Anyone who has spent some time reading or debating creationists is almost certain to hear the argument that “evolution is just a theory”. This is usually stated as if it were a blow against evolution, but in reality, nothing could be further from the truth. Though this statement is technically correct – evolution is indeed a theory – people who do not understand the implications of that word as it is used in science often think it is saying something different from what it actually means.
The important thing to keep in mind is that the everyday definition of the word “theory” is different from its scientific definition. In common usage, theory often means something like “guess” or “hunch”. However, in scientific circles, this is not the case. To scientists, a theory is an explanation of some feature of the world that meets three requirements: it is supported by evidence, is testable and falsifiable, and can be used to make predictions.
As the first requirement shows, “theory” in scientific use does not mean “guess” or anything similar. In fact, for a scientific explanation to be called a theory, it must be well-supported by evidence. When a scientist wishes to explain the cause of some object or event, they make an educated guess, usually called a hypothesis. This hypothesis is then tested by experiment and observation, and graduates to the status of theory if and only if enough evidence is found to support it and it repeatedly passes the tests it is subjected to. This is a standard the theory of evolution passes with flying colors: in a hundred and fifty years of scientific study of the natural world, evolution has never failed any crucial test, and an overwhelming amount of evidence has been found which supports it.
A scientific theory must also be, at least in principle, testable and falsifiable. If there is no imaginable test that could be performed to check a hypothesis, or if there is no evidence that could possibly prove it wrong, it can never become a theory. Evolution likewise meets both these requirements. To name some obvious examples, every discovery of a new fossil or a new species is a test of evolution. If a newly discovered species does not fit into the nested tree pattern used to classify all living things, or if a fossil is found in rock strata dramatically different from those where it should be, the theory of evolution would have to be drastically changed or discarded altogether.
The last requirement is whether a theory can be used to predict new discoveries we should make in the future. Anyone can patch together a hypothesis that explains a set of facts; the real test is whether we can take the organizing principles of that hypothesis and use them to deduce the existence of new evidence or phenomena not yet known. If such predictions cannot be made, or if they are made and then shown to be false, then the hypothesis fails to meet the qualifications for a theory and must be rejected. Evolution possesses great predictive power – not in the sense of predicting exactly how life will evolve in the future, because that depends on many chance factors too subtle for us to measure, but in the sense of predicting how new discoveries will fit into life’s established family tree. For example, if we possess part of a fossil series, we can reliably predict when in the rock record other members of that series will be found. See the Talk.Origins January 1997 Post of the Month for an example predicting where the ancestors of modern ants would be found. See here for a list of other verified predictions.
It is also important to point out the difference between a theory and a law. In science, a law is a description of some feature of the natural world. A theory is an explanation of that feature. In other words, laws say what happens, while theories explain why it happens. For example, Newton’s law of gravity states that two objects attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. It says what happens, but not why; it does not explain what gravity is or how it works. A theory of gravity, such as Albert Einstein’s theory of general relativity, explains why this happens. In astronomy, Hubble’s law states that the observed redshift of light coming from astronomical objects is proportional to their distance from the Earth; the theory of the Big Bang explains this observation by stating that the universe is expanding. In biology, Mendel’s laws of inheritance describe certain patterns in how traits are passed from parents to offspring, while theories of molecular genetics explain these observations by referring to the structure of chromosomes, genes and DNA. Theories do not change into laws as evidence accumulates to support them. Rather, theories are the “highest” one can get, and coming up with theories is the goal of every branch of science. If science consisted of nothing but discovering laws, it would be the activity sometimes derogatorily referred to as “stamp collecting”: listing natural phenomena without making any effort to explain them.
Though it is not usually phrased in these terms by biologists, it could be said that the “law of evolution” is that living things change over time. This is observable both in the fossil record and in the present day from one generation to the next. The theory of evolution explains this general pattern, as well as the specific details, by saying that living things experience differential reproductive success due to random mutation and natural selection. However, the evidence supporting evolution is so strong that biologists also generally, and correctly, consider it to be a fact, as obvious and unquestionable as heliocentrism or gravity. Therefore, evolution is both a theory and a fact.
An explanation of some feature of the world that is well-supported by evidence, that has passed every test it has been subjected to, and that has been used to make a great number of verified predictions – this is an accurate description of the status of evolution. To call it “just a theory” is not an argument against it, but an argument for it. Only the most powerful, best-tested scientific ideas ever earn this designation.
One final point should be raised. As this essay has shown, to call evolution “just a theory” is not a valid objection to it, but a compelling point in its favor. But another rapid rebuttal to anyone who makes that claim is this: Creationism is not even a theory! It has not been verified by observation and experiment, and in fact, the experiments that scientists have performed have disproved all of its central claims. It is neither testable nor falsifiable, because it is ultimately dependent on divine intervention and no test could disprove the idea of a miracle. In the few instances where it has been used to make predictions, those predictions have been shown to be wrong. And lastly, it does not truly explain any aspect of the natural world, because to say “God did it” explains nothing; it produces no genuine increase in our knowledge. Indeed, it closes the book on explanations altogether, because miracles are incomprehensible by definition, and once we decide that one has happened, no further conclusions can be drawn. Not only is creationism not a theory, it probably does not even qualify as a hypothesis. It is to creationism, not evolution, that the ordinary definition of “theory” as “guess” or “hunch” is more appropriate.