I think in discussions such as this it is important to establish what does or does not count as a legitimate argument against a scientific theory. Science has rules, and time and time again I find that arguments against evolution demonstrate serious misunderstanding—or even a total lack of understanding altogether—of what the rules actually are.
First of all, setting unrealistic standards of evidence is NOT a legitimate argument against a scientific theory. It’s a classic science denial technique to demand evidence that no scientific theory could possibly be expected to meet, such as a complete fossil record of every generation all the way back to the Cambrian, or continuous observation of the process over millions of years (the “were you there?” argument). But that’s just a case of saying “Heads I win, tails you lose.” It is often coupled with standards of evidence on the alternative that are so low that it would be near-impossible for them to fail to meet them. That is dishonest.
If you want to cite an absence of evidence as an argument against a scientific theory, you need to establish that it is evidence that we should realistically expect to see. You also need to account for the evidence that actually does exist, such as genetic similarities (which are far more complex and detailed than just a simple numerical percentage), or biogeographical distributions, and so on. And you need to apply the same standards of evidence to the scientific theory itself that you apply to the alternatives.
Second, assumptions are not a get-out-of-jail-free card,. The word “assumptions” gets bandied about all too often in arguments against evolution or deep geological time as if it were some sort of magic shibboleth. It isn’t. In science, assumptions have rules and so too do challenges to those assumptions. If you want to challenge a scientific theory on the grounds that there are assumptions involved, you need to do three things.
- State what the assumptions are.
- Make sure that the theory really does make the assumptions you are claiming that it makes, and that it hasn’t developed new techniques to test the assumptions or to avoid them altogether.
- Provide an explanation as to how the assumptions could have been violated while still producing the same evidence that we see as an end result. For example, if you are going to argue against “uniformitarianism” (in the sense of constant rates of change), you need to explain how unrelated correlating processes such as nuclear decay and continental drift could have varied in complete lock-step with each other.
Anything less is just vacuous hand-waving.
So how do you argue against a scientific theory then? You have basically two options.
On the one hand, you can provide evidence that contradicts it. However you need to make sure that your evidence contradicts its core fundamentals, and not just some incidental details. You don’t knock down a house by clearing out the furniture.
Alternatively, you can provide an alternative explanation that does a better job of explaining the evidence that exists. “Better” means specifically that it does so in greater detail, with fewer assumptions, fewer unknowns and greater mathematical precision.
For a more detailed treatment of these principles, see here: