Evolution: It’s the law(s)!
An interesting read in The Conversation last week about the language of science philosophy. What is a theory, what is a law and what is an hypothesis? These ought to be elementary definitions for anyone who studies science but I certainly learnt something during my read and on reflection, it changed my mind on some other science philosophy issues I have: the laws of evolution.
In the article, Peter Ellerton from the University of Queensland runs over some sci-philosophy 101 with a particular focus on popular misconceptions about the language of science.
A scientific law is a description of how nature behaves. While a law may describe a phenomenon, it does not explain it. Newton’s laws of motion describe how things move. The laws of thermodynamics describe how energy is transferred and transformed. But there is no explanation in these laws of why things move or where the energy comes from. Explanations beyond descriptions require theories.
Theories are models set up to explain why and how things work. Kepler gave us the laws of planetary motion but not an explanation of why they behave that way. The theories behind those laws require gravity and mass: theories about the nature of the objects obeying the laws. And theories have an ability to be tested or examined, the opportunity to be shown to be wrong; the concept of falsifiability. All the tests you do to examine your theory do not prove it correct because there could be some set of circumstances under which you could have been given a contrary result. But a single test against your theory demonstrates it is wrong. So no theory is proven or upheld to be right, it is only accepted as correct so long as it hasn’t been shown to be wrong.
Something that Ellerton brings up in his article that made me do a double take is the claim that laws do not develop from theories. There has been a dogmatic acceptance (and much to my chagrin, I have to say that I had gone along with this), that laws are established when theories are tested intensively and still not shown to be in error. But this is plainly wrong; go back to Kepler or Newton above and that was not how those laws were derived. Laws as a description of the universe should be supported by theory and the repeated observations from testing a theory may lead to the description of a law, but the two can exist in isolation.
Another thing that Ellerton challenged me with was the idea that an hypothesis can inform both laws and theories and in some ways is more closely aligned to laws. I had only really thought of hypotheses as untested models, initial propositions that could become theories if tested and supported. But clearly you can also hypothesise about the nature of a law as a description of nature, as Kepler and Newton did.
So all in all, a worthwhile read from Mr Ellerton and I wish him well in his PhD candidature. But it lead me to think about why there are no laws of evolution? I can certainly propose a few observations about the history of life that would constitute laws of evolution as laid out by Peter but we always talk about the theory of evolution, never the laws.
Well almost never! A quick Google search unearthed the fact that the great biologist Ernst Mayr had indeed proposed Darwin’s original proposition for the theory of evolution via natural selection could be reformulated into five laws:
1) That life evolves. That is, it changes through time.
2) That life is related by common descent.
3) That life diversifies. New species are generated and multiply through time.
4) That evolutionary change is gradual (and this is still true under models such as punctuated equilibrium which postulates varying rates of evolutionary change but all of them gradual).
5) Natural selection as the agent of evolutionary change.
There is some equivocating around some of these margins. There are, for example, agents of evolutionary change other than natural selection but it is by far the dominant mode. Rules one, two and three are so thoroughly demonstrated as to be beyond challenge and four requires clarification (as provided!) concerning more recent debates about its nature but is still firm. And all five can be held up as laws as Ellerton defines them in his article: they are all descriptions of natural phenomena and they can all be demonstrated to be applicable to the history of life.
So, if like me, you are occasionally afflicted with a creationist claiming that evolution is only a theory (a claim that apparently can be traced back to Ronald Reagan), now you have a powerful rebuttal. Yes, there are several theories of evolution and there are also five solid scientific laws that underpin it.
By Paul Willis, @Fossilcrox
Image credit: wallygrom