Irreducible Complexity (IC)
is a term invented by Michael Behe in his book 'Darwin’s Black Box', 1996.
Following Behe's own words
an IC is defined as follows:
complex I mean 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
The idea was immediately
challenged by H. Allan Orr in his book ‘Darwin v. Intelligent Design (Again)',
complex system can be built gradually by adding parts that, while initially
just advantageous, become - because of later changes - essential. The logic is
very simple. Some part (A) initially does some job (and not very well,
perhaps). Another part (B) later gets added because it helps A. This new part
isn’t essential, it merely improves things. But later on, A (or something else)
may change in such a way that B now becomes indispensable. This process
continues as further parts get folded into the system. And at the end of the
day, many parts may all be required."
In 2001, Behe responds to
this critique by changing his definition slightly. He now defines an IC as:
"... a single system
which is necessarily composed of several well-matched..."
and claims that this solves
What Behe fails to
recognize is the possibility of a change in function. The following scenario
can illustrate this:
A system has evolved in the way Orr proposes, Then some change in one or more
of the parts changes the function of the system in a way so that the new system
could not work unless build by 'several well-matched interacting parts'.
I will offer an example
connected to Behe's pet example of an IC: The bacterial flagellum.
The theoretical minimum flagellum has only three parts: A Stator anchoring it
to the membrane; A Rotor producing the rotation relative to the Stator and a
Filament rotated by the rotor and acting as a propeller. Therefor it is by Behe’s
new definition, an IC.
The evolution of such a system could look as follows:
- A membrane protein complex, A, made of several identical units serves the function
of a pore letting some molecules sieve in or out of the cell. This protein
complex is anchored to the membrane.
- Another protein complex, B, which fits within the first, and also made of
several identical units, enhances the specificity of the pore. B is loosely
anchored to A.
- B mutates resulting in rotation of B relative to A, and therefor relative to
the membrane. This adds to the efficiency or specificity of the pore.
- B mutates resulting in a change from passive pore to active pump.
- B mutates resulting in a change in what kind of molecules are pumped out of
the cell. The new molecules stick together when pumped out, thereby forming a
The result of this (admitted
highly speculative) scenario is a minimal flagellum that then can start its own
evolution towards higher and higher efficiency through the mechanism described
It should be noted that the term 'The bacterial flagellum' is somewhat incorrect, as several different structures exist. It should be 'The bacterial flagella' (in plural.)´.
A specific example of the evolution of an irriducbly complex system is found in Nature (vol 481 s. 270) and illustrated by this figure.
a) Shows a ring structure made up of 8 identical proteins.
A duplication result in two identical copies of the gene coding for the protein,. Mutations result in two different proteins, fitting together at random in the ring structure.
c) Futher mutations in one or both copies result in the formation of only one specific heteropolymer.
As a result we have an irriducibly complex structure. If any of the two copies of the gene mutated so that the proteins no longer fitted together, the structure wouldn't form.
List of references:
Behe, M.J.: 1996, Darwin’s Black Box: The Biochemical Challenge to Evolution, Free Press, New York
Behe M.J.: 2001, Reply to My Critics: A Response to Reviews of Darwin’s Black Box: The Biochemical Challenge to Evolution, Biology and Philosophy 16: 685-709, 2001
Orr,H.A.:1996-1997, ‘Darwin v. Intelligent Design (Again)’, Boston Review Dec/Jan,28-31.