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Chapter 38 : Irreducible Complexity? Not!
Many bacteria have a propeller device called a flagellum on their surface that allows them to swim from one place to another. Bacterial flagella are complicated structures that have been touted by creationists as an example of “irreducible complexity” in an effort to refute evolution by natural selection. Flagella of Escherichia coli and Salmonella consist of thin helical propellers turned by rotary motors in the cell membrane, thereby allowing the bacteria to move from one place to another in an aqueous environment. This motility provides an enormous survival advantage by allowing bacteria to move toward nutrients or away from harmful substances. The flagellar motor obtains energy from the membrane ion gradient, harnessing the flow of ions from outside to inside the cell to drive rotation of the propeller filament. The rigid flagellar filament is built from several thousand subunits of the protein flagellin (encoded by the fliC gene). In the time since the flagellum was nominated as an example of irreducible complexity, some important evolutionary relationships have come to light which show that the flagellum is not irreducibly complex and which provide the outlines of a credible pathway for flagellar evolution. The Exb proteins function in the active transport of vitamin B12 and other essential molecules across the outer membrane, while the Tol proteins function in maintenance of the outer membrane. Thus, rather than debunking Darwin, the bacterial flagellum actually provides further evidence for the evolution of complex structures via natural selection.