The terms "fact" and "theory" can be applied to evolution, just as they are to gravity Misuse and misunderstanding of how those terms are applied to evolution have been used to construct arguments disputing the validity of evolution.
There have been many theories that attempt to explain the fact of gravity. That is, scientists ask what gravity is, and what causes it. They develop a model to explain gravity, a theory of gravity. Many explanations of gravity that qualify as a Theory of Gravity have been proposed over the centuries: Aristotle's, Galileo's, Newton's, and now Einstein's. Confusion of the terms can arise when we use a single word to describe both the observed facts and the theory that explains it. The word ‘’gravity’’ can be used to refer to the observed facts (i.e., the observed attraction of masses) and the theory used to explain it (gravity is the reason why masses attract each other). Thus, gravity is both a "theory" and a "fact."
In the study of biological species, the facts include fossils and measurements of these fossils. The location of a fossil is an example of a fact (using the scientific meaning of the word fact). In species that rapidly reproduce, for example fruit flies, the process of evolutionary change has been observed in the laboratory. The observation of fruit fly populations changing character is also an example of a fact. So evolution is a fact just as the observations of gravity are a fact.
In biology, there have been many attempts to explain these observations over the years. Lamarckism, Transmutationism and Orthogenesis were all non-Darwinian theories that attempted to explain the observations of species and fossils and other evidence. However, the Theory of Evolution is the explanation for all relevant observations regarding the development of life, based on a model that explains all the available data and observations. Thus, evolution is not only a fact but also a theory, just as gravity is both a fact and a theory.
