Published 06/2019 Updated 09/16/2026
Irreducible complexity describes a system composed of several interacting parts that contribute to an essential function, where removing a critical part causes that function to cease.
The concept was popularized by biochemist Michael Behe in Darwin’s Black Box. Importantly, the system is claimed to be irreducible, not necessarily the individual parts. A component might perform another function by itself, yet that does not by itself explain how the complete integrated system arose.
This distinction is important.
Darwin’s Challenge
Charles Darwin recognized that his theory depended upon complex biological structures being attainable through successive modifications.
In On the Origin of Species, Darwin 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.”
Darwin immediately added that he knew of no such example and cautioned against assuming that unknown transitional stages never existed.¹ Nevertheless, Darwin’s statement identifies an important requirement of evolutionary theory: complex biological structures must ultimately have an evolutionary pathway.
Natural selection does not plan for a future biological system but acts upon existing heritable variation according to the reproductive consequences that variation produces in the present environment. Therefore, an evolutionary explanation for a complex biological machine must account for how its components and intermediate arrangements could arise, persist, and eventually become integrated into the system observed today.
The Problem of an Integrated System
Consider a machine consisting of numerous interacting components.
Having all of the components available somewhere is not the same thing as having a functioning machine. The components must also be assembled in the appropriate arrangement, interact properly, and collectively perform a useful biological function.
Behe summarizes this distinction: “Evolution does not occur at the machine level, it must occur at the nut-and-bolt level.” Darwin’s Black Box, p. 46.
This gets to the heart of the irreducible-complexity argument.
It is not enough to identify possible precursors to individual components. The evolutionary explanation must account for the development of the integrated system itself.
A protein that previously served another function could theoretically be recruited into a new system, a process known as co-option or exaptation. Gene duplication followed by modification can likewise produce proteins with related but different functions. These mechanisms are well-established areas of evolutionary biology.²
But identifying such mechanisms does not automatically reconstruct the evolutionary history of every complex molecular machine. The relevant question remains:
Can a plausible series of functional intermediate stages be demonstrated between a simpler ancestral system and the integrated biological machine that exists today?
The Bacterial Flagellum
One of Behe’s best-known examples is the bacterial flagellum.
The flagellum is a sophisticated molecular system involving numerous interacting proteins that collectively produce motility. Different proteins contribute to its basal structure, assembly, export apparatus, hook, filament, and rotational machinery.
Behe argues that the flagellum represents an irreducibly complex system because disrupting certain essential components destroys its motility function. Although evolutionary biologists largely all disagree with Behe’s conclusion and are convinced, although largely without evidence, that nature must have provided small step by step ways.
Researchers have identified similarities between components of bacterial flagella and other biological systems, particularly Type III secretion systems. Proposed evolutionary mechanisms include gene duplication, divergence, recombination, self-assembly, and recruitment of proteins that previously performed other functions.² ³
This evidence demonstrates that components of complex molecular machines can have relationships with components performing different functions elsewhere.
However, that observation should be distinguished from experimentally reproducing the complete historical pathway by which the flagellar system itself originated.
The central disagreement is therefore not simply whether individual flagellar proteins can perform other functions. The deeper question is whether mutation, selection, duplication, co-option, and other unguided processes adequately explain the origin and integration of the complete functional system.
Function at Every Stage
Richard Dawkins emphasized the importance of gradual development when explaining biological complexity:
“Evolution is very possibly not, in actual fact, always gradual. But it must be gradual when it is being used to explain the coming into existence of complicated, apparently designed objects, like eyes.” The Blind Watchmaker
The problem becomes particularly interesting when multiple interacting components are necessary for a particular biological function.
Evolution does not know what it is building toward. Natural selection cannot preserve a presently useless structure simply because that structure will become essential after several additional components arise generations later.
Therefore, proposed evolutionary pathways must provide biologically meaningful intermediate stages.
This does not require every intermediate to perform the same function as the final system. Evolutionary theory specifically proposes that components can change functions over time. But if that explanation is invoked, the proposed pathway still needs to account for how those components were recruited, modified, coordinated, regulated, assembled, and ultimately integrated into the new biological system.
Possible Is Not the Same as Demonstrated
This is where I believe irreducible complexity remains an important challenge.
Showing that an evolutionary pathway is conceivable is different from demonstrating that it actually occurred.
Likewise, discovering that one component resembles a protein in another biological system may provide evidence of a relationship, but it does not by itself establish every historical step necessary to construct the complete system.
Behe’s argument therefore should not be reduced to the claim that every individual component must have appeared simultaneously. His stronger argument concerns the origin of the integrated functional arrangement.
The question is not merely:
Could the individual parts have existed previously?
The more important question is:
How did the coordinated system arise through a sequence of biologically viable intermediate stages?
Irreducible complexity challenges evolutionary explanations to move beyond identifying similarities, possible precursors, or hypothetical pathways and demonstrate how highly integrated biological machinery can actually arise through unguided processes.
From a design perspective, systems containing numerous coordinated components, where the components work collectively toward a functional result, are precisely the kinds of arrangements we ordinarily associate with engineering.
For this reason, I believe irreducible complexity remains a significant challenge to the claim that mutation and natural selection adequately explain the origin of biological machinery.
Sources
1. Charles Darwin, On the Origin of Species, Chapter VI, “Difficulties on Theory.”
Darwin Online:
https://darwin-online.org.uk/converted/published/1859_Origin_F373/1859_Origin_F373.html
2. Pallen, M.J. & Gophna, U. “Bacterial flagella and Type III secretion: case studies in the evolution of complexity.” Genome Dynamics (2007).
PubMed:
https://pubmed.ncbi.nlm.nih.gov/18753783/
3. Pallen, M.J. & Matzke, N.J. “From The Origin of Species to the origin of bacterial flagella.” Nature Reviews Microbiology (2006).
PubMed:
https://pubmed.ncbi.nlm.nih.gov/16953248/
4. Michael Behe, Darwin’s Black Box: The Biochemical Challenge to Evolution.
Free Press.
5. Richard Dawkins, The Blind Watchmaker.
W. W. Norton & Company.
Therefore, an irreducibly complex system cannot be produced directly (that is, by continuous step by step Darwinian processes) by slight, successive modifications because any irreducibly complex system that has even one missing a part is by definition nonfunctional.
In other words: everything must be working from the beginning. An irreducibly complex biological system challenges Darwinian evolution at its core.
Natural Selection carries a heavy burden: “ 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 completely break down.”
Dawkins, Charles “The Origin of Species”
Natural selection in reality cannot “choose” or “select” anything other than genetic function and spontaneous mutations. Therefore within a functioning system which is already working, then the biological system cannot be produced gradually because all components are required to be fully functioning from the start, as an integrated unit, all built in one fell swoop.
“As the complexity of interacting (components of a) system increases…the likelihood of an indirect route drops precipitously.”
Dr. Behe, Michael “Darwin’s Black Box” page 40
Dr. Behe points out that while it might be welcoming for the naturalist to imagine that such a horrendous unlikelihood might occur through rapid simultaneous mutations—it is only “an appeal to brute luck that can never be refuted. Yet it is an empty argument…Luck is a metaphysical speculation; scientific explanations invoke causes.”
“Evolution is very possibly not, in actual fact, always gradual. But it must be gradual…to explain the existence of complicated, apparently designed objects, like eyes. Without gradualness…we are back to miracles, which is a synonym for the total absence of explanation.”
Richard Dawkins “The Blind Watchmaker”
“Evolution does not occur at the machine level—it must occur at the nut-and-bolt level.”
Dr. Behe, Michael “Darwin’s Black Box” page 46
Irreducible Complexity definition: “a single system which is composed of several well-matched, 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.” (39) “The system is irreducible not the parts” (261)
As an example, “bacterial flagellum is Irreducibly Complexity, could not have evolved by means of random mutation and natural selection. ”
Dr. Behe, Michael “Darwin’s Black Box”
Below is from “Evolution News”
“Behe’s original argument for irreducible complexity was always probabilistic, meaning that like all claims in science, one never gets 100% proof. You never absolutely rule out with 100% certainty the possibility of an indirect evolutionary route. But science doesn’t deal in the currency of absolutes. Is that Behe’s problem or the Neo-Darwinian evolutionists’ problem? It’s the evolutionists’ problem because they claim these structures evolved by unguided mechanisms, and then they promote wildly speculative, perhaps even untestable indirect evolutionary pathways, to back that claim.
Behe’s view doesn’t require absolute falsification. His view is that a direct Darwinian pathway is effectively falsified by the presence of irreducible complexity, not absolutely falsified. Perhaps indirect routes are possible (in the same way that anything is possible), but possible does not mean plausible, or likely.
If the indirect evolutionary pathway is so unlikely or impractical that it would never occur in nature, then evolutionists have only escaped refutation by irreducible complexity by promoting a widely speculative and untestable hypothesis. In essence, they have put Darwinian evolution in an unfalsifiable position.
This has been Behe’s point all along: the inability to achieve 100% refutation of a wildly speculative indirect evolutionary scenario does not mean that irreducible complexity is invalid. To argue such is to hold irreducible complexity to an unreasonably high standard. To the contrary, the fact that evolutionists are forced to respond to Behe with such tenuous and speculative stories of indirect evolution shows the strength of the argument for irreducible complexity. Behe makes this precise point in Science and Evidence for Design in the Universe, Proceedings of the Wethersfield Institute:
[O]ne needs to relax Darwin’s criterion from this: “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.” to something like this:
(Michael Behe, “Answering Scientific Criticisms of Intelligent Design,” Science and Evidence for Design in the Universe, Proceedings of the Wethersfield Institute, Vol 9:146-147 (Ignatius Press, 2000).)
If a complex organ exists which seems very unlikely to have been produced by numerous, successive, slight modifications, and if no experiments have shown that it or comparable structures can be so produced, then maybe we are barking up the wrong tree. So, LET’S BREAK SOME RULES!”
http://creation.com/refuting-evolution-2-chapter-10-argument-irreducible-complexity
https://evolutionnews.org/2011/03/michael_behes_critics_make_dar/
https://www.allaboutscience.org/darwins-theory-of-evolution.htm
