“For years I’ve listened to scientists say that evolution is BOTH a fact and a theory.”
This came about as a response to creationist claims that evolution was “only a theory”, with the definition of “theory” being a wild guess devoid of fact. In trying to deny that claim (which is false), Stephen Jay Gould first proposed the “fact vs theory” idea:
“Facts are the world’s data. Theories are structures of ideas that explain and interpret facts. Facts don’t go away when scientists debate rival theories to explain them. Einstein’s theory of gravitation replaced Newton’s in this century, but apples didn’t suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin’s proposed mechanism or by some other yet to be discovered.” Gould defines a scientific fact as “confirmed to such a degree that it would be perverse to withhold provisional assent.”
“Evolutionists have been clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory – natural selection – to explain the mechanism of evolution.” SJ Gould, Science and Creationism, ed. by Ashley Montagu, 1984.
Look at Gould’s definition of “scientific fact”: “confirmed to such a degree that it would be perverse to withhold provisional assent.”
This actually applies to some theories, as well. Take the statement “the earth is round.” Technically, that is a theory. Or take the statement “the planets of the solar system orbit the sun.” Again, a theory! This theory is known as “heliocentrism” Or, since we are talking about DNA, the statement “DNA exists as a double helix with the bases toward the center.” A theory.
Yet each of those theories are supported by so many observations and tests that everyone accepts them as factual. NASA plots the courses of planetary probes based upon the truth and factuality that the planets orbit the sun. The recent arrival of the spacecraft at Jupiter is another test, and confirmation, of the theory.
So, in that sense, evolution is a “fact”. A theory that is so well confirmed that it is “perverse to withhold provisional assent.”
Not all theories have reached that level of confirmation. Modified Newtonian Gravity (MOND) has not, for instance. Some theories have been tested and shown to be wrong. Geocentrism (sun and planets orbit the earth) certainly has. So has creationism.