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Both creationist and intelligent design writers have asserted that evolution is at best a poor scientific theory, because it is not "falsifiable," which in the parlance of scientific philosophy means that the theory is too flexible -- no test could be devised that decisively rejects its key tenets. Creationist Ken Ham, for instance, has argued that theories such as evolution and the big bang cannot be tested, because no scientists were present to directly observe whether or not the conjectured events really took place [Ham2011]. Similarly, creationist Henry Morris has asserted that virtually any observation of the natural world could, with some adjustment, be accommodated within the overall evolutionary framework, and thus evolution is not worthy to be termed a solid scientific theory [Morris2000, pg. 6-7].
So does evolution really qualify as a first-rate scientific theory, or not?
First of all, it should be noted that even some of Charles Darwin's original assertions have been falsified. For instance, he believed that organisms could acquire traits during a single lifespan and transmit these traits to offspring. But modern genetics has concluded otherwise -- acquired traits, with rare exceptions, are not passed on to offspring. The current evolutionary paradigm, often termed the "modern synthesis," reflects this conclusion.
Karl Popper, who more than any other scientific philosopher promoted falsifiability, initially regarded Darwinian evolution as only a metaphysical research program, because it was too difficult to test. Most of the claims by creationists and others regarding falsifiability derive from these comments by Popper. But subsequently Popper reversed his position, saying, "I have changed my mind about the testability and logical status of the theory of natural selection, and I am glad to have the opportunity to make a recantation." [Popper1978].
In any event, there are numerous ways in which evolutionary theory can be tested and, if found wanting, would have to be rejected. Here are just a few:
It should also be pointed out that strict adherence to "falsifiability" is not an accurate description of the process of modern science. For one thing, major theories are seldom falsified by a single experimental result. There are always questions regarding the underlying experimental design, measurement procedures, and data analysis techniques, as well as questions of whether the underlying theories have been properly applied. For example, if we were to strictly apply Popper's principle, Copernicus' heliocentric theory was falsified from the start and should not have been further considered, because it could not predict planetary motions as accurately as the traditional Ptolemaic system. It only after Kepler modified the theory to include elliptical orbits with time-varying speeds, and when Newton showed that this behavior could be mathematically derived from his laws of motion, that it gained widespread acceptance. It must also be kept in mind that in most cases, "falsified" theories continue to be extremely accurate models of reality within appropriate domains. Even today, over 100 years after Newton's mechanics and Maxwell's electromagnetic equations were "falsified," they remain the basis of almost all practical engineering and scientific computations, giving results virtually indistinguishable from those of more advanced theories in all but highly exotic circumstances.
For additional discussion, see Evolution, Theory and Postmodern.