As I and others have noted many times, facts, theories, and hypotheses are independent elements in the scientific process. Contrary to their vernacular meanings, they are not ranks indicating differential degrees of certainty in some claim.
Evolution is scientific fact, meaning that the numerous types of evidence point so overwhelmingly to shared ancestry that scientists have accepted it as true about the world, in the same provisional-but-extremely-likely sense that they accept other facts like gravity or the existence of atoms.
Evolution is also a theory, meaning that there is a cohesive body of mechanistic explanations that seeks to explain the historical fact of common descent. This includes, but is not limited to, random processes like genetic drift, quasi-random ones such as mutation, and absolutely non-random ones like natural selection.
In addition to being a fact (that species are related through common ancestry) and a theory (well supported mechanisms that explain how evolutionary change happens), evolution represents the unique historical path that living lineages have followed. Whereas there is no longer any real disagreement in biology (or indeed, in science in general) over the historical factuality of common relatedness, evolutionary biology is rife with heated debate regarding the mechanisms and their relative importance, the specific historical relationships linking related groups, and the intermediate steps that occurred in the origin of particular features. As such, hypotheses are also important in evolutionary biology, because they represent testable statements that are used to support or refute specific details of theory or path.
One of the more stinging criticisms that evolutionary biologists level at each other when they argue about evolution as path is to call a proposed account a “just-so story”. The phrase itself comes from Rudyard Kipling’s Just So Stories from the early 1900s, many of which included fantastical accounts of the origins of particular features, like the elephant’s trunk, the leopard’s spots, and the giraffe’s neck. According to Wikipedia,
A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, and social sciences. It describes an unverifiable and unfalsifiable narrative explanation for a cultural practice or a biological trait or behavior of humans or other animals. The use of the term is an implicit criticism that reminds the hearer of the essentially fictional and unprovable nature of such an explanation. Such tales are common in folklore and mythology.
The important criterion in whether something represents a just-so story rather than a hypothesis is a lack of testability and hence unfalsifiability. A lack of complete supporting data in itself is not an indicator that something is a just-so story, because many hypotheses also lack these prior to being subjected to testing. In other words, what matters is whether it can be tested, not whether it has already been tested, though obviously some supporting data must be provided eventually if the hypothesis is to be considered seriously.
Creationists, who seem unequipped with irony detectors, tend to dismiss all hypotheses about evolutionary path, even ones for which there is substantial supporting evidence, as just-so stories. In fact, they often demand an absurd amount of evidence and detail, such as an observable, repeatable, mutation-by-mutation demonstration of some feature evolving. When this obviously cannot be delivered (nor could its counterpart in any science), they believe this supports their unobservable, unrepeatable, vague explanation for the feature’s origins. Theirs is the ultimate just-so story, but that does not prevent them from projecting onto scientists.
As a case in point, consider the infamous bombardier beetles, which I mentioned briefly in a recent post. Along with eyes, blood clotting, the immune system, and bacterial flagella, the defence mechanism of these intriguing beetles supposedly represents an un-evolvable feature due to its irreducible complexity.
The claims made by creationists about these beetles relate to both fact and path. In terms of fact, they often suggest that the reagents used in its defensive system will explode when mixed together, and thus that their mechanism of storing them separately must have arisen fully formed. This is demonstrably false. A catalyst is required, as Dawkins showed by mixing the liquids.
With regard to path, they argue that a series of functional intermediates in the gradual evolution of the defensive apparatus is impossible in principle. This claim is also easily refuted by the presentation of plausible, testable hypotheses showing how functional intermediates could have occurred. One example is given in this video.
Now, is this the answer to how the spectacular defence system of bombardier beetles arose? I do not know, but I suspect probably not. A similar point of view is presented by Mark Isaak, who authored the Talk.Origins article Bombardier beetles and the argument of design:
The scenario above is hypothetical; the actual evolution of bombardier beetles probably did not happen exactly like that … Determining the actual sequence of development would require a great deal more research into the genetics, comparative anatomy, and paleontology of beetles. The scenario does show, however, that the evolution of a complex structure is far from impossible. The existence of alternative scenarios only strengthens that conclusion.
Are such scenarios just-so stories? No, they are hypotheses, and they are testable. For example, are the hypothetical intermediate stages found to be functional in any other species? Are the chemicals used in combination in the defence system also functional on their own elsewhere in the beetle’s body? Are there genetic differences between bombardier beetles and other Carabidae related to this system?
Without clear answers to these and other questions, the path of bombardier beetle evolution will remain an open and interesting question. However, biologists do not assume that these beetles did not evolve simply because the specific path has yet to be elucidated. And they certainly would not assume that an untestable, supernatural just-so story of cosmic proportions is the null hypothesis.
That, of course, is the difference between science and pseudoscience.