Evolution is not a fact

Image credit: Genetic Literacy Project https://geneticliteracyproject.org/

OK, I have to admit, the title I chose is a little 'click-baity'. But if you're coming in hot to set me straight, cool your jets for a moment, and listen.

I completely accept that biological evolution is the best available explanation we have for the diversity of lifeforms we have on this planet, including but by no means limited to the human lifeforms. From the time when life came from non-life (through a separate process of abiogenesis, which is a whole different discussion), to right now, the modern 'neo-Darwinian synthesis' understanding of evolutionary theory is supported by multiple independent lines of evidence. It continues to be modified and improved as our knowledge grows, but the broad outlines remain the same.

So what do I mean when I say that it is 'not a fact'? I'm making a point in the philosophy of science about what kind of thing evolution is, rather than about its truth (or, better, the evidence and arguments supporting it as our best available explanation).

In the philosophy of science, a fact is something we can measure and observe, and a theory is something that explains a fact. It is a fact that, if we ignore air resistance, objects dropped near the surface of Earth will accelerate toward the centre of gravity of the planet at about 9.81 m/s/s. That's a measurable fact. There are competing theories to explain that fact: Newton's theory of universal gravitation and Einstein's theory of general relativity. Both explain that fact satisfactorily, but if we instead want to drop an object near the event horizon of a black hole, Einstein's theory offers a better and more powerful explanation.

A theory never turns into a fact, no matter how much evidence supports it, because they are qualitatively different kinds of things. We might say that evolutionary theory is 'factual', in the sense that we mean it is true, or is the correct (though I would still want to insist on 'best available' as a better way of saying it) explanation for the diversity of life. But saying 'evolution is a fact' betrays a misunderstanding of the nature of science.

Evolution explains facts, such as the increasing complexity of the living things found in the fossil record and particular patterns in the genes of related species. These are facts. The fusion of Chromosome 2 in humans compared to its existence as two separate chromosomes in most primate and other hominid species, for example, is a fact.

But evolution is a theory. Not 'just a theory', in the lay sense of a 'best guess', or worse, a 'conspiracy theory'. It is a theory, like Newton's and Einstein's theories of gravity. It is the collective human scientific community's best current explanation, based on evidence, argument, experience and that special class of controlled experience called 'experiment', for the enormous body of facts derived from the fossil record, from the features and characteristics of living things (plant, animal, protista, fungi, monera) and from the DNA of living things.