Irreducible complexity

Arguments by proponents of irreducible complexity

In his book, "Darwin's Black Box[?]" (New York: The Free Press, 1996), Behe argues that neo-Darwinists have failed to explain the origin of complex molecular machines in living systems (Pp. 51-73). For example, 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 would make the flagella fail to function. Thus 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. According to the principle of natural selection, it would die out soon if not immediately.

Another example is the bombardier beetle (Genus Brachinus[?]), an organism that has become somewhat of a standard bearer for those who argue for intelligent design in their attempt to disprove natural selection as a potential mechanism for evolution. These beetles have three chambers in their abdomen, two which contain liquids that are chemically inert, but when mixed they create an incendiary combination. The third chamber is a reaction chamber into which the two chemicals are squeezed when danger is near, and then expelled explosively towards the perceived danger. Without all the components present, either the beetle would blow itself up or the remaining parts would serve no useful benefit.

It is important to note that Behe and other intelligent design proponents are not attempting to disprove natural selection, but rather natural selection as a mechanism for evolution. The reasoning is: Since irreducible complexity and natural selection seem to have irreconcilable differences, evolution can not always occur through natural selection. More simply put, the argument can be summarized as:

• Assume: Evolution's sole mechanism is natural selection (this is an assumption by some, but not all evolutionists).
• Statement: To work, evolution has to get around the problem of irreducible complexity.
• Statement: Natural selection, while a true part of nature, does not get around the problem of irreducible complexity.
• Conclusion: Therefore evolution can not occur.

According to Behe there is no imaginable way that the necessary combination of molecules could be built up piecemeal, as the theory of evolution requires; either they are all present, or the process does not work. Behe does accept the existence of an old earth[?] and also accepts the existence of common descent.

Arguments against irreducible complexity

The term irreducible complexity assumes that some pathways within a living cell function only if all their parts are present in good working order at the outset. For example, a mousetrap consists of several parts which work together to catch a mouse; if any part is missing or defective, the mousetrap fails to work at all. Similarly, the biochemistry of light detection requires complex interactions among many different molecules, each performing a very specialized job.

Evolutionists further state that there are flagella simpler in form than the type Behe cites which function perfectly well despite "missing parts". There are also, say biologists, even simpler forms that only have fully functioning components of Behe's flagellum that perform a myriad of functions apart from propulsion. For example, the bubonic plague bacterium Yersinia pestis has an organelle assembly very similar to a complex flagellum, except that it functions as a needle to inject toxins into host cells. The key, they say, is that the component parts of the complex flagellum Behe uses as an example can perform many roles in component form. Most significantly, say evolutionary biologists, is that most of those roles have nothing to do with propulsion and that evolution often works in this kind of blind, haphazard manner in which the function of an early form not necessarily being the same function of a later form.

Most biochemists do not believe that the concept is useful, because Behe ignores mechanisms by which complexity comes into being. One such mechanism is "scaffolding" as 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. Another mechanism is that a set of biochemical reactions start being used with something completely unrelated. For example, the fact that any step of light detection would render the pathway useless for light detection would not be important if the original adaptive benefit of the pathway was something unrelated.

Behe has also been accused of falling into the argument by lack of imagination fallacy. Just because he does not see how a pathway can develop, or just because no one sees how a pathway can come into being does not mean that there is no possible way for this to occur.

A potential mechanism for dealing with irreducible complexity can be found in the hypothesis of quantum evolution.

See: intelligent design, theory of evolution