Wednesday, March 30, 2011

Evolution Is Not "Just A Theory"

"Evolution is a fact amply demonstrated by the fossil record and by contemporary molecular biology. Natural selection is a successful theory devised to explain the fact of evolution." - Carl Sagan, The Dragons of Eden, p.6

You've been told that evolution is "just a theory", a guess, a hunch, and not a fact, not proven. You've been misled. Keep reading to discover why this is misleading.   I'm not going to try and change your mind about evolution.  I just want to point out that "it's just a theory" is not a valid argument.

The Theory of Evolution is a theory, but guess what?   When scientists use the word theory, it has a different meaning to normal everyday use.[1]   That's right, it all comes down to the multiple meanings of the word theory. If you said to a scientist that you didn't believe in evolution because it was "just a theory", they'd probably be a bit puzzled.

In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It's a well-substantiated, well-supported, well-documented explanation for our observations.[2]   It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It's as close to proven as anything in science can be.

Some people think that in science, you have a theory, and once it's proven, it becomes a law. That's not how it works. In science, we collect facts, or observations, we use laws to describe them, and a theory to explain them. You don't promote a theory to a law by proving it. A theory never becomes a law.

This bears repeating.   A theory never becomes a law.  In fact, if there was a hierarchy of science, theories would be higher than laws.  There is nothing higher, or better, than a theory.   Laws describe things, theories explain them. An example will help you to understand this. There's a law of gravity, which is the description of gravity. It basically says that if you let go of something it'll fall. It doesn't say why. Then there's the theory of gravity, which is an attempt to explain why. Actually, Newton's Theory of Gravity did a pretty good job, but Einstein's Theory of Relativity does a better job of explaining it. These explanations are called theories, and will always be theories. They can't be changed into laws, because laws are different things. Laws describe, and theories explain.

Just because it's called a theory of gravity, doesn't mean that it's just a guess. It's been tested. All our observations are supported by it, as well as its predictions that we've tested. Also, gravity is real! You can observe it for yourself. Just because it's real doesn't mean that the explanation is a law. The explanation, in scientific terms, is called a theory.

Evolution is the same. There's the fact of evolution. Evolution (genetic change over generations)3 happens, just like gravity does. Don't take my word for it.[4]   Ask your science teacher, or google it. But that's not the issue we are addressing here. The Theory of Evolution by Natural Selection is our best explanation for the fact of evolution. It has been tested and scrutinised for over 150 years, and is supported by all the relevant observations.

Next time someone tries to tell you that evolution is just a theory, as a way of dismissing it, as if it's just something someone guessed at, remember that they're using the non-scientific meaning of the word. If that person is a teacher, or minister, or some other figure of authority, they should know better. In fact, they probably do, and are trying to mislead you.[5]

Evolution is not just a theory, it's triumphantly a theory!


1. “Theory: A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.” American Heritage Dictionary (
2. “Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses.” Teaching About Evolution and the Nature of Science - National Academy Press (
3. A standard, scientific definition of evolution is: “In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next.” Biology - Helena Curtis and N. Sue Barnes, W H Freeman
4. “Evolutionists have been clear about this distinction between 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.” Evolution as Fact and Theory - Stephen Jay Gould (
5. The Cobb County School Board required a sticker with the following text to be placed on all biology textbooks: “This textbook contains material on evolution. Evolution is a theory, not a fact, regarding the origin of living things. This material should be approached with an open mind, studied carefully, and critically considered.” Decision of the Court Striking Down the Cobb County Evolution Disclaimer (



From the American Heritage Dictionary:

the·o·ry n. pl. the·o·ries

1.A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.
2.The branch of a science or art consisting of its explanatory statements, accepted principles, and methods of analysis, as opposed to practice: a fine musician who had never studied theory.
3.A set of theorems that constitute a systematic view of a branch of mathematics.
4.Abstract reasoning; speculation: a decision based on experience rather than theory.
5.A belief or principle that guides action or assists comprehension or judgment: staked out the house on the theory that criminals usually return to the scene of the crime.
6.An assumption based on limited information or knowledge; a conjecture.


From the National Academy Press:

Is evolution a fact or a theory?

The theory of evolution explains how life on earth has changed. In scientific terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses. Biological evolution is the best scientific explanation we have for the enormous range of observations about the living world.

Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.

Why isn't evolution called a law?

Laws are generalizations that describe phenomena, whereas theories explain phenomena. For example, the laws of thermodynamics describe what will happen under certain circumstances; thermodynamics theories explain why these events occur.

Laws, like facts and theories, can change with better data. But theories do not develop into laws with the accumulation of evidence. Rather, theories are the goal of science.


From the Talk Origins site:

"Evolution is a Fact and a Theory"

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.


Answers in Genesis, a web site which promotes Creationism, has a section on arguments that creationists shouldn't use. Whilst they correctly direct people not to use the "just a theory" argument, their alternative is no better:

"Evolution is just a theory."

What people usually mean when they say this is "Evolution is not proven fact, so it should not be promoted dogmatically." Therefore people should say that! The problem with using the word "theory" in this case is that scientists use it to mean a well-substantiated explanation of data. This includes well-known theories such as Einstein's Theory of Relativity and Newton's Theory of Gravity, as well as lesser-known ones such as the Debye-Hückel Theory of electrolyte solutions. It would be better to say that particles-to-people evolution is an unsubstantiated hypothesis or conjecture.


From the PBS series on evolution:

When we use the word "theory" in everyday life, we usually mean an idea or a guess, but the word has a much different meaning in science. This video examines the vocabulary essential for understanding the nature of science and evolution and illustrates how evolution is a powerful, well-supported scientific explanation for the relatedness of all life.


The book The Top 10 Myths About Evolution, by Cameron M. Smith and Charles Sullivan, has a chapter entitled "Myth Two: It's Just a Theory":

"...calling evolution "just a theory" involves a misunderstanding of what a scientific theory is. Evolution is a fact, and the three main processes that make up evolution - replication, variation, and selection - are observable and undeniable."



Talks Origins Archive:

Understanding Evolution:

National Center For Science Education:

Wikipedia article on Theory/Fact:

Not Just A Theory F.A.Q.:

No comments:

Post a Comment