Irreducible complexity
Irreducible complexity is the argument that certain biological systems with multiple interacting parts would not function if one of the parts were removed, so supposedly could not have evolved by successive small modifications from earlier less complex systems through natural selection, which would need all intermediate precursor systems to have been fully functional. This negative argument is then complemented by the claim that the only alternative explanation is a "purposeful arrangement of parts" inferring design by an intelligent agent. Irreducible complexity has become central to the creationist concept of intelligent design, but the concept of irreducible complexity has been rejected by the scientific community at large, which regards intelligent design as pseudoscience. Irreducible complexity and specified complexity are the two main arguments used by intelligent-design proponents to support their version of the theological argument from design.
The central concept that complex biological systems which require all their parts to function could not evolve by the incremental changes of natural selection, and so must have been produced by an intelligence, was already featured in creation science. The 1989 school textbook Of Pandas and People introduced the alternative terminology of intelligent design, a revised section in the 1993 edition of the textbook argued that a blood-clotting system demonstrated this concept.
This section was written by Michael Behe, a professor of biochemistry at Lehigh University. He subsequently introduced the expression irreducible complexity along with a full account of his arguments, in his 1996 book Darwin's Black Box, and said it made evolution through natural selection of random mutations impossible, or extremely improbable. This was based on the mistaken assumption that evolution relies on improvement of existing functions, ignoring how complex adaptations originate from changes in function, and disregarding published research. Evolutionary biologists have published rebuttals showing how systems discussed by Behe can evolve.
In the 2005 Kitzmiller v. Dover Area School District trial, Behe gave testimony on the subject of irreducible complexity. The court found that "Professor Behe's claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large."
Definitions
defined irreducible complexity in natural selection in terms of well-matched parts in his 1996 book Darwin's Black Box:... 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.
A second definition given by Behe in 2000 states:
An irreducibly complex evolutionary pathway is one that contains one or more unselected steps. The degree of irreducible complexity is the number of unselected steps in the pathway.
Intelligent-design advocate William A. Dembski assumed an "original function" in his 2002 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.
History
Forerunners
The argument from irreducible complexity is a descendant of the teleological argument for God. This states that complex functionality in the natural world which looks designed is evidence of an intelligent creator. William Paley famously argued, in his 1802 watchmaker analogy, that complexity in nature implies a God for the same reason that the existence of a watch implies the existence of a watchmaker. This argument has a long history, and one can trace it back at least as far as Cicero's De Natura Deorum ii.34, written in 45 BC.Up to the 18th century
wrote about the large number of parts of the body and their relationships, which observation was cited as evidence for creation. The idea that the interdependence between parts would have implications for the origins of living things was raised by writers starting with Pierre Gassendi in the mid-17th century and by John Wilkins, who wrote, "Now to imagine, that all these things, according to their several kinds, could be brought into this regular frame and order, to which such an infinite number of Intentions are required, without the contrivance of some wise Agent, must needs be irrational in the highest degree." In the late 17th-century, Thomas Burnet referred to "a multitude of pieces aptly joyn'd" to argue against the eternity of life. In the early 18th century, Nicolas Malebranche wrote "An organized body contains an infinity of parts that mutually depend upon one another in relation to particular ends, all of which must be actually formed in order to work as a whole", arguing in favor of preformation, rather than epigenesis, of the individual; and a similar argument about the origins of the individual was made by other 18th-century students of natural history. In his 1790 book, The Critique of Judgment, Kant is said by Guyer to argue that "we cannot conceive how a whole that comes into being only gradually from its parts can nevertheless be the cause of the properties of those parts".19th century
Chapter XV of Paley's Natural Theology discusses at length what he called "relations" of parts of living things as an indication of their design.Georges Cuvier applied his principle of the correlation of parts to describe an animal from fragmentary remains. For Cuvier, this related to another principle of his, the conditions of existence, which excluded the possibility of transmutation of species.
While he did not originate the term, Charles Darwin identified the argument as a possible way to falsify a prediction of the theory of evolution at the outset. In The Origin of Species, he wrote, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case." Darwin's theory of evolution challenges the teleological argument by postulating an alternative explanation to that of an intelligent designer—namely, evolution by natural selection. By showing how simple unintelligent forces can ratchet up designs of extraordinary complexity without invoking outside design, Darwin showed that an intelligent designer was not the necessary conclusion to draw from complexity in nature. The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of Darwinian evolution.
In the late 19th century, in a dispute between supporters of the adequacy of natural selection and those who held for inheritance of acquired characteristics, one of the arguments made repeatedly by Herbert Spencer, and followed by others, depended on what Spencer referred to as co-adaptation of co-operative parts, as in:
"We come now to Professor Weismann's endeavour to disprove my second thesis—that it is impossible to explain by natural selection alone the co-adaptation of co-operative parts. It is thirty years since this was set forth in 'The Principles of Biology.' In § 166, I instanced the enormous horns of the extinct Irish elk, and contended that in this and in kindred cases, where for the efficient use of some one enlarged part many other parts have to be simultaneously enlarged, it is out of the question to suppose that they can have all spontaneously varied in the required proportions."Darwin responded to Spencer's objections in chapter XXV of The Variation of Animals and Plants Under Domestication. The history of this concept in the dispute has been characterized: "An older and more religious tradition of idealist thinkers were committed to the explanation of complex adaptive contrivances by intelligent design.... Another line of thinkers, unified by the recurrent publications of Herbert Spencer, also saw co-adaptation as a composed, irreducible whole, but sought to explain it by the inheritance of acquired characteristics."
St. George Jackson Mivart raised the objection to natural selection that "Complex and simultaneous co-ordinations... until so far developed as to effect the requisite junctions, are useless". In the 2012 book Evolution and Belief, Confessions of a Religious Paleontologist, Robert J. Asher said this "amounts to the concept of 'irreducible complexity' as defined by... Michael Behe".
20th century
, in the early 20th century, discussed a concept similar to irreducible complexity. However, far from seeing this as a problem for evolution, he described the "interlocking" of biological features as a consequence to be expected of evolution, which would lead to irreversibility of some evolutionary changes. He wrote, "Being thus finally woven, as it were, into the most intimate fabric of the organism, the once novel character can no longer be withdrawn with impunity, and may have become vitally necessary."In 1975 Thomas H. Frazzetta published a book-length study of a concept similar to irreducible complexity, explained by gradual, step-wise, non-teleological evolution. Frazzetta wrote:
"A complex adaptation is one constructed of several components that must blend together operationally to make the adaptation 'work'. It is analogous to a machine whose performance depends upon careful cooperation among its parts. In the case of the machine, no single part can greatly be altered without changing the performance of the entire machine."The machine that he chose as an analog is the Peaucellier–Lipkin linkage, and one biological system given extended description was the jaw apparatus of a python. The conclusion of this investigation, rather than that evolution of a complex adaptation was impossible, "awed by the adaptations of living things, to be stunned by their complexity and suitability", was "to accept the inescapable but not humiliating fact that much of mankind can be seen in a tree or a lizard."
In 1985 Cairns-Smith wrote of "interlocking": "How can a complex collaboration between components evolve in small steps?" and used the analogy of the scaffolding called centering—used to build an arch then removed afterwards: "Surely there was 'scaffolding'. Before the multitudinous components of present biochemistry could come to lean together they had to lean on something else." However, neither Muller or Cairns-Smith claimed their ideas as evidence of something supernatural.
An early concept of irreducibly complex systems comes from Ludwig von Bertalanffy, an Austrian biologist. He believed that complex systems must be examined as complete, irreducible systems in order to fully understand how they work. He extended his work on biological complexity into a general theory of systems in a book titled General Systems Theory.
After James Watson and Francis Crick published the structure of DNA in the early 1950s, General Systems Theory lost many of its adherents in the physical and biological sciences.
However, systems theory remained popular in the social sciences long after its demise in the physical and biological sciences.