Some biochemical systems are irreducibly complex, meaning that the removal
of any one part of the system destroys the system's function. Irreducible
complexity rules out the possibility of a system having evolved, so it
must be designed.
Behe, Michael J. 1996. Darwin's Black Box, New York: The Free Press.
- Irreducible complexity can evolve. It is defined as a system that
loses its function if any one part is removed, so it only indicates
that the system did not evolve by the addition of single parts with no
change in function. That still leaves several evolutionary mechanisms:
- deletion of parts
- addition of multiple parts; for example, duplication of much or all
of the system (Pennisi 2001)
- change of function
- addition of a second function to a part (Aharoni et al. 2004)
- gradual modification of parts
All of these mechanisms have been observed in genetic mutations. In
particular, deletions and gene duplications are fairly common (Dujon et
al. 2004; Hooper and Berg 2003; Lynch and Conery 2000), and together
they make irreducible complexity not only possible but expected. In
fact, it was predicted by Nobel-prize-winning geneticist Hermann Muller
almost a century ago (Muller 1918, 463-464). Muller referred to it as
interlocking complexity (Muller 1939).
Evolutionary origins of some irreducibly complex systems have been
described in some detail. For example, the evolution of the Krebs
citric acid cycle has been well studied (Meléndez-Hevia et
al. 1996), and the evolution of an "irreducible" system of a
hormone-receptor system has been elucidated (Bridgham et al. 2006).
Irreducibility is no obstacle to their formation.
- Even if irreducible complexity did prohibit Darwinian evolution, the
conclusion of design does not follow. Other
have produced it. Irreducible complexity is an example of a failed
argument from incredulity.
- Irreducible complexity is poorly defined. It is defined in terms of
parts, but it is far from obvious what a "part" is. Logically, the
parts should be individual atoms, because they are the level of
organization that does not get subdivided further in biochemistry, and
they are the smallest level that biochemists consider in their
analysis. Behe, however, considered sets of molecules to be individual
parts, and he gave no indication of how he made his determinations.
- Systems that have been considered irreducibly complex might not be.
- The mousetrap that Behe used as an example of irreducible complexity
can be simplified by bending the holding arm slightly and removing the
- The bacterial flagellum is not irreducibly
it can lose many parts and still function, either as a simpler
flagellum or a secretion system. Many proteins of the eukaryotic
flagellum (also called a cilium or undulipodium) are known to be
dispensable, because functional swimming flagella that
lack these proteins are known to exist.
- In spite of the complexity of Behe's protein
example, there are other proteins for which no transport is necessary
(see Ussery 1999 for references).
- The immune system example that Behe includes
irreducibly complex because the antibodies that mark invading cells
for destruction might themselves hinder the function of those cells,
allowing the system to function (albeit not as well) without the
destroyer molecules of the complement system.
TalkOrigins Archive. n.d. Irreducible complexity and Michael Behe.
- Aharoni, A., L. Gaidukov, O. Khersonsky, S. McQ. Gould, C. Roodveldt
and D. S. Tawfik. 2004. The 'evolvability' of promiscuous protein
functions. Nature Genetics [Epub Nov. 28 ahead of print]
- Bridgham, Jamie T., Sean M. Carroll and Joseph W. Thornton. 2006.
Evolution of hormone-receptor complexity by molecular exploitation.
Science 312: 97-101. See also Adami, Christopher. 2006. Reducible
complexity. Science 312: 61-63.
- Dujon, B. et al. 2004. Genome evolution in yeasts.
Nature 430: 35-44.
- Hooper, S. D. and O. G. Berg. 2003. On the nature of gene innovation:
Duplication patterns in microbial genomes.
Molecular Biololgy and Evolution 20(6): 945-954.
- Lynch, M. and J. S. Conery. 2000. The evolutionary fate and
consequences of duplicate genes. Science 290: 1151-1155. See also
Pennisi, E., 2000. Twinned genes live life in the fast lane.
Science 290: 1065-1066.
- Meléndez-Hevia, Enrique, Thomas G. Waddell and Marta
Cascante. 1996. The puzzle of the Krebs citric acid cycle: Assembling
the pieces of chemically feasible reactions, and opportunism in the
design of metabolic pathways during evolution. Journal of Molecular
Evolution 43(3): 293-303.
- Muller, Hermann J. 1918. Genetic variability, twin hybrids and
constant hybrids, in a case of balanced lethal factors. Genetics 3:
- Muller, H. J. 1939. Reversibility in evolution considered from the
standpoint of genetics. Biological Reviews of the Cambridge
Philosophical Society 14: 261-280.
- Pennisi, Elizabeth. 2001. Genome duplications: The stuff of evolution?
Science 294: 2458-2460.
- Ussery, David. 1999. A biochemist's response to "The biochemical
challenge to evolution". Bios 70: 40-45.
Gray, Terry M.. 1999. Complexity--yes! Irreducible--maybe!
Unexplainable--no! A creationist criticism of irreducible complexity.
Lindsay, Don. 1996. Review: "Darwin's black box, the biochemical challenge
to evolution" by Michael Behe.
Miller, K. 1999. Finding Darwin's God. Harper-Collins, chap. 5.
Shanks, N. and K. H. Joplin. 1999. Redundant complexity: A
analysis of intelligent design in biochemistry. Philosophy of Science
Ussery, David. 1999. (see above)
created 2001-2-17, modified 2007-7-19