The junkyard tornado is an argument used to deride the probability of both abiogenesis and the evolution of higher lifeforms as comparable to "the chance that a tornado sweeping through a junkyard might assemble a Boeing 747." It was used originally by Fred Hoyle's statistical analysis applied to evolutionary origins, but similar observations predate Hoyle and have been found all the way back to Darwin's time. While Hoyle himself was an atheist, the argument has since become a mainstay of creationist and intelligent design criticisms of evolution.
This argument is rejected by the vast majority of biologists. From the modern evolutionary standpoint, while the odds of the sudden construction of higher lifeforms are indeed improbably remote, evolution proceeds in many smaller stages over a long period of time. The transition as a whole is plausible, as each step improves survivability; the 747 is not constructed in one single unlikely event, as the junkyard tornado posits.
According to Hoyle's analysis, the probability of cellular life evolving was about one-in-1040,000. 
The chance that higher life forms might have emerged in this way is comparable to the chance that a tornado sweeping through a junkyard might assemble a Boeing 747 from the materials therein.
This is a reflection of his stance reported elsewhere:
Life as we know it is, among other things, dependent on at least 2000 different enzymes. How could the blind forces of the primal sea manage to put together the correct chemical elements to build enzymes?
Hoyle used this to argue in favor of panspermia, that the origin of life on earth had come from preexisting life in space.
The junkyard tornado derives from arguments most popular in the 1920s, prior to the modern evolutionary synthesis, which are rejected by evolutionary biologists. A preliminary step is to establish that the phase space containing some biological entity (such as humans, working cells, or the eye) is enormous, something not contentious. The argument is then to infer from the huge size of the phase space that the probability that evolution yielded the entity is exceedingly low.
Sometimes, arguments invoking the junkyard tornado analogy also invoke Borel's Law, which claims that highly improbable events do not occur. The usual argument against Borel's "Law" is that if all possible outcomes of a natural process are highly improbable when taken individually, then a highly improbable outcome is certain. The true law being referenced is actually the Strong Law of large numbers, but creationists have taken a simple statement made by Borel in books written late in his life concerning probability theory and called this statement Borel's Law.
This "Borel's Law" is actually the universal probability bound, which when applied to evolution is axiomatically incorrect. The universal probability bound assumes that the event one is trying to measure is completely random, and some use this argument to prove that evolution could not possibly occur, since its probability would be much less than that of the universal probability bound. This, however, is fallacious, given that evolution is not a completely random effect (genetic drift), but rather proceeds with the aid of natural selection.
The junkyard tornado is also applied to cellular biochemistry. It is comparable to the older infinite monkey theorem but instead of the works of William Shakespeare, the claim is that the probability that a protein molecule could achieve a functional sequence of amino acids is too low to be realised by chance alone. The argument conflates the difference between the complexity that arises from living organisms that are able to reproduce themselves (and as such may change to become more complex over time) with the complexity of inanimate objects, unable to pass on any reproductive changes (such as the multitude of parts manufactured in Boeing 747). The comparison breaks down because of this important distinction.
According to Ian Musgrave in Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations:
These people, including Fred, have committed one or more of the following errors.
- They calculate the probability of the formation of a "modern" protein, or even a complete bacterium with all "modern" proteins, by random events. This is not the abiogenesis theory at all.
- They assume that there is a fixed number of proteins, with fixed sequences for each protein, that are required for life.
- They calculate the probability of sequential trials, rather than simultaneous trials.
- They misunderstand what is meant by a probability calculation.
- They underestimate the number of functional enzymes/ribozymes present in a group of random sequences.
The junkyard tornado argument is rejected by evolutionary biologists, since, as the late John Maynard Smith pointed out, "no biologist imagines that complex structures arise in a single step." The modern evolutionary synthesis explains how complex cellular structures evolved by analysing the intermediate steps required for precellular life. It is these intermediate steps that are omitted in creationist arguments, which is the cause of their overestimating of the improbability of the entire process.
Hoyle's argument is a mainstay of creationist, intelligent design, orthogenetic and other criticisms of evolution. It has been labeled a fallacy by Richard Dawkins in his two books The Blind Watchmaker and Climbing Mount Improbable. Dawkins argues that the existence of God, who under theistic uses of Hoyle's argument is implicitly responsible for the origin of life, defies probability far more than does the spontaneous origin of life even given Hoyle's assumptions, with Dawkins detailing his counter-argument in The God Delusion, describing God as the Ultimate Boeing 747 gambit.
- Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations – An explanation at the TalkOrigins Archive by Ian Musgrave Last Update: December 21, 1998
- George Johnson, Bright Scientists, Dim Notions NY Times, October 28, 2007
- Derek Gatherer, The Open Biology Journal, 2008, 1, 9–20, Finite Universe of Discourse: The Systems Biology of Walter Elsasser (1904–1991)
- A similar argument goes back at least to Cicero's De Natura Deorum 2.37
- Fred Hoyle(1983): “The Intelligent Universe”, page 17. The Boeing 747 metaphor is reported in Nature, 294 (1981), p.10
- Fred Hoyle, The Intelligent Universe (1983), ISBN 0-7181-2298-4
- John Maynard Smith, The Problems of Biology, p.49. (1986), ISBN 0-19-289198-7, "What is wrong with it? Essentially, it is that no biologist imagines that complex structures arise in a single step."
- "A memorable misunderstanding" Fred Hoyle's Boeing-story in the Evolution/Creation literature by Gert Korthof
- Evolution Encyclopedia Vol. 1 Chapter 10 Appendix Part 2 Contains a number of Hoyle quotations on evolution.