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Logic and rhetoric
Hoyle's fallacy, also known as the Junkyard tornado, describes a hypothetical tornado that passes through a hypothetical junkyard resulting in chaos. Proponents of Intelligent Design erroneously assume that because the ensuing chaos does not produce some sort of complex, man-made device (for example, a Boeing 747), that various processes of evolution, abiogenesis or other origins theory are equally unlikely.
The "Tornado in a Junkyard" analogy is an example of an argument by false analogy, a logical fallacy. It is also an example of denying the antecedent: when confronted with the claim that adding energy to a system can give rise to complexity, creationists simply present an example of a situation where adding energy to a system does not give rise to complexity.
The "Tornado in a Junkyard" analogy is credited to Sir Fred Hoyle, a British astronomer and writer. He originally used the comparison not as an analogy for evolution, but as an argument against abiogenesis. He felt that the improbability of even the simplest life form arising from non-living matter was too great. However, his analogy lives on in origins debate despite its original context.
Relation to abiogenesis
Abiogenesis is a well supported scientific hypothesis for the origin of life on earth. Arguing that abiogenesis is akin to jumbo jets appearing in a storm-stricken junkyard is a straw man, oversimplifying a complex theory. Current scientific theory in abiogenesis does not suggest that complex high-order beings appeared from primordial soup in one magical step.
Relation to evolution
The original context of Hoyle's argument was against abiogenesis, not evolution. Nevertheless, opponents of evolution occasionally use it when discussing aspects of evolutionary biology, largely because, for the most part, they don't know the difference. The analogy is exceptionally poor when compared to the process of evolution, as one of the main mechanisms of evolution is natural selection which is non-random.
Hoyle is rendered effectively irrelevant by the anthropic principle, in that life already exists, and our universe, solar system, and planet necessarily are able to support our existence. Whether Creationism is true or Evolution is true, our world contains all necessary conditions for our existence (as evidenced by our existence), therefore the Tornado analogy is irrelevant. Further examination of our origins can be left to science (which of course rejects creationism, because the evidence points to a different explanation).
It may very well be possible for a tornado in a junkyard to, by chance, create some complex instrument (probably not a Boeing 747). Since it has not yet been demonstrated by all the tornadoes that have passed through junkyards, it can be estimated the probability of it occurring is unfathomably small. However, if over the course of billions of years trillions of tornadoes went through endless fields of junk, a functional device that accomplishes something would probably get slapped together... if only to be destroyed by another tornado moments later.
At the molecular scale, however, the probability of an accidental precursor to life rises dramatically when bombarded with solar radiation and a constantly changing environment, especially when that environment is the size of the Earth and there are many millions of years to work with. While proteins are incredibly complex (like a Boeing 747), amino acids, carbohydrates and lipids are relatively simple and exist in incredible abundance and similarly simple chemicals are bound to occur. Simple chemicals can eventually combine into more complex ones, and so on.
Variety of forces
Another problem with the "Tornado in a Junkyard" analogy is that a tornado cannot realistically be expected to supply the variety of forces needed to create a complex assembly. Given that forces generated by a tornado tend to be aligned to the tangents of the tornado's cone, it is inconceivable that a tornado would be able to apply the diametrically opposed forces needed to, for instance, insert both wings into the fuselage of the hypothetical craft, let alone install bolts in the myriad different directions, including those which can only be accessed from inside the plane (for instance, those which affix the seats to the floor).
In contrast, abiogenesis only requires that a certain variety of spontaneous chemical reactions occur within an appropriate spatial and temporal vicinity, and so a more fitting analogy would be that of a tornado tearing through a junkyard and moving all the pieces required for assembly of a 747 into one corner of the yard, given that the pieces already existed in the junkyard.
Manufacture of parts
Yet another problem is that a Boeing 747 has very specific and highly complex parts such as the avionics or its engines, that are extremely unlikely to be found on your standard scrapyard unless it was one for planes, something that can also be said of what materials are needed to build one.
As commented above in abiogenesis, however, not only said parts (water, amino-acids, other organic compounds...) are already available, produced by inorganic processes, but also the aforementioned chemical reactions are all that is needed to assemble them into something (far) more complex. The tornado would here be far more akin to either a storm so virulent that disintegrated whatever junk present there into its component atoms and rearranged them into 747 pieces or a tornado that would have brought from aircraft boneyard(s) elsewhere and/or the Boeing factory all the parts of said airliner and reassembled them into one.
The ultimate tornado
In The God Delusion, Richard Dawkins expands upon Hoyle's fallacy of the tornado in a junkyard by applying it to the existence of God himself. Although creationists might argue that such an abstraction misrepresents God's nature, Dawkins primarily runs it as a thought experiment to convey the complexity of God. In light of the fact that God would not have developed from a refinement process like natural selection or the existing laws of chemistry and biology, it's probably more applicable to God anyway. Dawkins' expansion goes like this: God requires certain properties; He must know the entire universe, its past, present, future, all in implausibly precise detail, it must know the rules, how it interacts and have a deep intimate knowledge of the emergent properties within it (i.e., thoughts of every individual within the universe, not just the arrangement of neurons and their bio-electric connections) and in addition to all this, some information on how to be God. If life is comparable to a tornado running rampant through a junkyard and forming a Boeing 747 then God must be like a tornado running through a junkyard and forming the entire British Airways fleet and then some. Dawkins describes this as the "ultimate" tornado and the "ultimate" 747, but indeed, the bounds of the metaphor really can't quite get across the scale of the random organisation needed for this. A more accurate representation would be that if a tornado went through a junkyard and two scrap wheels fell within 10 metres of each other, that would be the organisation equivalent to forming life; but if the tornado ripped through the junkyard and left the entire universe in its wake, several times, that would be just a fraction of what God would need to be.