Evolution of the Bacterial Flagellum

P roponents of the intelligent design (ID) explanation for how organisms developed claim that the bacterial flagellum (BF) is irreducibly complex. They argue that this structure is so complicated that it could not have emerged through random selection but had to be designed by an intelligent entity. One part of this claim is that each flagellar component is used solely for the purpose of making a flagellum that, in turn, is used only for motility. Further, each flagellar protein is assumed to have appeared independently of the other component proteins. Here, we summarize evidence from hundreds of laboratories, including our own, showing that these assumptions are false. Instead of by design, BF developed as modular systems, with components deriving from many different sources. Each BF module evolved independently from various primordial systems, which, in most cases, had nothing to do with cell motility. Complexity within BF arose by domain and protein recruitment, by intragenic and extragenic duplication events, and by superimposition of various modules onto others. The net result was coevolution of many types of structurally and functionally distinct flagella in various bacterial species. Although these different flagella are all used for motility, they share only about half of their protein constituents.