It is certainly the case that neo-Darwinian evolution has not been proved.  But we probably shouldn’t say that it is “just a theory” because a theory (in the primary, scientific sense of the word) has supporting evidence, whereas neo-Darwinian evolution does not.  When people say, “evolution is just a theory,” they probably mean it is unproved.  But a scientist will be inclined to interpret the word “theory” in the scientific sense of something with supporting evidence – perhaps something that should be believed like the theory of relativity.  But the theory of relativity is truly a theory in the scientific sense and thus has supporting evidence.  Neo-Darwinian evolution does not.  

Carl Sagan once said, “Evolution is a fact, not a theory.”[1]  And he was partly right; evolution is not a theory.  He’s not right about evolution being a fact of course, unless he meant evolution in the most generic sense of change over time.  We all agree that things change over time.  But Sagan was clearly referring to a specific type of change: neo-Darwinian evolution.  This is the idea that all life on earth is descended from a common microbial ancestor: that the information in our DNA is entirely the result of mutations over time that happened to convey survival advantage.  It is this version of evolution that is neither a fact nor a theory.

What is a Theory?

Some creationists like to say, “evolution is just a theory” by which they mean it has not been proved.  I would certainly agree that neo-Darwinian evolution has not been proved, and yet I would discourage creationists from saying, “evolution is just a theory.”  Why?  The problem pertains to the word ‘theory.’  The word can indeed mean an unproved assumption: conjecture.  But this is not the most common definition.  It is listed as 3b in the Merriam-Webster dictionary.  ‘Theory’ can mean contemplation or speculation.   But this is number 6 in the dictionary.com list of definitions.  In other words, the use of the word ‘theory’ to mean “unproved, conjecture, or speculation” is legitimate, but is not the most common meaning of the word, and therefore can lead to misunderstanding.

What is the most common meaning of the word ‘theory?’  The first definition listed in dictionary.com is “a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena.”  The listed example is: “Einstein’s theory of relativity.”  This is the definition scientists generally have in mind when they speak of a theory.  They are not referring to something that is mere conjecture or speculation.  Rather, they refer to something that has been tested, has passed the test, makes accurate predictions, and therefore has supporting evidence.

The physics discovered by Einstein is great example of a scientific theory.  It is hardly a mere assumption, conjecture, or speculation.  It is well-tested.  The theory of relativity makes very specific claims about the rates at which clocks tick, lengths contract, and masses increase as an object approaches the speed of light.  There have been hundreds if not thousands of experiments which have tested these claims, and the claims of Einstein were correct in every case.  Furthermore, no competing theory has been able to pass these tests.  So, we have very good reasons to believe that Einstein’s theory of relativity is correct, or is at least a very good approximation of the way the universe behaves.  It is well-supported by evidence.

Is Neo-Darwinian Evolution a Theory?

Is the idea of particles-to-people evolution a theory in this primary sense?  Is it something that has been tested by the scientific method and passed?  Does it make successful predictions for a class of phenomena?  What kinds of tests and predictions qualify as supporting evidence?

Evolutionists often list three kinds of evidence that they believe supports evolution.  First, organisms have certain similarities (and differences) in their anatomy.  These similarities can be arranged into a hierarchy, which is what makes taxonomy possible (phylum, class, order, family, genus, species).  Second, all organisms have basically the same genetic language.  Furthermore, the DNA of two different organisms tends to be similar if those organisms are anatomically similar.  Finally, the fossil record shows variations of organisms that no longer exist.  Some of these are claimed to be transitional between organisms that do exist today, thereby apparently confirming that they are all related.

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