The phrase “theory of evolution” is sometimes used pejoratively by people who do not want to acknowledge that populations and species of biological organisms evolve. In fact, the legislature of the state in which I reside – New Hampshire – is now considering legislation (e.g., see here and here) “requiring the teaching of evolution as a theory in public schools”. As any scientist will tell you, the meaning of the word theory in science is very different from its common usage.
Pondering this difference got me to thinking about how the theory of evolution stacks up to other more readily accepted scientific theories, at least in the public’s mind. We all have an intuitive understanding of many physical phenomena, and so I wanted to compare how the theory of evolution stacked up against our scientific understanding of something as familiar as, say, gravity.
Gravitation, the attraction of physical objects to one another, is one of the four fundamental interactions in nature. No one would question the phenomenon of gravity. Objects fall back to earth when not supported. But what is the scientific explanation for what causes this phenomenon? Scientific theories are statements of these explanations.
I think most non-scientists would be dumbfounded to find that physics has no single theory of gravitation. Currently, two theories are the main competitors as the explanation for why two objects are attracted to one another, but a number of other theories have been proposed as alternatives to these two, as well. (For a fuller and more nuanced description, see a number of popular books on the latest theories of physics (e.g., books by Brian Greene).)
One theory of gravitation is derived from Albert Einstein‘s general theory of relativity. In general relativity, gravity is imagined to be created by the presence of an object with mass warping the surrounding spacetime, which will in turn change the trajectory and velocity of other objects (even massless particles like photons of light) in the affected part of spacetime. Thus, gravity in Einstein’s conception is the result of how objects influence the fundamental geometry of space and time.
The other main theory of gravitation is derived from quantum mechanics and postulates that gravity results from the exchange of gravitons between two masses. The graviton is an as-yet hypothetical but elementary particle, which means it has never been observed, and no experimental evidence has been generated to suggest its existence.
How could two conceptions of a phenomenon be more different? One re-imagines the geometry of the universe, and the other invokes a particle. The existence of these seemingly incongruous formulations of gravity in the scientific community and the fact that scientists disagree about how to understand gravity does not negate the phenomenon of gravity or the fact that gravity exists.
Now, let’s turn to the theory of evolution. The mechanisms causing evolution are pretty simple and easily understood by anyone. Four different mechanisms cause evolution: genetic mutations, genetic drift, gene flow, and selection in its various forms. In fact, most “theories” describing various specific mechanisms of evolution are simply accounting problems.
When people use the theory of evolution as a pejorative, what they are denying is the phenomenon of evolution and not the theory. Their issue is “does evolution happen” – not “how does evolution happen”. The comparable issue with respect to gravity would be to question whether a rock would fall to the earth if you released your grip on it.
The phenomenon of evolution is slightly more difficult to detect than the phenomenon of gravity, because it must be observed over a somewhat greater expanse of time than a few seconds. However, the evidence is all around us. For example, the new flu strains that infect humans each year result from the evolution of the flu virus. Similarly, viruses and bacteria evolve resistance to drugs continually. Individuals in many species today are quite different than the individuals in those species a century ago, and we know those differences are genetically based and so must have evolved (the example of the soapberry bug being an exemplar). We can even make other species evolve ourselves. Scientists do such things in the laboratory and in controlled populations in the field all the time, and humans have also been making other species evolve for literally thousands of years. Look at any domesticated animal or plant, and ask where this creature existed in the wild before humans domesticated it. The phenomenon of evolution is irrefutable.
The theory of evolution, like the theory of gravitation or any other scientific theory, is certainly a work in progress. Should this then behoove the legislature to pass a law “requiring the teaching of gravity as a theory in public schools” or the teaching of anything in science “as a theory”? The scientist in me says, “well of course, theories have a hallowed and revered place in science, and no progress in science is made without well reasoned and well supported theories like those that explain the phenomena of evolutionary change in populations and species.” But the citizen in me knows that’s not the point.