The burden of proof is always on the arguer, creationist or evolutionist, who is making an assertion. The difficulty for creationists lies in making logical arguments against evidence, and for evolutionists in sifting through voluminous evidence for a pertinent example to illustrate their point. [searching for "biological evolution" on PubMed produced 117942 hits on 5/1/07]
The construction of a cogent deductive/inductive argument begins with acceptable premises (supported by evidence) and proceeds according to the laws of deductive/inductive reasoning to a conclusion supported by the premises.
Shifting the burden of proof is a subtype of the argument from ignorance fallacy that shifts the burden of proof onto the person whose argument is attacked. This is the "prove it, or I am correct!" challenge that underlies many creationist taunts.
The source of the fallacy in arguments from ignorance is the assumption that something is true unless proven otherwise. The implied conclusion of creationist demands is that unless the evolutionist can provide an adequate explanation for specifically selected creationist questions, then evolutionary theory is false, biological evolution is no longer a fact, and the existence of God is proven (at another level of implication).
The argument runs like this:
creationist challenge: demonstrate exactly how the bacterial flagellum evolved!
premise: the bacterial flagellum is a functional multi-component system
premise: if scientists cannot demonstrate exactly how the bacterial flagellum evolved, then (implied or stated fallacious conclusions)
conclusion E: Biological complexity cannot have evolved
conclusion T: evolutionary theory fails
conclusion G: God must have designed the flagellum
conclusion X: God exists
conclusion M: We are the products of special creation, are not related to apes, and must possess innate moral absolutes
The argument above demonstrates the fallacious argumentum ad ignorantiam because the conclusions, in employing a false dichotomy, are not supported by the premise. That is, even if scientists could never demonstrate exactly how the bacterial flagellum evolved, it would not necessarily follow that evolutionary processes were not responsible for the structure.
Further, even if this structure arose by some mechanism other than evolution, this mechanism need not by necessity have been designed by some mythical supernatural intelligence.
In fact, on the basis of the fallacious, not to mention devious, reasoning behind creationist demands for specific scientific explanations, "irreducible complexity" demands fail because the claims of scientific merit regarding questions of divine design behoves the creationists to demonstrated exactly how God designed the apparatus. If creationists wish to make the claim that God designed the bacterial flagellum, then the claimers must provide experimental or empirical verification for the hypothesis of divine design. This is particularly so when creationist are demanding inclusion of these speculations within the science curriculum.
Creationists' earlier demands for an explanation of the evolution of the eye have been abandoned because much more research has been devoted to the evolution of vision, and evolutionary scientists can provided an account for the reducible complexity of the eye. That ploy did not work, so creationist demands have moved to the microscopic bacterial flagellum because much less funding has been devoted to microbiology.
If one follows 'creation vs evolution' debates, one will quickly observe that creationists do claim to prove the existence of God, while evolutionists do not attempt to disprove the existence of God. Evolutionists merely defend the experimental support for evolutionary theory. Creationists mistakenly assume that because modern science provides much more probable explanations for natural phenomena than does the God of the Gaps then the aim of science is to disprove the existence of God. Mainstream science does not concern itself with questions concerning the existence or nonexistence of purported supernatural phenomena.
On the other hand, scientists investigate the physical world and defend, to other scientists, their conclusions based upon experimental results. Scientific reports follow a format of introduction to ideas based upon past research, the question to be addressed, experimental methods, experimental results, and conclusions based upon both current scientific understanding and experimental results. Science proceeds by incremental gains in understanding. It is the very nature of science that conclusions are acknowledged not to be the final word upon explanation, and that, once a scientific theory is widely accepted, research will move on to yet unanswered questions. Scientists also recognize something of which creationists appear to be unaware – that inductive reasoning cannot, by the rules of logic, yield proof. This impossibility applies to scientific inductive reasoning and to theological induction from any misinterpreted facts.
Creationists utilize, in their religiously-motivated attacks on science, one of the chief virtues of science – the reluctance of the community of scientists to accept any hypothesis or theory until burden of 'proof' (probability) moves to the level of general acceptance.