By: Michael J. Behe
Discovery Institute
July 31, 2000


Introduction

In Darwin's Black Box: The Biochemical Challenge to Evolution I coined the term
"irreducible complexity" in order to point out an apparent problem for the
Darwinian evolution of some biochemical and cellular systems. In brief, an
irreducibly complex system is one that needs several well-matched parts, all
working together, to perform its function. The reason that such systems are
headaches for Darwinism is that it is a gradualistic theory, wherein
improvements can only be made step by tiny step,(1) with no thought for their
future utility. I argued that a number of biochemical systems, such as the blood
clotting cascade, intracellular transport system, and bacterial flagellum are
irreducibly complex and therefore recalcitrant to gradual construction, and so
they fit poorly within a Darwinian framework. Instead I argued they are best
explained as the products of deliberate intelligent design.

In order to communicate the concept to a general audience, I used a mousetrap as an example of an irreducibly complex system in everyday life. The mousetrap I
pictured in my book had a number of parts that all had to work together to catch
mice. The usefulness of the mousetrap example was that it captured the essence
of the problem I saw for gradualistic evolution at a level that could be
understood by people who were unfamiliar with the fine points of protein
structure and function--that is, nearly everyone. For that same reason,
defenders of Darwinism have assailed it. Although it may seem silly to argue
over a mousetrap, it is actually critical to allowing people who are not
professional scientists to understand the issues involved. In this article I
defend the mousetrap as an example of irreducible complexity that can't be put
together by a series of small, undirected steps.

Mousetrap rebuttals have popped up in a variety of situations including national
television, but most recently (June 2000) was at a conference I attended at
Concordia University in Wisconsin where Kenneth Miller, professor of biology at
Brown University, spent several minutes during his presentation attacking the
mousetrap. In doing so he used images of mousetraps that were drawn by Professor John McDonald of the University of Delaware and can be seen on his web site(2) (reproduced below with permission). In defense of the mousetrap I will make a number of points, including: (1) McDonald's reduced-component traps are not single-step intermediates in the building of the mousetrap I showed; (2) intelligence was intimately involved in constructing the series of traps; (3) if
intelligence is necessary to make something as simple as a mousetrap, we have
strong reason to think it is necessary to make the much more complicated
machinery of the cell.

Conceptual precursors vs. physical precursors

On his web site Professor McDonald was careful to make a critical distinction.
He clearly stated "the reduced-complexity mousetraps . . . are intended to point
out the logical flaw in the intelligent design argument; they're not intended as
an analogy of how evolution works." Nonetheless Kenneth Miller discussed
McDonald's examples in a way that would lead an audience to think that they were indeed relevant to Darwinian evolution. Only at the end of the presentation did he briefly mention the disanalogy. I believe such tactics are disingenuous at
best, like tagging a brief warning onto the end of a cigarette commercial
containing attractive images. The purpose of the images is to get you to buy the
cigarettes, despite the warning. The purpose of citing McDonald's drawings is to
get people to buy Darwinian evolution, despite the brief disclaimer.

The logical point Professor McDonald wished to make was that there are
mousetraps that can work with fewer parts than the trap I pictured in my book.
Let me say that I agree completely; in fact, I said so in my book (see below).
For example, one can dig a steep hole in the ground for mice to fall into and
starve to death. Arguably that has zero parts. One can catch mice with a glue
trap, which has only one part. One can prop up a box with a stick, hoping a
mouse will bump the stick and the box will fall on top of it. That has two
parts. And so forth. There is no end to possible variation in mousetrap design.
But, as I tried to emphasize in my book, the point that is relevant to Darwinian
evolution is not whether one can make variant structures, but whether those
structures lead, step-by-excruciatingly-tedious-Darwinian-step, to the structure
I showed. I wrote(3):

To feel the full force of the conclusion that a system is irreducibly complex
and therefore has no functional precursors we need to distinguish between a
physical precursor and a conceptual precursor. The trap described above is not
the only system that can immobilize a mouse. On other occasions my family has
used a glue trap. In theory at least, one can use a box propped open with a
stick that could be tripped. Or one can simply shoot the mouse with a BB gun.
However, these are not physical precursors to the standard mousetrap since they
cannot be transformed, step-by-Darwinian-step, into a trap with a base, hammer,
spring, catch, and holding bar.

Since I agree with Professor McDonald that there could be mousetraps with fewer parts, the only relevant question is whether the mousetraps he drew are physical precursors, or merely conceptual precursors. Can they "be transformed,
step-by-Darwinian-step" into the trap I pictured (essentially the same structure
as the fifth trap shown below), as some people have been led to believe? No,
they can't.

From the first trap to the second

Professor McDonald started with a complete mousetrap and then showed ones with fewer parts. I will reverse that order, start with his simplest trap, and show
the steps that would be necessary to convert it into the next more complex trap
in his series. That, after all, is the way Darwinian evolution would have to
work. If we are to picture this as a Darwinian process, then each separate
adjustment must count as a "mutation." If several separate mutations have to
occur before we go from one functional trap to the next, then a Darwinian
process is effectively ruled out, because the probability of getting multiple
unselected mutations that eventually lead to a specific complex structure is
prohibitive. Shown below are the simplest and next-to-simplest traps.

Figure 1. The first trap (top) and second trap (bottom).

The single-piece trap, consisting of just a spring with extended arms, is
supposed to have one arm, under tension, propped up on the other arm. When a
mouse jiggles it, the arm is released and comes down, pinning the mouse's paw
against the other arm. Now, the first thing to notice is that the single piece
trap isn't a simple spring--it's got a very specific structure. If the lengths
of the extended ends varied by much before their first bend, or if the angle of
the bends differed somewhat, the trap wouldn't work. What's more, the strength
of the material out of which the spring is made has to be consonant with the
purpose of catching a mouse (for example, if it were made from an old Slinky it
likely wouldn't work). It is not a simple starting point; it was intelligently
selected. Nonetheless, I realize that in coming up with an analogy we have to
start somewhere. So I will not complain about an intelligently-selected starting
point. However, the involvement of intelligence at any other point along the way
invalidates the entire exercise as an analogy to a Darwinian process. Because
Darwinism wholly rejects intelligent direction, Darwinists must agree that the
involvement of intelligence at any point in a scenario (after the agreed-on
starting point) is fatal. That point occurs immediately for our mousetrap.

The second mousetrap (above) has a spring and a platform. One of the extended
arms stands under tension at the very edge of the platform. The idea is that if
a mouse in the vicinity jiggles the trap, the end of the arm slips over the edge
and comes rushing down, and may pin the mouse's paw or tail against the
platform. Now, the first thing to notice is that the arms of the spring are in a
different relationship to each other than in the first trap. To get to the
configuration of the spring in the second trap from the configuration in the
first, it seems to me one would have to proceed through the following steps (4):
(1) twist the arm that has one bend through about 90* so that the end segment is
perpendicular to the axis of the spring and points toward the platform; (2)
twist the other arm through about 180* so the first segment is pointing opposite
to where it originally pointed (the exact value of the rotations depend on the
lengths of the arms); (3) shorten one arm so that its length is less than the
distance from the top of the platform to the floor (so that the end doesn't
first hit the floor before pinning the mouse). While the arms were being rotated
and adjusted, the original one-piece trap would have lost function, and the
second trap would not yet be working.

At this point we bring in a new piece, the platform, which is a simple piece of
wood. One now has a spring resting on top of a platform. However, the spring
cannot be under tension in this configuration unless it is fixed in place.
Notice that in the second mousetrap, not only has a platform been added, but two
(barely visible) staples have been added as well. Thus we have gone not from a
one piece to a two-piece trap, but from a one to a four piece trap. Two staples
are needed; if there were only one staple positioned as drawn, the tensed spring
would be able to rotate out of position. The staples have to be positioned
carefully with respect to the platform. They have to be arranged within a very
narrow tolerance so that one arm of the spring teeters perilously on the edge of
the platform or the trap doesn't work. If either of the staples is moved
significantly from where they are drawn, the trap won't function. I should add
that I did not emphasize the staples in my book because I was trying to make a
simple point and didn't want to exhaust the readers with tedium. However,
someone who wishes to seriously propose that the mousetrap I pictured is
approachable in the tiny steps required by Darwinian processes would indeed have to deal with all the details, including the staples.

It is important to remember that the placement, size, shape, or any important
feature (not just "piece") of a system can't just be chosen to fit the purposes
of a person who wishes to simulate a Darwinian process. Rather, each significant
feature has to be justified as being a small improvement. In the real world the
occasional unselected feature might occur which serendipitously will be useful
in the future, but invoking more than one unselected (neutral, nonadaptive)
event in a Darwinian scenario seems to me impermissible because the
improbability of the joint events starts to soar. In our current case the
unselected event we are allowed was used up when we began with a special
starting point.

I think the problems of rearranging the already-functioning first mousetrap
shows the general difficulties one expects in trying to re-arrange an
already-functioning system into something else. The requirements ("selection
pressures") that make a component suitable for one specialized system will
generally make it unsuitable for another system without significant
modification. Another problem we can note is that the second mousetrap is not an
obvious improvement over the first; it is difficult to see how it would function
any better than the one-piece trap. It's just that it's on the road to where we
want to see the system end up--on the road to a distant target. That, of course,
is intelligent direction.

The transition from the first to the second mousetrap is not analogous to a
Darwinian process because: (1) a number of separate steps are required to make
the transition; (2) each step has to fall within a narrow range of tolerance to
get to the target trap; and (3) function is lost until the transition is
completed. In fact, the situation of going from the first trap to the second
trap is best viewed not as a transition, but as building a different kind of
trap using some old materials from the first trap (with major modifications) and
some new materials. Far from being an analogy to a Darwinian process, the
construction of the second trap is an example of intelligent design.

From the second trap to the third

The way the traps are drawn (below), the transition from the second to the third
trap doesn't seem to be a big step. Both drawings are superficially similar. But
when one thinks about the transition in detail problems crop up. The first
problem is that a new piece is added--the hammer. Unlike the platform that was
added in the last transition (which I did not object to), the hammer is not a
simple object. Rather it contains several bends. The angles of the bends have to
be within relatively narrow tolerances for the end of the hammer to be
positioned precisely at the edge of the platform, otherwise the system doesn't
work. For the same reason, the length of the second segment of the hammer has to be within a narrow range of values. How does the hammer get into the third trap? It would seem that the extended arm of the second trap has to be held up while the newly-fashioned hammer is inserted through the tunnel of the spring. Thus an intelligent agent has to actively push parts around to get to the configuration of the third trap. Again, there is no obvious improvement in function of the third trap compared to the second or first. Both the second and third traps
appear to do the same thing as the first, but require more parts. Such an event
is not expected in a Darwinian scenario. It seems the only reason they attract
our attention is because they appear to be along the path we wish the process
would go. That is intelligent design.

Figure 2. The second trap (top) and third trap (bottom).

From the third trap to the fourth

Going from the third trap to the fourth requires major rearrangements. The
hammer is bent, lengthened, and an extra segment is added to it. Two new pieces
are added: the "hold-down bar" and a staple to hold down the hold-down bar. The end of the hold-down bar is endowed with a closed curl so that the staple has something to hang on to. The staple again has to be positioned in a specific
region of the trap. Depending on details, this configuration may be an
improvement over the first three traps because it appears that, depending on the
tension of the spring, the trap could kill a mouse outright, rather than just
pinning it (yet that feature could probably easily be built into the earlier
versions). On the other hand the arm of the spring is now being pushed through a
much greater displacement in the fourth trap than previous versions. It seems
unlikely a spring optimized for use in earlier traps would work well in the
fourth trap (unless of course we are "looking ahead"). Rather than a
"transition," this process is again better viewed as building a new trap using
refashioned parts of the old trap plus new ones. This is intelligent design.

Figure 3. The third trap (top) and fourth trap (bottom).

From the fourth trap to the fifth

This is left as an exercise for the reader.

Figure 4. The fourth trap (top) and fifth trap (bottom).

Discussion

I have to admit that even I find it tedious to discuss mousetraps in such
excruciating detail. But the critical point is that that is exactly the level at
which Darwinian evolution would have to work in the cell. Every relevant detail
has to fit or the system fails. If an arm is too long or an angle not right or a
staple placed incorrectly, the mouse dances free. If you want to get to a
certain system, but the road there isn't a series of continual improvements,
Darwinism won't take you there. It's important for those interested in these
issues to realize that, when evaluating descriptive evolutionary scenarios (as
opposed to experiments--see below), one has to attend to the tiniest details (as
I did here) to see if intelligence is directing the show. On the other hand, if
one doesn't pay the strictest attention, Darwinian scenarios look much more
plausible because one sees only the possibilities, not the problems. It's easy
for a speaker to persuade an audience that the McDonald mousetraps represent a
series of Darwinian intermediates on the way to a standard trap--that they show
irreducible complexity is no big deal. All one has to do is gloss over the
difficulties. But although our minds can skip over details, nature can't.

In the real world of biology the staples, bends, and so forth would be features
of molecules, of proteins in particular. If two proteins don't bind each other
in the correct orientation (aren't stapled right), if they aren't placed in the
right positions, if their new activity isn't regulated correctly, if many
details aren't exactly correct, then the putative Darwinian pathway is blocked.
Now, it's hard, almost impossible, for persons without the appropriate science
background to tell where such difficulties would occur in Darwinian scenarios
for blood clotting or ciliary function or other biological systems. When they
read Darwinian stories in a book or hear them in lectures, they generally have
no independent information to judge the scenario. In such a situation one should
ask oneself, "If a simple mousetrap requires intelligent design, what is the
likelihood that the much more complicated molecular machines of the cell could
be built step-by-tiny-Darwinian-step?" Keeping that question in mind will foster
a healthy skepticism toward optimistic scenarios.

Why do the McDonald mousetraps look persuasive to some people? Certainly one reason is the way they are drawn. Drawings of four of the five traps are
dominated by the image of the large rectangular platform and prominent spring in
the center. That makes them all look pretty much the same. The staples are
barely visible and the various metal bars protruding here and there seem like
insignificant details. In fact, they are critical. Another reason is that the
scenario starts with the completed mousetrap. Any question about the placement
of the parts, their size, stiffness, and so on doesn't easily arise because the
parts were already placed where they needed to be for the ultimate goal in the
original drawing (that is, the fifth mousetrap here, which is the first drawing
in McDonald's series) and their properties could be inferred from the fact we
started with a working trap. The universe of possibilities was tightly but
implicitly circumscribed by the already-completed starting point. A third reason
it seems persuasive is that the series is always presented as parts being
removed from the complete mousetrap. Looking at it in such a backward
manner--the reverse of what evolution would have to do--obscures the teleology
of the building process. Going in a forward direction there is strong reason to
think we would not end up at the fifth mousetrap when starting from the first,
because the first works as well as the second and third, so greater complexity
would be disfavored. In going backwards, however, lesser complexity is favored
so it seems "natural" to move to simpler traps. Yet Darwinian evolution can't
work like that.

A final reason for the persuasiveness of the example we can call the "Clever
Hans effect." Clever Hans was the name of a horse who seemed to be pretty good at arithmetic. Its owner would give Hans a simple math problem such as 5+5, and the horse would stamp his hoof ten times, then stop. It eventually turned out that Clever Hans could pick up unconscious cues from its owner, who might raise his eyebrows or tilt his head when the horse's stamping reached the right value. The horse could even pick up unintentional cues from other people, not just the owner, who also apparently gave telltale reactions. In the case of Clever Hans, the human intelligence of the owner was inadvertently attributed to the horse. In my experience the same is invariably true of Darwinian scenarios--human intelligence is critical to guiding the scenario through difficulties toward the
"proper" goal, but the intelligence is then attributed to natural selection. As
with Clever Hans, the guidance is usually unconscious, but is intelligent
nonetheless.

Clever Hans was exposed as mathematically clueless by carefully controlled
experiments. To see whether natural selection can work wonders on its
own--without the aid of human intelligence-- we also have to do carefully
controlled experiments. One way to do this is to ask bacteria in the laboratory
if they can evolve irreducibly complex biochemical systems. (Kenneth Miller has
called this the "acid test.") Bacteria are a good choice because they can be
grown in huge numbers with short generation times--just what Darwinian evolution needs. However, when this was repeatedly tried over the course of 25 years for bacteria missing a comparatively simple biochemical system (called the "lac operon") natural selection came up empty (see "The Acid Test" on this website). It could make the small changes typically termed "microevolution," but whenever it had to do a couple things at once, such as would have to be done to make irreducibly complex systems, it got stuck.(5) Like Clever Hans on his own,
natural selection seems to have much less intelligence than we had given it
credit for. There is currently no experimental evidence to show that natural
selection can get around irreducible complexity.

Darwinian scenarios, either for building mousetraps or biochemical systems, are
very easy to believe if we aren't willing or able to scrutinize the smallest
details, or to ask for experimental evidence. They invite us to admire the
intelligence of natural selection. But the intelligence we are admiring is our
own.


Endnotes

1. One has to be sophisticated about what is regarded as a "step." One
mutational step in a biological organism might seem to have large effects, such
as the famous antennapedia mutation in fruit flies. Although such a change may
impress us, it involves only the rearrangement of existing structures; no new
structures are made. When thinking about what's involved in making a new
structure it's best to think of how many lines of instructions (analogous to
lines of computer code) would be needed to build it. See my discussion of this
topic in Darwin's Black Box, pp. 39-41.

2. http://udel.edu/~mcdonald/mousetrap.html

3. Behe, M. J. (1996). Darwin's black box: the biochemical challenge to
evolution. (The Free Press: New York), p. 43.

4. To play the game right, one has to compare the probability of these events
happening with the probability of any slight "mutation" happening. To give a
flavor of what that might mean, a mutation might involve bending the spring in
the middle, changing the size of the platform, changing the tension on the
spring, extending the end of a metal piece, and so on. A crude feel for the
probabilities of the events can be obtained by examining the precision a feature
must have for the trap to work. To get the probability for two or more
unselected events (ones that don't improve the function), one multiplies the
probabilities for each.

5. The work has been done by Barry Hall of the University of Rochester. He could get selection to replace one piece of the lac operon (the B-galactosidase) but had to intelligently intervene to keep the bacteria alive by adding the
artificial chemical induce IPTG. The bacteria could not replace two required
protein parts deleted simultaneously, showing the severe problem of irreducible
complexity. For a review of his work see: Hall, B. G. (1999). Experimental
evolution of Ebg enzyme provides clues about the evolution of catalysis and to
evolutionary potential. FEMS Microbiology Letters 174, 1-8.