Evolution: Fact Or Theory?

How Can It Be Both? What's the Difference?

Progress of Evolution. James Cotier/Getty

There is some confusion about evolution as a fact and evolution as a theory. Often you can find critics claiming that evolution is “just a theory” rather than a fact, as if this demonstrated that it shouldn’t be given serious consideration. Such arguments are based upon a misunderstanding of both the nature of science and the nature of evolution.

In reality, evolution is both a fact and a theory.

To understand how it can be both, it is necessary to understand that evolution can be used in more than one way in biology.

A common way to use the term evolution is simply to describe the change in the gene pool of a population over time; that this occurs is an indisputable fact. Such changes have been observed in the laboratory and in nature. Even most (although not all, unfortunately) creationists accept this aspect of evolution as a fact.

Another way the term evolution is used in biology is to refer to the idea of “common descent,” that all species alive today and which have ever existed descend from a single ancestor which existed at some time in the past. Obviously this process of descent has not been observed, but there exists so much overwhelming evidence supporting it that most scientists (and probably all scientists in the life sciences) consider it a fact as well.

So, what does it mean to say that evolution is also a theory? For scientists, evolutionary theory deals with how evolution occurs, not whether it occurs — this is an important distinction lost upon creationists.

There are different theories of evolution which can contradict or compete with each other in various ways and there can be strong and sometimes quite acrimonious disagreement between evolutionary scientists regarding their ideas.

The distinction between fact and theory in evolutionary studies is probably best explained by Stephen Jay Gould:

In the American vernacular, “theory” often means “imperfect fact” — part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is “only” a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can’t even make up their minds about the theory, then what confidence can we have in it? Indeed, President Reagan echoed this argument before an evangelical group in Dallas when he said (in what I devoutly hope was campaign rhetoric): “Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science — that is, not believed in the scientific community to be as infallible as it once was.

Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. 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.

Moreover, “fact” doesn’t mean “absolute certainty”; there ain’t no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are NOT about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us falsely for a style of argument that they themselves favor). In science “fact” can only mean “confirmed to such a degree that it would be perverse to withhold provisional consent.” I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.

Evolutionists have been very 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.

Sometimes creationists or those not familiar with evolutionary science will misquote or take scientists’ quotes out of context to make disagreements over the mechanisms of evolution seem like disagreements over whether evolution has occurred. This is indicative either of a failure to understand evolution or of dishonesty.

No evolutionary scientist questions whether evolution (in any of the senses mentioned) occurs and has occurred. The actual scientific debate is over how evolution occurs, not whether it occurs.


Lance F. contributed information for this.