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The falsifiability of evolution is an important part of establishing evolution as a scientific theory on the principles of falsifiability. Creationists and others sometimes claim that evolution and common descent cannot be falsified and consequently are not science. In other cases, intelligent design advocates insist that there is no way to falsify evolution because any new evidence just becomes "evolution in action" anyway.
This, however, is simply not the case as there are numerous pieces of evidence that could falsify evolution. The assertions made by creationists may be due to a misunderstanding between the concept of "falsifiability" and "has been falsified". More intelligent criticisms of the falsifiability of evolution focus on how new evidence (such as new fossil or climatology data) always leads to evolution and never towards design, even when theories about specifics of a species' development are changed significantly in light of evidence. However, this is a case of the science was wrong before gambit, and has little to do with falsifiability itself.
 What would falsify evolution?
A central characteristic of science is that it must be falsifiable; this feature of a theory is attributed to Karl Popper, who mentioned it in a criticism of Darwinism. Scientific theories cannot be proved outright – they can only fail to be disproved, and this means pointing out what evidence could disprove the theory. If a theory cannot be disproved, such as with Russell's Teapot, it makes no difference whether it is true either way.
Disproving evolution first requires to look at what the theory predicts and see where it can be shown to make incorrect predictions. It is easy to be side-tracked by specifics of the theory, such as individual evolutionary pathways of certain features, and confuse these with what would falsify the overall theory of evolution by natural selection. Indeed, many creationists do this whenever a new discovery is made in biology that causes scientists to rethink some pieces of evolution. To avoid this problem, it is best to be clear what evolution is. It is based on three main principles: variation, heritability and selection. Given these three principles, evolution must occur, and many features of evolution appear given only these three guiding principles. If any of these were shown to be flawed then the theory would be untenable.
Consequently any of the following would destroy the theory:
- If it could be shown that organisms with identical DNA have different genetic traits.
- If it could be shown that mutations do not occur.
- If it could be shown that when mutations do occur, they are not passed down through the generations.
- If it could be shown that although mutations are passed down, no mutation could produce the sort of phenotypic changes that drive natural selection.
- If it could be shown that selection or environmental pressures do not favor the reproductive success of better adapted individuals.
- If it could be shown that even though selection or environmental pressures favor the reproductive success of better adapted individuals, "better adapted individuals" (at any one time) are not shown to change into other species.
Charles Darwin made the case a little differently when he said, "If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case."
 Precambrian rabbit
J.B.S. Haldane famously stated that "fossil rabbits in the Precambrian" would disprove evolution - and this has been a talking point in philosophy of science for some time. This phrase is reported to be a rebuttal to the accusations that evolution is not falsifiable. However, the reality of disproving evolution in this manner is quite complicated. As science is based on an interplay between theory and evidence a single point of data is not enough to completely destroy a theory - just as much as an excellent theory can't win out against overwhelming data. Such a thing as finding fossilised rabbits wouldn't cause scientists to throw the theory of evolution out completely and immediately, so a little more explanation is needed.
First of all, it must be remembered that the fossil record is supporting evidence for evolution. This is contrary to the ideas put forward by creationists that state the gaps in the fossil record prove evolution to be false. If the fossil record simply did not exist it would make no difference to the validity of the theory of evolution — indeed, natural selection was initially formulated without the aid of fossil record, and subsequent DNA evidence can stand completely without it. The simple truth is that a single strange fossil would probably not make much difference. In practice, the evidence in the fossil record which supports evolution is so overwhelming that a single fossil would be regarded as curious certainly, but compared to the mountain of evidence in favor of evolution it would probably be regarded as an anomaly while more data was awaited. Imagining the fossil rabbit in the Precambrian as disproving all of natural selection would confuse the specifics of an individual evolutionary pathway with the falsification of the whole theory itself, as mentioned above.
However, the existence of entire groups of anomalous fossils would be a different thing - Haldane did say rabbits after all. Again, in practice an effort would initially be made to fit the new data into the existing framework - this is not cheating but simply the way that science works. But still, in principle some quite major revisions to the theory may be needed to explain them. Such a situation would not immediately and conclusively prove a special creation over a naturalistic evolution, however, a key point that creation proponents tend to overlook. Eventually, a new theory would develop to include these oddities, but this isn't necessarily a special young Earth creation as this assertion would also require supporting evidence, and lots of it. Perhaps this anomalous group was due to a now extinct second genesis, which would be a remarkable find, but unlikely to disprove evolution outright. Regardless of what it was, this new theory would explain both the evidence we have now and the hypothetical rabbit fossils and would indeed be science fully supported by evidence.
- The argument is non sequitur. Evolution can be predictive, and yet still only describe what happened in the past. We do not have omniscient knowledge of the past, so we make predictions about what has already occurred. While this may sound nonsensical on its face, we make predictions about the past all the time in day-to-day life: "If he hadn't had car trouble he'd have been on time", for instance.
- Many predictions are possible based on the theory of evolution (e.g., predicting germ/pest resistances to antibiotics/pesticides, predicting general traits/locations of future fossil finds, predicting population responses to selective breeding attempts, etc.)
- Just like meteorology only predicts effectively maybe a week into the future, evolution cannot be used to precisely predict what will happen in the future. It does, however, tell us what kind of changes can be expected to happen.
- There are many predictions concerning the future that can be extrapolated from the theory of evolution even if they are not explicitly stated by Darwin. Here are some examples of predictions that one could argue would be extrapolated from the theory of evolution.
- Species with high reproductive rates will have a better chance of surviving environmental change than species with low reproductive rates.
- Species from a common gene pool separated into group A and group B and isolated for generations over a longer period of time and exposed to different environmental conditions will eventually become unable to reproduce with members of the other group.
- It is possible to predict the outcome of a macro-evolutional experiment, but it's a bit difficult to verify it. However, it is equally possible and valid to predict certain paleontological findings. Some of these are actually verified by now (like some kinds of fossils).
- Darwinian evolution predicts that all organisms on Earth derive from a common ancestor. This ancestor has not been identified through fossil records (and probably cannot be), but its existence is a necessary consequence of the fundamental genetic unity of all life on Earth.
- Creationists who use this argument fail to realize that there have been no scientific predictions made by Scientific Creationism.
- Evolution requires a huge amount of time to operate and thus effectively predicted that the Sun was powered in a far more efficient manner than previously thought — a prediction that was verified by the discovery of nuclear fusion.
- Mendeleev only described what had happened in the past when devising the periodic table. One prediction of evolution, amongst others, was that transition species would be found and added to the fossil record, as they have been.
 What would falsify common descent?
Common descent is the theory that all life evolved from one common ancestor, or groups of ancestors (although it is slightly more complicated than the straw man version of "one day a cell appeared and everything is technically its descendent"). Morphological and, more recently, genetic evidence puts common descent pretty close to being outright proven, but there are still ways to falsify it. Common descent could easily be disproved (without even seriously challenging the theory of evolution) if we discovered/made a form of life that was not related to all the life we know - most simply, by finding life that does not use the nucleic acids (DNA and RNA) for information storage and retrieval as known biological life does. This would be exemplified in the situation of a second genesis, where two different forms of life began at two completely different points.
Also, if we were ever to discover life elsewhere in the universe, the odds lean towards it not having a common ancestor with life on earth. This would really only require that "common descent" be modified to "life on earth", and similarly, the second genesis on Earth would signify that two forms have two common descents.
 See also
- Claims CA210 (lack of predictions), CA211 (falsifiability), and CA211.1 (Popper) at Talk Origins
- "29+ Evidences for Macroevolution: The Scientific Case for the Theory of Common Descent with Gradual Modification" (1998), Douglas Theobald
- Evolution and Philosophy: Predictions and Explanations (1997), John Wilkins
- Proof Evolution Is False