Assorted creationists claim variously that creation theories are falsifiable and that evolutionary theories are not falsifiable. Here, I want to quickly point out a few flaws that I see in their arguments. I will discuss these issues in a more general manner in another post that I am currently writing.
Creationism is falsifiable: This essay from the Discovery Institute provides two arguments, neither of which are satisfying.
First, they look at the idea of irreducible complexity (IC, a component of Intelligent Design theory), and claim that it can be falsified by finding an evolutionary explanation for the structure in question. This does not match my understanding of "falsifiability" because IC does not make any predictions of its own, it just asks to be compared against an evolutionary explanation. As such, they are actually asking "What is more plausible, evolution or design?". This can be a productive question, but is not the gold standard of falsifiability. Even within this framework, the proponents of IC rely on an unjustified bias, assuming that as long as there is not an airtight evolutionary explanation, the IC explanation is valid (i.e. this is a "god of the gaps" argument). However, if we are just working with plausibility, the evolutionary explanation already trumps the IC explanation simply because we've observed countless instances of mutation, but no instances of intelligent design. Mutation is a more plausible explanation in all situations, without any further research. (Update: To clarify, with no known mechanism of ID, the explanation is completely implausible and therefore there is no way that we could examine existing life to see if its structure is more consistent with ID or evolution). Scientists gain nothing by considering the possibility of IC. For what it's worth, an evolutionary explanation has been proposed for their example of the bacterial flagellum.
The second example they give, the Privileged Planet hypothesis, may be theoretically falsifiable, but seems totally impotent as science. Based on what is written in the Discovery Institute essay (I haven't read the book or watched the movie), there is no reason to expect any of these predictions to be tested within my lifetime. Furthermore, the predictions all seem to boil down to "we expect alien life to be like known life", which is not a risky prediction, and therefore is not informative. At best, the privileged planet hypothesis is a dormant scientific hypothesis waiting for that century when we can test it. At worst, it is a scam to draw money from people who really want to legitimize ID.
Evolution cannot be falsified: This article at CreationWiki is just a mess, but what can you expect from a small wiki?
First, they claim that historical theories cannot be tested. This is untrue. Historical theories are regularly tested by a back-and-forth process that involves new observations of the patterns created by historical processes, and predictions based on processes known to occur in the modern world. For instance, a hypothesis about the relatedness of two organisms can be tested by making a prediction about how similar their DNA sequences will be (based on observed mutational processes) and then sequencing their genomes (new information) and seeing whether the prediction is upheld. For what it's worth, evolution is not the only historical science--astronomy and geology both rely on historical models. I also don't know why historical sciences would be treated differently than history in general, as if the testimony of witnesses is trivial to interpret (despite changes in languages, the diverse motives that people have when they write, and even the limits of human memory).
Second, when saying that only "subtheories" can be tested, they totally misrepresent what subtheories are (i.e. hypotheses). In evolutionary theories, the "subtheories" are not limited to things like evolution in a test tube (actually a fact, not a theory), they include things like asking whether birds are more closely related to mammals or dinosaurs (or bacteria). They could even include questions such as "Are birds related to mammals or dinosaurs at all". To use their "rocketship" analogy, this is like asking whether the ship landed in Asia or North America, and whether there were multiple ships.
The rest of the article seems to be nit-picking over details of evolutionary theory. The authors treat the evolving nature of evolutionary theory as evidence that it cannot be falsified. This viewpoint ignores the fact that the theory is not just refined to explain some fact and then left alone; any new hypothesis makes a variety of predictions which are then tested. These predictions involve both the historical artifacts (e.g. fossils and genome sequences), and the ongoing processes that allow populations to change (e.g. population growth, how quickly forms can change).
In the end, creationists complain that when one evolutionary hypothesis is rejected, it is replaced with another. They see this as evidence that "evolution" as a whole cannot be falsified. They are misguided on two points. First, a theory is not tested as a whole--it is tested in parts and the theory fails when so many of its parts have been demolished (or co-opted into other theories) that it can no longer be recognized as a theory. This is what happened with Spontaneous Generation, which required a few centuries between the recognition that it doesn't apply to flies, and its final death when it was conclusively demonstrated not to apply to microbes either. Their second error is their failure to recognize that evolution simply requires that life can change and that it has had sufficient time to change (i.e. the Earth is old).
As long as those conditions are met, and there is no plausible alternative mechanism for generating the diversity of life, some form of evolutionary theory will have a central place in biology.
Confusions On Evolution, Creationism, And Falsifiability