The Theory of Evolution is a theory and therefore... a verifiable hypothesis.

One of the most common accusations heard from creationists is that "evolution is only a theory and hasn't been proven". Such assertions are also heard from conservatives who give political support to the creationists. For instance, during the 1980 Presidential campaign, Ronald Reagan told an audience, concerning evolution, "Well, it's a theory--it is a scientific theory only, and it has in recent years been challenged in the world of science and is not yet believed in the scientific community to be as infallible as it was once believed." (cited in Berra 1990, p. 123, Wills 1990 p. 120, and Eldredge 1982 p. 28)

This accusation demonstrates a basic ignorance of the methods and principles of science. The scientific method holds as a matter of course that all conclusions are tentative, and that nothing can ever be absolutely proven to a certainty. Every conclusion reached by any scientist must always include, even if it is only assumed, the unspoken preface that "This is true only to the best of our current knowledge". Science does not deal with absolute truths; it deals with hypotheses, theories and models. The distinction between these is important in understanding and in countering creationist arguments, since the word "theory" also has a popular usage that is quite different from its scientific meaning (the vast majority of the US population--some studies have indicated as high as 95%--are in essence scientifically illiterate, and have only the vaguest grasp of modern scientific thinking, and the creationists always make a point of appealing to this popular ignorance).

In the popular view, the word "theory" means simply something that is unproven--an assertion which may or may not be true. It is this meaning which the creationists refer to when they assert that evolution is "just a theory", the implication being that, if evolution hasn't been proven, then it should have no more standing than creation "science". In science, however, the word "theory" has a very definite meaning. Under the scientific method, the first step in investigation is to gather data and information, in the form of verifiable evidence. Once data has been gathered, the next step is to form a hypothesis which would explain the data. This hypothesis is, quite simply, nothing more than an intelligent guess. (A hypothesis is, in fact, the closest scientific term to what most people mean when they say "theory").

Once a hypothesis has been formed, it is compared against the data (both old and new) to see how well it fits with the established facts. If the hypothesis is contradicted by the data, then it must be either modified and tested again, or discarded completely and a new hypothesis formed. Once a hypothesis has passed the test of verification through data, it becomes a scientific theory--i.e., it becomes an established framework within which to interpret the relationship of various bits of raw data. On the basis of this theory, new hypotheses are formed, and areas in which new data may be gathered are identified. If the theory continues to correctly explain new data (and indeed serves to correctly predict the outcome of scientific experiments), it is said to have a high degree of reliability. Such a theory is not a mere supposition or guess; it is a hypothesis that has been verified by direct experimentation and which has demonstrated a high degree of predictive ability.

When a related group of theories are correlated to one another and demonstrate the ability to be predictive and to explain the data, they form a scientific model. Models are the intellectual framework within which vast areas of particular data are explained and described. They also serve to indicate potential new areas of research and new hypotheses which can be tested to see if they can be integrated into the model.

An example may help to illustrate these distinctions. Observational data indicates to us that we can see the masts of tall ships while they are still far out on the horizon, before we can see the deck or the hull. We can also observe that the shadow of the earth, cast upon the moon during a rare eclipse, appears to be circular. We can therefore formulate the hypothesis that the earth is round. This would explain all of our data. Using this hypothesis, we can predict that, if the earth is indeed a sphere, we should be able to sail completely around the earth without falling off or coming to an edge. And, if this experiment is performed, we find that we can indeed do so. Our hypothesis has now been verified by experimentation, shows itself capable of correlating a variety of disparate data, and shows an ability to be predictive, and is therefore established as a scientific theory, the Theory of the Round Earth.

If we combine our theory of the round earth with other theories such as the theory of a round moon and a theory of heliocentrism, we can formulate a model--the moon orbits around the earth, the earth orbits around the sun, and all are part of a system of planets orbiting around a central star. This is the model of the heliocentric solar system.

Please note that none of this is to be treated as an absolute fact. It is entirely possible that some later observation or data will completely upset our model. Many times, a model must be modified and altered in order to explain new data or to expand its explanatory power. No scientific model can be viewed as an absolute proof. Perhaps at some point in time the shadow of the earth upon the moon will be seen to be a square, or perhaps one day we will see that the moon does not really revolve around the earth. However, based upon all of the data we possess currently, we can conclude that neither of these possibilities is very likely, and we are justified in having a high degree of confidence in the solar system model. Although it has not been (and cannot logically be) proven to an absolute certainty, it has been verified by every experiment we have conducted so far, and it has proven to have profound predictive power.

This model then becomes a basis on which to formulate new hypotheses and to investigate new areas of research. As various scientists produce new data and formulate new theories and hypotheses, a consensus will be reached about which theories are better suited to the data and which have a higher degree of confidence. In this manner, the model is constantly being modified, improved and expanded in order to encompass more and more data. Scientific models can never be stagnant--they are constantly changing and expanding as our knowledge of the universe increases.

Thus, scientific models can never be viewed as "the truth". At best, they are an approximation to truth, and these approximations become progressively closer to "the truth" as more testing of new evidence and data is done. However, no scientific model can ever reach "the truth", since no one will ever possess knowledge of ALL facts and data. As long as we do not have perfect and complete knowledge, our scientific models must be considered tentative, and valid only within the current limits of what we know.

The current theories of evolutionary mechanisms (Darwinian gradualism through natural selection, punctuated equilibria and neutralist evolution) together constitute a scientific model. This model has survived (with some modifications) every experimental test, and has not been invalidated by any data or evidence we now possess. Evolutionary theory has demonstrated an ability to correlate and explain a wide variety of disparate data with a high degree of confidence, and has proven to have the ability to predict experimental results and to point out new areas that may be investigated for new data. As a scientific theory, the theory of evolution has the same standing and authority that atomic theory, the theory of relativity and the theory of quantum physics possess.

excerpted from Creation Science Debunked: http://www.oocities.org/CapeCanaveral/Hangar/2437/theory.htm