argument from ignorance, that Behe fails to provide a testable hypothesis, and that there is a lack of evidence in support of the concept. As such irreducible complexity is seen as an example of creationist pseudoscience.



The term "irreducible complexity" is 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". (Michael Behe, Molecular Machines: Experimental Support for the Design Inference)

Supporters of the intelligent design theory use this term to refer to biological systems and organs that they believe could not have come about by an incremental 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.


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 (in William Paley's famous argument of 1802). This argument has a long history and can be traced back at least 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 by natural and sexual selection. The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of Darwinian evolution.

Possible examples

Behe and others, including some evolutionists, have suggested a number of biological features that they believe may be irreducibly complex.

The bombardier beetle

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 a defence mechanism that works thus: secretory cells produce hydroquinones and hydrogen peroxide (and perhaps other chemicals, depending on the species), which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones. These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it. Under pressure of the released gasses, the valve is forced closed, and the chemicals are expelled explosively through openings at the tip of the abdomen.. 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, but creationists' presentation of the facts has been criticized and there are, in fact, a number of ways in which this mechanism could have evolved [1] (

The bird lung

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.

Recently, conventional wisdom has held that birds are direct descendants of theropod dinosaurs. However, the apparently steadfast maintenance of hepatic-piston diaphragmatic lung ventilation in theropods throughout the Mesozoic poses a fundamental problem for such a relationship. The earliest stages in the derivation of the avian abdominal airsac system from a diaphragmatic-ventilating ancestor would have necessitated selection for a diaphragmatic hernia [i.e. hole] in taxa transitional between theropods and birds. Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage. (Michael Denton)

This comment appears to have been taken from a paper by Ruben et. al. entitled "Lung Structure and Ventilation in Theropod Dinosaurs and Early Birds" (1997) [2] ( [3] (, which deals with the evolutionary relationship of theropod dinosaurs and birds and does not lend any support to the argument regarding irreducible complexity [4] (

See Handicaps and Sexual Selection section below.


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.

This topic is discussed in the article on the Evolution of flagella.

Certain biological pathways

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, but for a detailed discussion see the article "Behe and the Blood Clotting Cascade" by George Acton [5] (

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.

Other examples

  • The cilium (for a discussion see [6] (
  • The ATP synthase molecule
  • The woodpecker's hyoid (for a discussion see "Anatomy and Evolution of the Woodpecker's Tongue" by Rusty Ryan [7] (
  • The eyes of Strepsiptera
  • Antibody example (see below)
  • The vertebrate eye (for a discussion see "Evolution of the eye: Lessons from freshman physics and Richard Dawkins" by Sonya Bahar [8] (
  • Feathers (for a discussion see "Feathers: Created or Evolved?" by Paul Keck [9] (

In social and economic history

Apparent irreducible complexity is a phenomenon not confined to biology. In accounting for the reawakening of Europe in the 12th century and the rise of the towns, historian Fernand Braudel instances the demographic expansion that itself needs to be explained, the progress in agricultural techniques which began in the eleventh century with the improved design of the plough, triennial crop rotation and the open field system for stock farming, the progress made in trade, the spread of a money economy, agricultural over-production and the accumulation of surpluses... "All the explanations must in the end be combined," Braudel notes (in The Perspective of the World 1984, p. 95). "How could there have been any growth unless everything progressed at more or less the same pace? A larger population, the perfection of agricultural techniques, the revival of trade and the first wave of craft industry were all essential factors if the area known as Europe was to develop an urban network." And yet the historic renaissance of medieval Europe is a fact.


There has been much scientific opposition to the irreducible complexity, with one science writer calling it a "full-blown intellectual surrender strategy." [10] ( 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.

Gradual replacement

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:

A = original killer and marker
K = second killer
M = replacement marker
  1. A
  2. AK
  3. AMK
  4. MK

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.

Handicaps and Sexual Selection

Another overlooked source of "irreducibly complex" features in a sexually reproducing organism is the Handicap Theory. Sexual selection often favors those who can demonstrate to their mates a surplus of energy by maintaining a feature or behavior that is unnecessary for basic survival -- sometimes even a hinderance. Examples include certain horns and antlers, display feathers, skin or hair colors and patterns, bony structure, scents, songs, symmetry, and elaborate ritualistic behavior. It is not unreasonable to imagine a handicapping feature eventually developing a useful purpose in a changing environment or for two or more handicapping features to become useful when combined. Conversely, a useful feature may evolve to become a handicapping feature, but through sexual selection the feature is passed through generations to again become useful in a completely different context. In this new context it may seem impossible to us that it was naturally selected to its purpose.

Imagine that a spontaneous hole formed in a pre-bird lizard's lung and quickly became a demonstration to potental mates that it has "energy to burn" because it was successful despite its handicap. Perhaps it evolved as a mating display because it made a distinctive sound like a frog's mating display. That feature could have been maintained by sexual selection long enough to have evolved into the modern bird lung we see today.

Falsifiability and experimental evidence

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 asserts is evidence that "Behe is wrong."

It should, however, be noted that although non-falisifiability may make something non-scientific, it does not make it wrong.

Significance (if found)

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. 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:

External links

In support

In opposition


  • Behe, Michael (1996). Darwin's Black Box. New York: The Free Press. ISBN 0684834936
  • Denton, Michael (1996). Evolution: A Theory in Crisis. Adler & Adler.
  • Ruben, J.A.; Jones, T.D.; Geist, N.R.; & Hillenius, W.J. (November 14, 1997). Lung Structure and Ventilation in Theropod Dinosaurs and Early Birds. Science 278 (5341) 1267–1270.
  • Sunderland, Luther D. (March 1976). Miraculous Design in Woodpeckers. Creation Research Society Quarterly.
  • Testing Darwin ( Discover Magazine Vol. 26 No. 02 ( | February 2005

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