Is the principle of irreducible complexity new and valid objection against evolutionary theory?
This is once again an argument that has arisen in various forms since the time of Darwin. Originally, anti-evolutionists liked to put forth these arguments in connection with the structure and function of the eyes of vertebrates. The eye has a great many components, where any of them would see to be essential for the creation of an image. However, the eye is a very unsuitable example for this type of argument. It is quite true that all the present-day components are necessary for perfect vision. However, it is simultaneously obvious that even the simple ability to differentiate light and dark is very useful for the survival of an organism; it is somewhat better to recognize the direction from which the light is coming and even better to roughly differentiate the shape of objects in the field of vision, etc. Thus, a perfect eye can be very easily formed by the gradual evolution of its components, where every evolutionary step put its bearer at an advantage compared with its less perfect predecessor.
The opponents of the theory of evolution tend at the present time to concentrate on the purported irreducible complexity of molecular structures, such as the molecular apparatus employed for rotating a bacterial flagellum (Behe 2001a; Behe 2001b). These structures are usually less complicated than macroscopic structures, still their functionality , such as the ability to rotate a flagellum, is dependent on the existence and functioning of all the components. Anti-evolutionists argue that these components could not have evolved gradually in the process of evolution, as one of them makes no sense in the organism without the other.
A fundamental error in this argument is that it ignores the existence of exaptations, i.e. structures that developed in evolution in a different functional context than that in which they function in modern organisms (see I.7.1). The individual components of the flagellum apparatus most probably developed independently of one another, under the effect of various selection pressures and originally performed a completely different functions. At some point, the finished components were ordered in a functional apparatus, that began to be capable of performing, originally probably rather imperfectly, a new function, here rotating a flagellum and thus allowing the bacteria to move. The molecular apparatus of a bacterial flagellum has a great many components in common with the molecular apparatus employed by a great many bacteria for injecting toxin into the cells of an attacked organism, and also with the F1-ATPase molecular apparatus, i.e. the molecular complex synthesizing ATP at the expense of the transmembrane proton gradient (Block 1997; Noji et al. 1997).