Only a theory

Hypotheses vs theories

When someone says they have a theory, in everyday life, it could be completely crazy. I have a theory that my rabbit is being a jerk on purpose, which you may or may not think is completely nuts. But when someone has a theory in science, that means something different. If something is being called a theory in science, it has the weight of evidence behind it already. It’s only a theory because there may still be factors we don’t understand, but it has enormous and proven predictive power.

Take, for example, the theory of gravity. You might think that’s a given by now; we can observe it every minute of every day, and it’s proven consistent over the long term. There are equations describing gravity, and we can predict its effects. We know that when there’s a body of a certain size, we’ll observe a certain amount of influence on the part of gravity. That makes it a fact, right?

Well, in everyday terms, yes, it does. But in science, it’s still only a theory. There’s even ideas about modifying how we model gravity, so that we can better resolve questions about how the universe works and where it came from. While we can be certain that if we drop a ball on Earth it’s going to fall toward Earth (although you might think otherwise watching a ball come off Leigh Halfpenny’s boot), we can’t be certain that gravity works in the same way throughout the universe, in every situation.

The ball will come down… eventually. Image via WalesOnline.

That’s why it’s important to distinguish between theories and hypotheses. A hypothesis is a prediction about the world which can be tested: I hypothesise that if I go and pick ten dandelions and feed them to my (non-jerk) rabbit, she’ll prefer the larger ones. I can test that now, provided I can find dandelions, and I can repeat the test. I can even statistically analyse the results, and eventually, with enough data, I can come up with a full theory which explains everything I’ve seen by actually testing — and you can repeat my experiment and come to your own conclusions as well. (Please don’t feed my rabbits anything weird.)

Theories can be challenged with new data: that’s an important part of the scientific process. So when someone argues that evolution is “only a theory”, they’re not wrong. It just happens to be a theory which holds up against every test we’ve come up with. The evidence for evolution is stronger than my evidence that my blood type is O-positive; evolution has been tested again and again, while my blood type has only been tested twice, and I could have inherited A-type from my mother. Evolution is a theory which fits in with every observation we can make of the world around us (although there remain arguments about how exactly evolution operates). Any rival theory has to be even more convincing.

Post last edited at 17.16 CEST 22.04.2017: Rephrased final paragraph to remove references to natural selection.

17 thoughts on “Only a theory

  1. Evolution by natural selection doesn’t explain “every observation we can make of the world around us” even if you restrict that to what you obviously meant: Genetic drift and epigenetics are on a very sound footing and contradict the idea that evolution is solely driven by natural selection. Additionally, see the still controversial Dawkins vs. Gould debate.


        • Why? Evolution by natural selection does fit with every observation I know of.

          If there’s a (scientific) observation which contradicts natural selection (rather than fitting in alongside it), I’d like a link to the paper, please!


          • It’s misleading because it implies that natural selection is the only thing you need to know about to understand evolution, which is false. As for papers that contradict the idea of natural selection, I’ve repeatedly mentioned the entire Dawkins vs. Gould debate, which you’ve refused to acknowledge exists. How evolution works is not the clear cut and closed topic you seem to think it is.


            • No, it doesn’t? It implies exactly what it says: that evolution by natural selection fits with every observation we can make; there is nothing which contradicts it.

              On a micro level, scientists like Dawkins and Gould have disagreed on how exactly natural selection works, without saying that it doesn’t fit with observations, but I don’t think that’s relevant to explaining the way scientific theories work. If I was doing a whole post on evolution, I probably would go into more of the intricacies. As I’ve said before, not every post is going to cover all of science. Think of it as being on the level of New Scientist magazine, whose tone and level I am somewhat indebted to.

              If you’re looking for something which will go into every detail, I suggest that a year’s subscription to Science magazine might work better for you!


              • You’ve mis-understood the nature of the debate if you think Dawkins vs.Gould was merely about how natural selection works. Also, Dawkins wouldn’t know science if it bit him on the arse.

                As a scientist I have a responsibility to call people out when they spout bullshit, even well-meaning bullshit. It’s entirely possible to explain the difference between a hypothesis and a theory without misleading your readers about the true state of understanding of evolution.

                It’s not acceptable to say, “My intended audience is poorly educated, so I can get away with not being rigorous.” See e.g. Feynman on popular science communication.


                • It is not wrong (or “bullshit”) to say that natural selection fits with our observations. It does (along with, obviously, evolution by artificial selection and by chance, etc). Every further edit I’ve tried to make seems to needlessly complicate the point, given I’m not going to get into explaining artificial selection, genetic drift or anything else in a post which mentioned evolution as an aside because it’s one of those things commonly denigrated with the words “it’s only a theory”! If you have a suggestion for wording that would satisfy you, I’ll consider it.

                  From what I’ve read, Dawkins emphasises selection on the level of the gene; Gould generally on the level of an organism within a local population (although in practice, at a number of different and interrelated levels). Neither of those views are consistent with dismissing natural selection as far as I can see, and I can’t find any reference via the OU’s library to anything scientific which claims natural selection is wrong.


                  • Yes – the point I’m making is that it IS complicated. It’s a dis-service to the public to pretend it isn’t. Trying to encapsulate the situation in a sentence is doomed to failure. This argument, “Theory A is right but a bunch of other theories are right, too, but since A is right I don’t need to so much as mention the existence of any other theories,” misleads the reader in several ways.

                    Natural selection operates at the level of individuals. Suggesting that group selection or group de-selection (e.g. random meteorite strike wipes out a sub-species) is not significantly different is false. Hence there is a HUGE body of work suggesting that natural selection may be wrong, not dominate or at the very least is not the whole story. Since the geneticists now also agree with this, for reasons already mentioned, the Dawkins position is completely untenable. it is disingenuous to carry on talking about evolution in such simplistic terms and hence misleading people.


                    • but since A is right I don’t need to so much as mention the existence of any other theories

                      I see that you did not, in fact, read the edit I made to the post before saying it was still misleading. Perhaps your browser contained a cached version of the page?

                      As I said, I will consider alternate wording if you can suggest something you believe will be more clear.

                      From my perspective, you’re making vague statements which contradict everything I know from my studies, the OU library, etc. Please provide me with exact references — titles of books, papers, etc, which explain it. I’ll go and read whatever I can get my hands on.


                      are good starting points, though no doubt at least somewhat out of date by now. They at least show there’s a case to answer, though.

                      It’s widely perceived in the palaeontology community that undergrad biology classes usually fail miserably to even acknowledge the existence of arguments from palaeontology that challenge the conventional wisdom of the Neo-Darwinist crowd, mainly because geneticist don’t talk to anyone but other geneticists.

                      Your latest changes answer all my concerns!


                    • Ordered Reinventing Darwin, but The Structure of Evolutionary Theory is £50. Eep! I’ve also raised the question on the national forum of one of my courses, so we’ll see if anywhere there comes up with anything more up to date.


                    • Oh yes! I forgot that the Gould book is a monster. I wish I could introduce you to the folks in Bath working on evolution; they’re fascinating to talk to and could do a much better job than I can of telling you what the difficulties are. I remember there was an issue about algorithms based on genetic mutation theory predicting rates of evolution that don’t match the fossil record. There are several possible explanations that don’t involve natural selection being non-dominant, though. No idea where that led; it was back in 2014.


                    • I’ve now made further edits to remove the reference to natural selection altogether, since it’s often evolution as a whole people disagree with as “only a theory”. That may somewhat help with your quibble, since I’m fairly sure evolution as a whole is uncontested by actual scientists…


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