Irreducible Complexity Demystified by Pete Dunkelberg
[Posted: 26 April 2003] "Evolution is cleverer than you are."
-biologists' proverb Introduction
The Argument That Irreducible Complexity Cannot Evolve
How Might Irreducible Complexity Evolve?
Irreducible Complexity in Nature
Venus' Flytrap
How to Eat Pentachlorophenol
Hemoglobin for the Active Life
The Blood Clotting System: is it IC?
Swimming Systems
The Eukaryote Cilium
The Archaeal Flagellum
The Bacterial Flagellum
IC Cores
How Does Irreducible Complexity Get Its Charm?
IC, ID, and Creationism
Conclusions
References
Introduction
 new term, irreducibly complex, (IC) has been introduced into public discussions of evolution. The term was defined by Michael Behe in 1996 in his book Darwin's Black Box: The Biochemical Challenge to Evolution ( 1). Irreducible complexity (also denoted IC) has gained prominence as the evidence for the intelligent design (ID) movement, which argues that life is so complicated that it must be the work of an intelligent designer (aka God) rather than the result of evolution. As you may have heard, the ID movement wants this taught in public schools as a new scientific theory. This essay will, I hope, prove helpful to any school teachers, boards of education, legislators and members of the press who may be wondering about it.
The argument from IC to ID is simply:
- IC things cannot evolve
- If it can't have evolved it must have been designed
This article just looks at the first part, the argument that irreducibly complex systems cannot be produced by evolution, either because they just can't evolve, or because their evolution is so improbable that the possibility can be ignored.
Let's take a look at the definition of IC, and then see if we can figure out its relation to evolution, and why scientists are so unimpressed. Here is the definition, from page 39 (page numbers refer to Darwin's Black Box unless otherwise noted):
"By irreducibly complex I mean a single system 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." [emphasis in original] IC is now a single defined term. The new definition, not the ordinary meaning of the words, is now our guide. IC refers to an organism doing something (the function) in such a way that the system (that portion of the organism that directly performs the function) has no more parts than are strictly necessary.
How do we decide when the term IC applies? Organisms don't come with parts, functions and systems labeled, nor are 'part', 'system' and 'function' technical terms in biology. They are terms of convenience. We might say, for instance, that the function of a leg is to walk, and call legs walking systems. But what are the parts? If we divide a leg into three major parts, removal of any part results in loss of the function. Thus legs are IC. On the other hand, if we count each bone as a part then several parts, even a whole toe, may be removed and we still have a walking system. We will see later that Behe's treatment of cilia and flagella follows this pattern.
What about the boundary of the system? This too is up to us. Take the digestive system for example. We may be interested only in the action of acids and enzymes in the stomach, or we may include saliva and chewing, or the lower intestine where some extraction of water and nutrients continues.
As a mental exercise, try before reading on to formulate an argument to prove that IC systems cannot evolve. IC is supposed to be the biochemical challenge to evolution, and thus the case when the parts are molecules, usually proteins, is the important case. So of course there may be multiple copies of a part. Losing a part means losing all copies of it, or at least so many that the function is effectively lost.
The Argument That Irreducible Complexity Cannot Evolve Behe's argument that IC cannot evolve is central to ID, so it deserves our attention. His method is to divide evolution into what he calls 'direct', which he defines in a special way, and 'indirect' (everything else). He finds that direct evolution of IC is logically impossible, and indirect evolution of IC is too improbable. The argument against 'direct' evolution of IC is contained in this long sentence right after the definition:
"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." The last part of the sentence, "...because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional." is why we should agree to the rest of the sentence. There are some problems:
- The first part of the sentence refers to slight changes. Removing a whole part is a major change;
this is a major 'disconnect' between the parts of Behe's argument. - It is not true that a precursor missing a part must be nonfunctional. It need only lack the function we specified. Even a single protein does something.
- The actual precursor may have had more parts, not fewer.
- If the individual parts evolve, the precursor may have had the same number of parts, not yet codependent. We will learn more about this possibility shortly.
How can one construct a valid argument that IC cannot be produced directly? ID proponents have not found a way. Yet it's easy (and left as an exercise for the reader) once you realize that a valid argument from definitions requires carefully defining the terms so that the argument becomes a tautology. This may be accomplished by redefining 'direct' or 'IC', or (best, I think) by defining Behe's expression 'be produced' which he uses in place of 'evolve'.
A precursor to IC lacking a part can have any functions except the specified one, which brings us to 'indirect' evolution. Consider a cow's tail. So far as I know, the main thing a cow uses its tail for is to swat flies. Did tails originally evolve for this function? Hardly. There were tails before there were flies. Long ago, tails helped early chordates to swim. Going back still farther, some very early animals started to have two distinct ends; one end for food intake (with sense organs for locating food) and the other end for excretions. As a consequence, this back end, and muscular extensions of it, could also be used to help the animal move. This illustrates yet another important facet of evolution: not only single mutations, but even large organs may begin more or less accidentally. It also illustrates that biological functions evolve. Indeed organisms and ecosystems evolve. It may not even make sense to expect a precursor to have had the same function.
The long term evolution of most features of life has not been what Behe, or indeed most people, would call direct. And even short term evolution can be indirect in Behe's terms. So it is surprising to read, on page 40, Behe's argument against indirect evolution of IC systems. Here is the crux of it:
"Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one can not definitely rule out the possibility of an indirect, circuitous route. As the complexity of an interacting system increases, though, the likelihood of such an indirect route drops precipitously." (page 40) He simply asserts that evolution of irreducible complexity by an indirect route is so improbable as to be virtually out of the question, except in simple cases. He makes no special connection between indirect evolution and IC. He offers no evidence. He just asserts that it is too improbable.
Actually, a more complex system probably has a long evolutionary history. Since evolution does not aim at anything in advance, the longer the history, the more circuitous it may be. And his very limited meaning of 'direct' renders much indirect that is not circuitous at all. Yet he insists:
"An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution." (page 39) Here's another exercise: before reading on, try to think of ways that IC systems, including biochemical ones, might evolve after all.
How Might Irreducible Complexity Evolve? How might an IC system evolve? One possibility is that in the past, the function may have been done with more parts than are strictly necessary. Then an 'extra' part may be lost, leaving an IC system. Or the parts may become co-adapted to perform even better, but become unable to perform the specified function at all without each other. This brings up another point: the parts themselves evolve. Behe's parts are usually whole proteins or even larger. A protein is made up of hundreds of smaller parts called amino acids, of which twenty different kinds may be used. Evolution usually changes these one by one. Another important fact is that DNA evolves. What difference does this make, compared to saying that proteins evolve? If you think about it, each protein that your body makes is made at just the right time, in just the right place and in just the right amount. These details are also coded in your DNA (with timing and quantity susceptible to outside influences) and so are subject to mutation and evolution. For our purposes we can refer to this as deployment of parts. When a protein is deployed out of its usual context, it may be co-opted for a different function. A fourth noteworthy possibility is that brand new parts are created. This typically comes from gene duplication, which is well known in biology. At first the duplicate genes make the same protein, but these genes may evolve to make slightly different proteins that depend on each other.
We can summarize these four possibilities this way:
- Previously using more parts than necessary for the function.
- The parts themselves evolve.
- Deployment of parts (gene regulation) evolves.
- New parts are created (gene duplication) and may then evolve.
The first of these only comes up if we are looking for IC. The others are the major forms of molecular evolution observed by biologists, phrased in terms of parts. They can lead to new protein functions, sometimes slowly and sometimes, especially when parts are redeployed, abruptly. Gene duplication and changes in protein deployment may introduce a new protein 'part' into a system. Then the parts may coevolve to do something better, but in a codependent manner so that all are required, without further change in the number of parts. But what happens in nature? |
