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What is a theory?
In the creation vs. evolution debate, proponents of Biblical Creationism often assert that evolution is "only a theory, not a fact." This statement is based on the common, vernacular meaning of the word "theory" and is a misunderstanding of what this term means in science.
"Science" comes from the Latin word for knowledge, and science is the study not of isolated facts, but of ideas. Thus it has a rich terminology for describing ideas. A hypothesis (literally, "put under") is the idea underlying an investigation such as an experiment. A law is a phenomenon that has been observed to take place consistently under particular conditions. A model is a set of interrelated ideas, based on laws and other observations, which together form a representation with predictive and explanatory value. A theory is a model that has stood up under repeated testing. Theories explain factual, empirical observations.
If a fact is "something that is definitely true," then no type of idea in science (such as a hypothesis, law, model, or theory) is excluded from also being a fact. An idea such as evolution can be both a theory and a fact.
However, in common usage, the word theory has a dramatically different meaning; it typically refers to a conjecture or speculation. It is a type of idea that is not even good enough to be called a hypothesis. A simple conjecture can never be said to be "true" in the sense that a fact is true, so the sound bite "evolution is only a theory" was coined to suggest that evolution is no more than an unsupported scientific speculation.
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Evolution fact, evolution theory
If evolution is a fact-based theory, there must be plenty of evolution evidence. And indeed, there is a wealth of observational evidence for evolution. As a theory, evolution is expected to have predictive value, so a good test would be to make a prediction using the theory of evolution, then test the prediction.
Cladistics, the study of phylogeny, allows such a prediction to be tested. If evolution is true, then it should be possible to develop a cladogram that shows the evolutionary history of a group of organisms. And it should be possible to derive that same cladogram by more than one method.
The first cladograms were developed using morphological characteristics -- that is, they were based on the physical appearance of organisms. These studies produced cladograms showing certain relationships, which are the "evolution facts" of the organisms' histories. Now researchers are able to create cladograms based on molecular characteristics. These cladograms are consistent with the previously devised cladograms based on morphological characteristics. The cladograms were tested against each other and found to be consistent, which is strong evidence that they contain real facts. The theory of evolution underpins the science of cladistics, so this observation supports evolution.
If evolution were not based on facts, then observations such as those made by cladistics researchers would uncover discrepancies. Instead, they uncover consistencies. The theory of evolution has been continuously bolstered as a good predictive model when tested by empirical observations.
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What do scientists say?
Stephen Jay Gould, who did major work on the theory of evolution, was adamant that evolution is both a fact in the historical sense and a theory in the scientific sense. Evolutionary biologists Richard E. Lenski and Douglas Futuyma are equally unyielding on this point. Evolution is a well-supported scientific theory, and the fact that it occurred is as certain as any other scientific fact.