In the recent Republican debates, three of the Republican candidates raised their hands when asked if any of the candidates did not believe in evolution. In the ensuing media attention to this, more than once I heard the old canard “It’s not a law, it’s just a theory.”
Why do I say this is a canard? Because it shows a profound mis-understanding of scientific terminology, a misunderstanding that the opponents of evolution deliberately encourage.
Just what is a scientific law and how does it differ from a scientific theory? A scientific law is an observation about the behavior objects in the physical world. Newton formulated the law of gravity. He observed how objects attracted each other and was able to deduce formulas that quantitatively described how they interact. This is actually a very weak assertion. What a physical law says is that we have seen a particular behavior and have not seen any exceptions, so we guess that it always holds and we probably can use the law to predict future behavior.
A scientific theory, on the other hand, is an attempt to understand why and how a particular behavior occurs. Thus we have a single “law of gravity” but many “theories of gravitation”. Once a phenomena is understood and explained, then we can make much stronger
assertions about the future behavior regarding that phenomena.
The only reason that we do not have a “law of evolution” is that evolution is more subtle and
happens over a longer time frame than most phenomena that have laws associated with them. Because of this, the law and the theory came simultaneously, and since a theory is the stronger claim, we now have a theory of evolution rather than a law of evolution.
In truth, now that we know where to look, evolution can be observed in action. New species have formed, existing species have changed. As the then-not-yet-famous Dr. Richard Dawkins said, “we have stronger evidence for the existence of evolution than we have for the existence of Julius Caesar.”