While watching the classic "Cosmos", Carl Sagan makes the statement "evolution is a fact". Obviously, living creatures can change over time, this is a fact. Don’t drug-resistant bacteria and the silver fox constitute examples of evolution?
Am I incorrect in believing evolution is a fact, or do I need to re-classify evolution as theory?
As you point out, this is a matter of the definition of the relevant words.
The "fact" of evolution -- that individual species change over time, or alternatively that the gene pool for a species changes over time -- is as incontrovertible as the "fact" that when I just dropped my pencil, it fell to the floor in such-and-such a trajectory.
The "theory" of evolution is a well-tested generalization of the observed facts about evolution that covers things both seen and unseen; we've seen drug-resistant bacteria and silver foxes, and we generalize from them to the origins of human beings from monkey-like protoprimates. In this regard, it's as incontrovertible as the "theory" that when I drop my coffee cup, it will fall to the floor in a trajectory as calculated by Newton's laws of motion, modified if necessary by relativistic corrections and local modifications such as air resistance.
Of course, most people would consider it, loosely speaking, a "fact" that my coffee cup will fall if dropped, even thought it's technically not been observed (otherwise my cup would be broken already) and so is merely a conjecture based on a well-tested set of evidence. No one would seriously entertain the idea that my cup will not fall, merely because no one has yet observed
that particular cup falling when dropped at
that particular time in
that particular place. So the Theory of Evolution is an well-thought-out explanatory framework for the various factual evolution-events that we have observed, along with a lot of secondary evidence that are not actual evolution events.
In this sense, a "theory" is a well-tested explanation of a wide variety of phenomena. As the National Center for Science Education puts it, '
cience teachers, however, use scientific terminology, in which "theory" means a logical, tested, well-supported explanation for a great variety of facts. In a physics class, students will learn that the theory of gravity explains such facts as the rate of acceleration of falling objects; in chemistry class, they learn that atomic theory explains the structure and behavior of elements and compounds;' No one with an ounce more sense than an onion would object to Dalton's "Atomic Theory" or "The Germ Theory of Disease" as "only a theory," and suggest that we bring back the classical four elements, or Galen's humours, as alternatives to be taught in class.
There's also, of course, an informal meaning of the word "theory," akin to "wild-assed guess." As in "Well, my theory of the crime is that the victim shot himself and then his wife hid the suicide note, but that's just a theory and we'll have to wait for the ballistics report to see if that holds up." Evolution is specifically not a "just a theory" in this weak sense.
