At the risk of doing this subject to death (for those who haven’t noticed, I’m fairly set on bacterial regulation of gene expression and, in particular, flagellar assembly), I recently encountered a somewhat revealing quote from one of my favourite overviews of the flagellum — that is, Pili and Flagella: Current Research and Future Trends. In chapter 6 of that book (entitled, “What Is Essential For Flagellar Assembly?”), contributer Shin-Ichi Aizawa writes,
Since the flagellum is so well designed and beautifully constructed by an ordered assembly pathway, even I, who am not a creationist, get an awe-inspiring feeling from its ‘divine’ beauty … However, if the flagellum evolved from a primitive form, where are the remnants of its ancestor? Why don’t we see any intermediate or simpler forms of flagella than what they are today? How was it possible that the flagella have evolved without leaving traces in history?
Just in case some are tempted to charge me with quote mining, Aizawa goes on to justify the sheer lack of intermediates on the basis of our ignorance of the vast majority of flagellar systems (actually, we know the assembly system for only about 20 bacterial species).
While this may or may not be considered a plausible rationalisation of the current state of evidence, one of course needs to be careful that an ad hoc rationalisation for a lack of evidence is not transfigured into evidence for Darwinism (as it so often is in this discipline). It’s not just a lack of known physical intermediates which is the problem, but the sheer lack of evidence that there even plausibly exists a step-wise Darwinian pathway leading to the base of the fitness peak for even the simplest of flagella (actually, the evidence runs quite to the contrary).
Now, there are indeed simpler flagella than the one found in Salmonella (as Aizawa quickly points out). For example, in Caulobacter, the anti-sigma factor FlgM is absent, as is the FliA gene which encodes for the sigma factor σ28. But the flagellar assembly system in Caulobacter is really somewhat different from the one found in Salmonella. There are simpler ways to make a flagellum, yes. But there are also simpler ways to make a mouse trap. Dispensability of a particular component in one mousetrap does not necessarily entail dispensability in a different mouse trap. Flagella actually seem to have evolved convergently in some instances.
Francis Crick, of DNA-double-helix fame, once said that “Biologists must constantly keep in mind that what they see was not designed, but rather evolved.” Well, since we are all agreed that biological systems give us the overwhelming impression of being designed systems, then what is the problem with one supposing that these systems might represent real — and not just apparent — design? If blind and undirected processes can so readily and obviously account for such marvels of engineering, why the need for the constant reminders?
Indeed, computational cell biologist Kathryn Appelgate (an ardent Darwinist, many essays of whom feature on the Biologos website), of “flagella assembly is like evolution” fame, wrote an essay last year entitled “Bacterial Flagellum: Appearances Can Be Deceiving”, noting:
The resemblance is so striking, we find it difficult to resist extending the analogy to how the flagellum originated. We know that all outboard motors are designed by intelligent engineers; the parts are carefully crafted to work together for an intended purpose. The bacterial flagellum also has many well-matched components. Together they perform the same job as the outboard motor—swimming. Since the flagellum wasn’t designed by human engineers, it seems only reasonable to infer that it was designed by Someone Else.
Meanwhile, the legitimate question has been raised as to how such a system arose: Is this system actually designed? Or is the design merely apparent? Well, since we all — at least most of us — are in agreement that the flagellum looks like a designed system, it would be unwise to rule out that proposition a priori. By closing off one possible answer, we potentially limit ourselves to a set of false choices. Since all of the purported naturalistic “explanations” of this system fail, and since we have positive reason (i.e. our uniform and repeated experience of cause-and-effect) to suspect that this system might really be the product of intelligent causality, it seems to stand to reason that the design inference be regarded as a scientifically respectable point of view to take.