This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post, we discuss what a scientific theory is, and how scientists use them to make predictions about how the world works.
Not a hunch, just a theory
In common English usage, “theory” means something like “guess” or “hunch”. It means something speculative, uncertain. In science, however, the meaning is almost exactly the opposite. In science, a theory is an idea that has stood the test of time. This difference between the common usage and the scientific usage of the word is a frequent source of confusion for nonscientists. In science, a theory is a well-tested idea – an explanatory framework that makes sense of the current facts available, and continues to make accurate predictions about the natural world.
Theories get their start as merely an idea, or hypothesis (plural = hypotheses). This literally means “less than” (hypo) a theory (thesis)”, and the name is appropriate. What scientists call a hypothesis is basically what nonscientists call a “theory” in the common English sense we discussed above. It’s an idea that makes sense, and fits with what we already know, but as such does not yet have much (or even any) experimental support. Here is where science departs from other approaches to knowledge: the key feature that distinguishes science from other activities is hypothesis testing. Rather than merely entertain a hypothesis as an interesting idea, scientists use a hypothesis to make specific predictions about the natural world, and then test to see if these predictions can be supported with experimental evidence. If the prediction is supported by the results of one experiment, scientists will use the same hypothesis to make (and test) more predictions. If the hypothesis is in fact an accurate idea about the way things really are, then this hypothesis will continue to make accurate predictions. Over time, as the idea gains more and more experimental support, scientists eventually drop the “hypo” prefix from hypothesis and start referring to the idea as a theory – a well-tested explanatory framework that continues to make accurate predictions about the natural world.
Theories: well-tested, but provisional
Despite being well-tested ideas, however, theories in science are never accepted as absolutely true. During hypothesis testing, only two results are possible: the scientist can reject the hypothesis if it did not make an accurate prediction, or the scientists can fail to reject the hypothesis if it did make an accurate prediction. The important point is that the scientist cannot accept the hypothesis. Put another way, science can show that certain ideas are “wrong” (in that they cannot be used to make accurate predictions about the natural world), but science cannot show that a given idea is “right” or “true.” To say that a hypothesis is “right” would be to imply that it will withstand all future tests of predictions it makes – something that is not possible, since there are always more tests that can be done. All science can say is that an idea has not yet been shown to be wrong. As such, all theories in science are seen as provisional, and are revised as new information comes in. The point here is this: theories in science remain theories – they don’t graduate to become something else (like a “law” for example).
So, a theory is an interesting entity in science – at the same time it is known to be both a powerful explanatory framework and a provisional one, subject to future revision (or even abandonment, should an even better idea be found). In practice, some scientific theories are so well supported that it is highly unlikely that their core ideas will be significantly changed in the future. These theories are ideas that are very close approximations of the way things really are, and as such they won’t change appreciably. Once a theory gets to this level, science accepts it as a given and moves on to other areas, nearer the fringes of what we do not know.
Learning from the past
Perhaps an example from history would be useful here. Take the theory of heliocentrism – the idea that the sun is the center of our solar system. (If it surprises you to hear this idea referred to as a theory recall that we are using the scientific meaning for theory here. Obviously heliocentrism is a very well-supported idea, and it’s not likely going to change in the future, but it remains a theory in the scientific sense). When heliocentrism was first conceived as an idea in contrast to an Earth-centered solar system there was precious little evidence to support it. Indeed, it had popularity only among mathematicians, who were attracted to the idea based on its simplicity and elegance. Once the idea was articulated, however, evidence came to light that supported it: Galileo’s observation that Venus had phases, like the moon (an observation incompatible with the standard geocentric model of the time) and his observation that Jupiter was orbited by four moons (a model in the heavens of bodies in motion around a larger body).
Now, Galileo’s observations allowed science to discard the standard geocentric model, but not an alternative geocentric model advanced by Tycho Brahe. Heliocentrism did make a key prediction, however. In Brahe’s model, like all geocentric models, the earth was predicted to be stationary. In the heliocentric model, the earth was in motion, orbiting the sun. This key prediction (and, at the time, the lack of evidence supporting it) was not lost on those commenting on this issue in the years after Galileo:
Again, I argue thus, the Motion of the Earth can be felt, or it cannot: If they hold it cannot, they are confuted by Earth-quakes … I mean the gentler Tremblings of the Earth, of which there are abundant Instances in History, and we our selves have had one not long since; so that by too true an experiment we are taught that the Earth’s Motion may be felt. If this were not a thing that had been frequently experienc’d, I confess they might have something to say, they put us off with this, that it is not possible to perceive the moving of the Earth: But now they cannot evade it thus; they must be forc’d to ackowlegd the motion of it is sensible. If then they hold this, I ask why this Motion also which they speak of is not perceived by us? Can a Man perswade himself that the light Trepidation of this Element can be felt, and yet the rapid Circumvolution of it cannot? Are we presently apprehensive of the Earth’s shaking never so little under us? And yet have no apprehension at all of our continual capering about the Sun?1
Unfortunately for Galileo, direct physical evidence of the earth’s motion would have to wait until the 1720s, when stellar aberration (the effect of the earth’s motion on starlight) was first observed. It would take over hundred years more (the 1830s) for the first successful measurement of stellar parallax, the slight shifting of the relative positions of stars as observed from earth due to our change in perspective as the earth moves through space. By the time this observation was made, heliocentrism was a theory—a well-tested framework that made accurate predictions, including predicting stellar parallax. Of course, by the 1830s, heliocentrism had come a long way from its humble beginnings, and it continued to be modified in accordance with new evidence afterwards as well. Still, as an idea, it stood the test of time since it was a reasonably accurate representation of the way things really are. We accept it (yes, provisionally) since it is a productive, useful framework. Its core ideas are not likely to change, even if we add nuances to it now that Galileo could not have imagined. While it’s difficult to imagine, we might even discard it some day, should an even better framework come along—but any competition will have a very tough battle ahead of it.
Evolution as theory
So, what does any of this have to do with evolution? Simply this: despite what many evangelical Christians have been told, evolution is a theory in the scientific sense. It started off as a hypothesis, and scientists have been trying to reject that hypothesis to no avail. In the present day evolution is an explanatory framework that has withstood 150 years of testing, and continues to make accurate predictions about the natural world. Like heliocentrism, our ideas about evolution have developed significantly since the 1850s. In the next post in this series, we’ll sketch out some of the lines of evidence that Darwin offered in his Origin of Species, before going on to examine the state of the evidence in the present day.