Well, its only a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science and is not yet believed in the scientific community to be as infallible as it once was believed.
—presidential candidate Ronald Reagan, addressing a group of evangelicals in Texas in1980 regarding Darwinian evolution
The implication in that statement is that there is something not quite right about a theory—that it is a mere speculation, and very likely wrong. Indeed, the everyday connotation of “theory” is “guess”, as in, “My theory is that Fred is crazy about Sue”. But in science the word “theory” means something completely different, conveying far more assurance and rigor than the notion of a simple guess.
According to the Oxford English Dictionary, a scientific theory is “a statement of what are held to be the general laws, principles, or causes of something known or observed.” Thus we can speak of the “theory of gravity” as the proposition that all objects with mass attract one another according to a strict relationship involving the distance between them. Or we talk of the “theory of relativity”, which makes specific claims about the speed of light and the curvature of space-time.
In science, a theory is much more than just a speculation about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world. “Atomic theory” isn’t just the statement that “atoms exist”; it’s a statement about how atoms interact with one another, form compounds, and behave chemically. Similarly, the theory of evolution is more than just the statement that “evolution happened”: it is an extensively documented set of principles that explain how and why evolution happens.
For a theory to be considered scientific, it must be testable and make verifiable predictions. That is, we must be able to make observations about the real world that either support it or disprove it. Atomic theory was initially speculative, but gained more and more credibility as data from chemistry piled up supporting the existence of atoms. Although we couldn’t actually see atoms until scanning-probe microscopy was invented in 1981 (and under the microscope they do look like the little balls we envision), scientists were already convinced long before that atoms were real. A good theory makes predictions about what we should find if we look more closely at nature. And if those predictions are met, it gives us more confidence that the theory is true. Einstein’s general theory of relativity, proposed in 1916, predicted that light would be bent as it passed by a large celestial body. Sure enough, Arthur Eddington verified this prediction in 1919 by showing, during a solar eclipse, that light coming from distant stars was bent as it went by the sun, shifting the stars’ apparent positions. It was only when this prediction was verified that Einstein’s theory began to be widely accepted.
Because a [scientific] theory is accepted as “true” only when its assertions and predictions are tested over and over again, and confirmed repeatedly, there is no one moment when a scientific theory suddenly becomes a scientific fact.
A theory becomes a fact (or a “truth”) when so much evidence has accumulated in its favor—and there is no decisive evidence against it—that virtually all reasonable people will accept it. That does not mean that a “true” theory will never be falsified. All scientific truth is provisional, subject to modification in light of new evidence.
In Darwin’s day, the evidence for his theories [about evolution] was compelling but not completely decisive. We can say, then, that evolution was a theory (albeit a strongly supported one) when first proposed by Darwin, and since 1859 has graduated to “facthood” as more and more supporting evidence has piled up. Evolution is called a “theory” just like the theory of gravity, but it’s a theory that is also a fact.
—excerpts from Why Evolution is True, chapter 1, by Jerry Coyne, professor of Evolution at the University of Chicago
To read the next post in this series on Evolution click: here
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