The bacterial flagellum is an example of a supposedly irreducibly complex trait. So what is a flagellum and what does it do? Bacteria need both to find nutrients and to avoid noxious substances and in order to do this they need to be motile. One way for bacteria to move around is effectively to stick a propeller to one end of themselves. This propeller is made of one or more long filaments which are shaped a bit like corkscrews. This then turns and propels the bacterium forward or, by reversing direction, allows the cell to turn around.
This ingenious apparatus is the bacterial flagellum and the modern argument for the presence of a designer often hinges on the assertion that this tiny molecular motor is irreducibly complex. That is to say that if even one component was absent, it would no longer function and so could not have developed incrementally over millions of years. This would provide a strong argument against natural selection if it wasn’t flawed in several ways.
Firstly some proteins in bacterial flagella have been shown to be helpful but non-essential to the flagella’s function. Several of these structures have been removed through genetic manipulation and the cells, although they may not be quite as happy, can still swim showing, that the flagellum is functional. In addition, different bacteria have different flagella which require different and unique proteins. As such this irreducibly complex structure shows a large amount of variability and requires a range of different proteins in different organisms arguing against a single irreducibly complex structure.
Secondly the proteins making up the flagellum are very often similar to each other and to proteins used for other functions within the cell. This allows a model based on gene duplication to be built. In this model a section of DNA is duplicated resulting in a second copy of a gene within an organism, and this second copy is not maintained by selection since the first copy is still fulfilling the original role. Subsequently, therefore, this gene may diverge and can be co-opted into the flagellum. The existence of these similar proteins provides an indication that the flagellum did arise from existing structures rather than being produced in fully functional form by an intelligent external creator.
The gradual formation by recruitment of proteins previously used for other structures might still seem incredible. Several groups of scientists have now, however, developed models of flagella evolution relying on the formation of intermediate structures which had other functions. These often start with a passive pore structure allowing transport of molecules across the bacterial membrane. Then this pore developed into a form of secretory apparatus, similar to those still observed in some bacteria. Finally this ‘proto-filament’ could be coupled to ion pumps in the membrane allowing rotation; this would be a fairly crude flagellum but would be expected to provide some selective advantage by the small degree of movement it would allow. This small amount of movement would, for example, allow a bacterium to move from areas where it has excreted waste products into areas where nutrients haven’t been depleted. This simple system could then be elaborated producing the flagellum seen today.
Finally scaffolding is another possible explanation for how a flagellum could appear to be irreducibly complex. What does that mean? Well there may have been a more unwieldy and inefficient flagellum which developed over time bit by bit. This in itself was not irreducibly complex and it could easily be seen how it built up. This could then be stripped down over time, as proteins became adapted to be more efficient whilst others were lost, making a sleeker and more efficient motor. This new motor may not look like anything could be removed from it if it is to remain a working motor, but then who knows what was there holding it together before?