From the Critical Cafe:
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Evolution is an incomplete theory. The task of science regarding this theory is to add statements that would make it predictable.
This is possible thanks to research in genetics and molecular biology that unveils the structure of organic compounds and life.
Once the evolution mechanism is unveil, prediction mechanism could be part of it to test future evolution of species.
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What do you mean by “incomplete”? All theories are incomplete in the sense that it is impossible to predict every possible state of affairs of a real system in the future even if they are fully determined by the theory. For example, the evolution of the solar system cannot be predicted indefinitely in the future, even though (omitting smaller effects such as interplanetary gas and dust and solar wind) there is no reason to suppose the state of the system is not determined by gravitational theory.
“Prediction” in the sense of testing a theory has nothing whatsoever to do with time. It is simply a statement of logical implication between a general, explanatory statement (theory) and a particular state of affairs that that theory implies could exist and could not exist if that theory were not true. The state of affairs could have existed in the past. Indeed, geological theories are tested against rock formations that mostly were formed long before man existed. Someone used “retrodiction” for this, but it is in fact a timeless logical relationship.
Theories are not testable in themselves. In order to test any part of a theory, you have to build a “model”. A model instantiates the theory with physical situations and particular values of parameters. It may be designed to isolate a particular implication of the theory for testing, such as the relationship between two variables. The kind of model that most people will find familiar is probably a physical model. If you have done physics labs you may remember such things as using springs to find the relationship between a force and the extension of the spring, or inclined planes to find the laws governing the acceleration of a body under gravity.
In the case of evolution, the model is a conceptual model. It consists of all the relationships that have been hypothesised between species, their fossils, the geological strata they are found in, their structural and DNA relationships and all the other relevant evidence currently available (including mathematical models of evolutionary processes). This model has been being built for over two centuries, and is much too big for any one person to hold in their head – it is spread through the scientific literature, some of it long forgotten or ignored.
Nevertheless, it renders the theory falsifiable because potentially somewhere in this model you could find relationships between the parts that are not compatible with the theory of evolution. Creationists are perfectly well aware of this and are always trying to interpret (and often fake) evidence to show that, for example, dinosaurs and humans are recorded in the same rock formations. (Suspicions should be aroused by the fact that they always find this evidence in spectacular and newsworthy species like humans and dinosaurs – you don’t find much creationist research on the scientific staples of E. coli and fruit flies, let alone obscure fossils. In fact, creationists tend to mock real research, vide Palin and McCain.)
Adding new evidence to this conceptual model supports, or corroborates the theory, but it does not amount to proof, because the theory remains potentially falsifiable. In fact, new evidence makes the theory potentially more falsifiable, as there are more relationships within the model that have to be true for the theory to be true.
Models, of course, have imperfections, which means that when a relationship does not fit you don’t immediately throw out the theory. Our physics experiments rarely fitted the textbook relationships, because of friction, air resistance, boredom and other factors. In advanced work you may have to use statistical techniques to study the relationship you want. In a conceptual model, there are always going to be problems over interpretation of the available evidence. As the theory is well-corroborated, people will review the interpretation of the evidence before rejecting the theory, and to an outsider this can look superficially like re-interpreting the data to fit a dogma, which is what creationists do.