Irreducible Complexity: Definition & Evaluation
Defining by Analogy: To illustrate the principle of irreducible complexity that is caused by functional interdependencies between parts in a system, Michael Behe asks us to think about a common mousetrap with five interacting parts: a base, hammer, spring, catch, and holding bar. Each part is necessary, and there is no mouse-catching function unless all five parts are present. A trap with only four parts doesn't just catch mice poorly, it doesn't catch them at all.
Four Definitions of Irreducible Complexity
1. Michael Behe's Original Definition — [an irreducibly complex system is] "a single system composed of several well-matched, interacting parts that contribute to the basic function of the system, wherein the removal of any one of the parts causes the system to effectively cease functioning." (Darwin's Black Box, page 39, 1996)
2. William Dembski's Enhanced Definition — "A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, nonarbitrarily individuated parts such that each part in the set is indispensable to maintaining the system's basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system." (No Free Lunch, page 285, 2001)
3. Michael Behe's "Evolutionary" Definition — "An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway." (A Response to Critics of Darwin's Black Box, 2002)
4. My Revision of Behe's Original Definition — A system is irreducibly complex if there is no function for any system that is missing one part, i.e. if all "subsystems with one less part" are functionless. { This revision, suggested in 2001, corrects a minor error in Behe's original definition; the error does not affect the logic of claims about irreducible complexity if we use Definitions 2, 3 or 4. }
Irreducible Complexity is not just Complexity
To understand the difference between complexity that is and isn't irreducible, imagine a system with 20 enzymes (1, 2, 3,..., 19, 20) that evolves because it can be assembled one enzyme at a time, and at each stage there is a useful function that can be selected for: a 1-2 combination is functionally useful (so it offers an advantage and is selected for), but 1-2-3 is even better (so it is selected for), and then 1-2-3-4, and so on. This process of gradually building complexity, in a step-by-step process guided by natural selection, continues until the whole system, 1-2-3-....19-20, has naturally evolved. The 20-enzyme system is cumulatively complex but is not irreducibly complex.
By contrast, a system with 5 enzymes (ABCDE) is much less complex, but is irreducibly complex — according to Definitions 2 and 4 — if every subsystem with 4 enzymes (ABCD, ABCE, ABDE, ACDE, and BCDE) is nonfunctional. Because there is no functional 4-enzyme system, the evolution of ABCDE would require at least one unselected step in going from 3 to 4 enzymes, so ABCDE is also irreducibly complex according to the Evolutionary Definition (#3) which asks: Would it be difficult, or even impossible, for evolution to produce the 5-enzyme system of ABCDE in a step-by-step process of Darwinian natural selection?
http://dennisdjones.wordpress.com/2011/05/07/irreducible-complexity/
There exists in nature highly conserved and unevolved biochemical systems and protein structures that operate as machines for a specific intended purpose, assembled by parts of which any component of said parts would otherwise not exist but for serving its unique function as a member of the machine; and that machine would cease to operate upon the removal of any of those vital parts. The IC system cannot be reverse engineered or generated via bioengineering in a lab, and the degree of irreducible complexity is measurable by the number of unselected steps difference between the engine and its closest phylogenetic ancestral counterpart.
http://www.evolutionnews.org/2011/03/michael_behes_critics_make_dar044511.html
Behe's definition of irreducible complexity was always intended to test a Darwinian explanation where some function is built up gradually over time -- a direct evolutionary pathway. Boudry, Blancke and Braekman rightly observe that critics have claimed in response to Behe that indirect evolutionary pathways were possible.
Like their other bluffs, it might sound like the critics explained the origin of irreducible complexity -- but that's only true if such indirect pathways are plausible. In fact, Behe never "switches" arguments: somehow Boudry, Blancke and Braekman miss the conspicuous fact that Behe addresses this very objection on the very next page of Darwin's Black Box after he defines irreducible complexity. As Behe wrote:
Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one can not definitively 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. And as the number of unexplained, irreducibly complex biological systems increases, our confidence that Darwin's criterion of failure has been met skyrockets toward the maximum that science allows
(Michael Behe, Darwin's Black Box, p. 40 (Free Press, 1996).))
http://content.csbs.utah.edu/~rogers/evidevolcrs/ircomp/
The argument was first used to attack evolution by Gustave Cuvier in the early 19th century. As Cuvier put it,
The entirety of an organic being forms a coordinated whole, a unique and closed system, in which the parts mutually correspond and work together in the same specific action through a reciprocal relationship. None of these parts can change without the others changing as well. (Cuvier, 1831, p 59)
Cuvier's critique was influential in discouraging evolutionary thinking in the decades before 1859.
As Darwin went on to point out, a single function may be served by several organs, and a single organ may have several functions at the same time or at different times. This allows selection to construct organs that exhibit irreducible complexity. Darwin provided several examples. Modern evolutionists have discovered others.
Defining by Analogy: To illustrate the principle of irreducible complexity that is caused by functional interdependencies between parts in a system, Michael Behe asks us to think about a common mousetrap with five interacting parts: a base, hammer, spring, catch, and holding bar. Each part is necessary, and there is no mouse-catching function unless all five parts are present. A trap with only four parts doesn't just catch mice poorly, it doesn't catch them at all.
Four Definitions of Irreducible Complexity
1. Michael Behe's Original Definition — [an irreducibly complex system is] "a single system composed of several well-matched, interacting parts that contribute to the basic function of the system, wherein the removal of any one of the parts causes the system to effectively cease functioning." (Darwin's Black Box, page 39, 1996)
2. William Dembski's Enhanced Definition — "A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, nonarbitrarily individuated parts such that each part in the set is indispensable to maintaining the system's basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system." (No Free Lunch, page 285, 2001)
3. Michael Behe's "Evolutionary" Definition — "An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway." (A Response to Critics of Darwin's Black Box, 2002)
4. My Revision of Behe's Original Definition — A system is irreducibly complex if there is no function for any system that is missing one part, i.e. if all "subsystems with one less part" are functionless. { This revision, suggested in 2001, corrects a minor error in Behe's original definition; the error does not affect the logic of claims about irreducible complexity if we use Definitions 2, 3 or 4. }
Irreducible Complexity is not just Complexity
To understand the difference between complexity that is and isn't irreducible, imagine a system with 20 enzymes (1, 2, 3,..., 19, 20) that evolves because it can be assembled one enzyme at a time, and at each stage there is a useful function that can be selected for: a 1-2 combination is functionally useful (so it offers an advantage and is selected for), but 1-2-3 is even better (so it is selected for), and then 1-2-3-4, and so on. This process of gradually building complexity, in a step-by-step process guided by natural selection, continues until the whole system, 1-2-3-....19-20, has naturally evolved. The 20-enzyme system is cumulatively complex but is not irreducibly complex.
By contrast, a system with 5 enzymes (ABCDE) is much less complex, but is irreducibly complex — according to Definitions 2 and 4 — if every subsystem with 4 enzymes (ABCD, ABCE, ABDE, ACDE, and BCDE) is nonfunctional. Because there is no functional 4-enzyme system, the evolution of ABCDE would require at least one unselected step in going from 3 to 4 enzymes, so ABCDE is also irreducibly complex according to the Evolutionary Definition (#3) which asks: Would it be difficult, or even impossible, for evolution to produce the 5-enzyme system of ABCDE in a step-by-step process of Darwinian natural selection?
http://dennisdjones.wordpress.com/2011/05/07/irreducible-complexity/
There exists in nature highly conserved and unevolved biochemical systems and protein structures that operate as machines for a specific intended purpose, assembled by parts of which any component of said parts would otherwise not exist but for serving its unique function as a member of the machine; and that machine would cease to operate upon the removal of any of those vital parts. The IC system cannot be reverse engineered or generated via bioengineering in a lab, and the degree of irreducible complexity is measurable by the number of unselected steps difference between the engine and its closest phylogenetic ancestral counterpart.
http://www.evolutionnews.org/2011/03/michael_behes_critics_make_dar044511.html
Behe's definition of irreducible complexity was always intended to test a Darwinian explanation where some function is built up gradually over time -- a direct evolutionary pathway. Boudry, Blancke and Braekman rightly observe that critics have claimed in response to Behe that indirect evolutionary pathways were possible.
Like their other bluffs, it might sound like the critics explained the origin of irreducible complexity -- but that's only true if such indirect pathways are plausible. In fact, Behe never "switches" arguments: somehow Boudry, Blancke and Braekman miss the conspicuous fact that Behe addresses this very objection on the very next page of Darwin's Black Box after he defines irreducible complexity. As Behe wrote:
Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one can not definitively 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. And as the number of unexplained, irreducibly complex biological systems increases, our confidence that Darwin's criterion of failure has been met skyrockets toward the maximum that science allows
(Michael Behe, Darwin's Black Box, p. 40 (Free Press, 1996).))
http://content.csbs.utah.edu/~rogers/evidevolcrs/ircomp/
The argument was first used to attack evolution by Gustave Cuvier in the early 19th century. As Cuvier put it,
The entirety of an organic being forms a coordinated whole, a unique and closed system, in which the parts mutually correspond and work together in the same specific action through a reciprocal relationship. None of these parts can change without the others changing as well. (Cuvier, 1831, p 59)
Cuvier's critique was influential in discouraging evolutionary thinking in the decades before 1859.
As Darwin went on to point out, a single function may be served by several organs, and a single organ may have several functions at the same time or at different times. This allows selection to construct organs that exhibit irreducible complexity. Darwin provided several examples. Modern evolutionists have discovered others.
