Surely some things are too complex to evolve. Isn’t it true that the human eye and bacteria flagellum are examples of irreducible complexity?
Irreducible complexity, as defined by Michael Behe in Darwin’s Black Box:
“By irreducible complexity I mean 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.” But there are ways in which the eye and bacteria flagellum could have evolved. There is no need to posit a supernatural designer. The eye is also not irreducibly complex, in which the removal of one part causes complete blindness. Many people have problems with their eyes, but some sight is better than none. The human eye is the result of millions of eyes of gradual change of pre-existing structures. We don’t have fossilized eyes, since soft parts rarely fossilize, but we can look at the different eyes in nature to figure out how the eye could have evolved. First there is the simple eyespot where a few light sensitive cells give the organism the location of light sources; a recessed eyespot where a small indentation in the surface provides additional information to the location of the direction of the light; a deeply recessed eyespot where even more light sensitive cells which provides even more accurate information to the organism; a pinhole camera eye that is able to focus an image on the back of a layer of deeply recessed of light sensitive cells; a pinhole lens eye that is actually able to focus the image; and finally the complex eye found in humans and other modern animals. (Shermer, “Science Friction”, p. 183-186)
The eye is not that intelligently designed. The retina is configured in three layers, with the light sensitive rods and cones at the bottom. They face away from the light, underneath a layer of bipolar, horizontal, and amacrine cells, which are under a layer of ganglion cells. They help carry the transduced light signal to the brain by way of nerve impulses. This entire structure sets below a series of blood vessels. Basically the eye is backwards and upsides down. Evolution can explain this, since evolution is a bottom up designer, while Intelligent Design cannot. (Shermer, “Science Friction”, p. 183-186)
The bacteria flagellum is another of the Intelligent Design movement’s favorite examples of Irreducible complexity. Flagellums are whip like structures that some bacteria use for locomotion, while for others it is secretion. Some use it to attach themselves to cell walls. Simple bacteria didn’t use the flagellum for propulsion, but for the other uses mentioned. More complex bacteria co-opted the use of the flagellum for propulsion.
(Shermer, “Science Friction”, p. 186-187) (Dawkins, “The Ancestor’s Tale”, p. 549-553)
There is an analogy that applies to irreducible complexity. An arch is irreducible in that if you remove one part of it the whole collapses. But it is possible to build it by means of scaffolding. The removal of the scaffolding after the arch is completed can give the appearance that the arch could not have been built as it is. We have no more reason to suspect that arches are supernaturally designed than we do the human eye or the bacteria flagellum.
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“By irreducible complexity I mean 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.”