The bacterial flagellum (above) is a fascinating device. It contains a molecular motor which rapidly rotates the filament. Whipping around, the filament drives the bacterium forward. In some bacteria, running the motor in reverse causes a random tumble. Amazingly, the combination of forward motion, random tumbling, and a simple sensor allows a bacterium to “home in” on a target (see four simulated example runs below). The idea is to do a random tumble whenever the sensor shows the bacterium heading in the wrong direction. An actual steering mechanism is not necessary – the bacterium gets to the target in the end.
William Dembski and Michael Behe famously argued (via the somewhat informally articulated concepts of specified complexity and irreducible complexity) that the flagellum was too complex to have evolved. Their argument fell apart with the discovery that the flagellum shares components with other bacterial gadgets, such as the injectisome, and thus could potentially have evolved in stages (although in fact the injectisome seems to have evolved as a simplification of the flagellum, and the evolutionary history of the flagellum remains a mystery).
The fundamental point that Dembski and Behe were attempting to make can be illustrated by the simple experiment summarised in the chart above. This experiment presupposes three genes (A, B, and C) all created by single point mutations on copies of existing genes, such that the combination of all three genes creates a useful widget. In the “flat landscape” case, this combination must arise entirely by chance. This takes a very long time (on the experimental assumptions used, an average of almost 14,000 generations). Dembski and Behe were probably right to suggest that, if the bacterial flagellum had to arise that way, it could not have evolved in the time available since the earth was formed.
In the “parallel evolution” case, however, each of the genes A, B, and C are assumed to be independently beneficial. The A-B-C combination then evolves very quickly. Evolution of the bacterial flagellum may have included aspects of parallel evolution, if components of multiple older widgets were “co-opted” for the flagellum.
The evolution of the bacterial flagellum is generally assumed to have instead been a case of “sequential evolution” (gene A is beneficial on its own, gene B is beneficial in the presence of gene A, gene C is beneficial in the presence of genes A and B, etc.). However, it is not at all clear what the sequence of genes producing the bacterial flagellum might have been (suggestions on this topic by Liu and Ochman have been criticised), nor is it clear what the sequence of intermediate benefits might have been (given that the injectisome was not an intermediate stage). Further research on the humble, but amazing, bacterial flagellum is clearly still required.
Scientific Gems 