It is important to note that this statement is most often construed as referring to the ontological status of the theory. In other words, it is a statement about whether the theory agrees with what is happening in the natural world. Now some people like to say that evolution is a fact because it is based on factual data like fossils and genetic data. I would argue that that saying evolution is a fact because it is based on factual data is a misleading argument and should be avoided.
Fine. Now let’s move on.
There are two major categories of research in science; there is theory explanation and there is theory testing (justification). Sometimes these two categories overlap and they share similar research characteristics, but they are very different. I bring up these two categories (theory testing and theory building) because sometimes evolutionists and others confuse them. This confusion leads to unjustifiable truth claims about evolution (more on this in a bit).
When talking about the factuality of evolution, it is essential to distinguish between two types of evolution. Now I know that some evolutionists don’t like to distinguish between micro and macroevolutionary processes, and that’s fine; the distinction may not be important in their everyday work. However, the distinction is important when exploring the question “Is evolution a fact?” It turns out that one type of evolution has been subjected to rigorous testing and passed while the other has not.
Microevolutionary processes within a species is a demonstrable fact. It has been proven beyond a reasonable doubt with rigorous testing. Specifically, random genomic mutations and environmental selection have been shown to create change within a species.
However, macroevolutionary processes across life forms is not a demonstrable fact. It has not been proven beyond a reasonable doubt. Then why do so many evolutionists and others claim that macroevolution is a fact? I think there are two main reasons for this mistake. First they confuse the theory description and theory testing processes, and second, they do not understand the kind of testing that justifies “beyond a reasonable doubt” truth claims.
Much of the evidence for macroevolution is descriptive in nature. Here is what I mean. Macroevolution involves a lot of descriptive research wherein one goes out into the real world, gathers data, and best explains that data in a manner consistent with macroevolution. Gathering real world data and explaining that data in a manner consistent with a theory is an inductive process. Sometimes it is called making an inference to the best explanation or it is sometimes called ‘abduction’. Much of the real world data “fits” into the macroevolutionary theoretical framework, however, because the data are inductive in nature, they do not provide certain evidence of macroevolution.
If you are feeling confused, the following example should help.
Sherlock Holmes used induction when he collected observable facts and used those facts to reconstruct what happened at a crime scene. (It is interesting to note that Holmes said he was using ‘deduction’ - actually he was using ‘induction’.) In philosophical terms induction is going from particulars to a single explanation, and that is what Holmes did; he went from particular evidence to a single conclusion about who committed the crime. In the same way evolutionists collect facts in the natural world and explain or describe them using macroevolution (common descent). This too involves moving from particular field evidence to a single conclusion (macroevolution). Although the evidence may appear to support a conclusion, we cannot be certain that either conclusion is correct. Holmes never saw the crime being committed and an evolutionist never witnessed common descent (evolution from one life form to another). This sort of evidence does not lead to certainty.
Yet evolutionists argue that macroevolution is on the same footing as relativity and gravity, because, they claim, relativity and gravity were built using the same inductive methodology. This is true. Relativity and gravity were built using the same inductive methods; however, evolution is not on par with these two well established theories. Why not? The answer is that macroevolution lacks rigorous testing; it has not been subjected to, and passed, tests of hypotheses to the same extent that relativity and gravity have.
What macroevolution lacks are controlled crucial tests of its core tenets, and assuming that such tests could be carried out, the theory would also have to survive those tests. A major barrier to carrying out a controlled, crucial test is time – too many years are required to observe change from one life form to another. (Note that some evolutionists like to point to one species of salamander evolving into another “species”. I am talking about going from a salamander to another significantly different life form like a platypus.)
On the other hand, relativity and gravity have been subjected to crucial tests of their core tenets, and survived. The last time you accidentally dropped something you unwittingly submitted gravity to a crucial test, and I am sure it passed (the item fell). In 1919 a research team led by Sir Arthur Eddington submitted relativity to a crucial experiment by measuring shifts in the Hyades star field near the sun during an eclipse. And in 1971 Hafele and Keating submitted relativity to a crucial experiment by measuring time differences in previously synchronized atomic clocks flown in jet airplanes. Relativity passed both tests. Because of these and other successes, we've elevated gravity and relativity to law-like status. For the time being they have been proven beyond a reasonable doubt – macroevolution has not.
So the bottom line is that microevolution is a fact, but macroevolution is not. This does not mean that macroevolution is necessarily false; it does mean, however, that it has not been proven beyond a reasonable doubt. In a manner of speaking, yes, it is still just a theory.