Irreducible complexity
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[edit] Definition
According to the biochemist Michael Behe, who invented the phrase, a system displays irreducible complexity if it is:
- 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.
People who use the phrase are usually creationists who maintain that the existence of irreducibly complex biological systems could not be accounted for by means of evolution, and so use their existence as a form of the argument from design.
[edit] Origins
Although the Argument from Design has a lengthy intellectual history, the argument for Irreducible Complexity can be said to be fairly recent, going back to Michael Behe's 1996 publication of Darwin's Black Box: The Biochemical Challenge to Evolution.
[edit] Discussion
The flaw in the argument seems obvious enough: the fact that a biological system is irreducibly complex doesn't mean it can't have evolved; it just means that it can't have evolved by a process the last stage of which was one of the parts suddenly popping into existence. But of course, this is not how evolution works. To quote professor of philosophy and zoology Dr. Michael Ruse:
- No one is denying that in natural processes there may well be parts which, if removed, would lead at once to the non-functioning of the systems in which they occur. The point however is not whether the parts now in place could not be removed without collapse, but whether they could have been put in place by natural selection. [1]
[edit] Example: The Mammalian Ear
Take for example the bones of the mammalian ear. The stapes, malleus and incus form a chain conducting sound. If you remove just one, the remaining structure becomes useless.
If you ask biologists how this structure evolved, they will tell you that redundant anterior gill arches in agnathan fish evolved into the jaw bones of bony fish; that a lineage of bony fish evolved into amphbians; that amphibians "listened" to the vibrations in the ground through their jaw muscles; that the evolution of a jaw joint allowed the hyomandibula to gradually lose its function as a jaw support and take on a role as a conductor of sound; how the upper end of the hyomandibula separated from it to become the columella, how the columella was the precursor of the reptilian stapes; that reptiles have only the stapes as an inner ear bone; and that parts of the reptile jaw gradually change roles from jawbones to organs of hearing in mammals, a fact that they can illustrate with a beautiful series of intermediate forms (some of which may be found here); and in particular how the reptilian articular evolved into the mammalian malleus, and how the quadrate evolved into the incus.
Now you may believe that or not, but you will note that at no point is it claimed that some part just pops into being; the last stage in the process is not the wholesale addition of the stapes, the malleus, or the incus. Instead, the whole process takes place by a process of the adaptation of redundant parts, of one part fulfilling two functions, of gradual adaptation of function, of co-evolution of parts --- in short, by evolutionary mechanisms which have been well understood ever since the publication of the Origin of Species. The fact that the middle ear is irreducibly complex has no bearing on whether it could have evolved as it is claimed to have evolved, that is, by actual evolutionary mechanisms, not by parts appearing out of nowhere by magic.
[edit] Example: obligate symbiosis
Perhaps the simplest example of irreducible complexity in nature is a symbiotic relationship which is obligate for both parties. The process of co-adaptation by which such relationships evolve from a relationship which is neither obligate nor symbiotic is obvious to everyone with the barest modicum of biological knowledge, and as such is completely incomprehensible to creationists. For information on the evolution of symbiotic relationships, see the main article on Symbiosis.
[edit] Example: The Venus Fly-Trap
A Venus Fly Trap is sometimes claimed to be irreducibly complex: it has many co-adapted features include producing a sweet smell to attract the insects, having leaves that close on an insect, having sensing hairs that detect movement, fine teeth on the leaves that prevent small insects from escaping, and digestive enzymes to consume use of the insect. Some creationists claim that this system fits Behe's definition of irreducible complexity, as any removal of these parts would destroy the bug-eating function of the plant altogether.
It is not clear that this is so. If you count the "function" of these features as "doing exactly what a Venus fly-trap does", then they are all necessary to this end. If, on the other hand, we define their function as "catching flies", then it is clear that some of these features, though useful, are not essential to this function, and that they cannot be considered an irreducibly complex system. Other plants, such as pitcher plants[2] manage to catch flies without having all the specializations of a Venus fly-trap.
The following account (summarized from Irreducible Complexity Demystified) has been offered to account for the evolution of the Venus Fly-trap:
- In the beginning there were plants in nitrogen-deficient soil
- Then, one of those plants produced from its leaves a sticky substance that captured insects, where their decomposed and dissolved remains contributed to enrich the nitrogen as well as protecting the plant from being eaten before it had time to reproduce (good selective advantage)
- An improvement can be made in which the plant, if it doesn't already smell nice, produces a sweet smell to attract more insects to eat
- Then, in some plants, it formed a dish or tube shape from its leaves to trap more insects more efficiently
- In those plants, the leaves would then form in such a way that they close around insects in case some insects escaped before being rendered totally immobile (especially large insects). All plants, it should be noted, have some power of movement, and many plants open and close their leaves and petals. A visual illustration of steps 3 and 4 can be found here.
- The plant is already a pretty effective killing machine, and as the leaves closing around the insect accomplishes the same task as holding the insect in place with sticky glue, it would be beneficial for the plant to conserve energy by ridding itself of the need to produce glue while retaining its ability for the leaves to close.
- Naturally, the leaves of the plant can be improved by simple natural selection, such as developing fine "teeth" to prevent small insects from escaping. The end result looks like this.
[edit] Example: the bacterial flagellum
Much touted by creationists, this example appears, on examination, not to be irreducibly complex. For a closer look at it, we recommend The Flagellum Unspun, from Professor Kenneth Miller's book, Finding Darwin's God, and Liu and Ochman's paper Stepwise formation of the bacterial flagellar system .
[edit] References and Resources
Pro-Irreducible Complexity
- Behe, Michael. Darwin's Black Box: The Biochemical Challenge to Evolution. Free Press Publications: 1996.
Critical of Irreducible Complexity
- [3] TalkDesign - Irreducible Complexity Demystified
