"Evolution is only a theory" is one of their favourite slogans, aimed, as it invariably is, at a scientifically illiterate audience to whom they are selling their latest book or to whom they are pushing an extreme right-wing, anti-democratic, anti-egalitarian theocratic political agenda disguised as a moral crusade and intended to keep on side those whom they plan to keep at the bottom of the social peck-order.
It is intended to mislead by playing on the different meanings of the word 'theory' as used by science and by a lay person respectively. It is normally followed by ' - a guess with no supporting evidence', 'not a fact', or 'which has not been proved', all of which intentionally reinforce the misinformation with disinformation. In common parlance, lies.
The Theory of Evolution (which explains the fact of evolution) is of course, as has been repeatedly said, no less a scientific theory than is the Theory of Gravity which explains the fact of gravity. In fact, in the early part of the 20th century, Isaac Newton's Theory of Gravity was overthrown by Einsteinian Relativity. But never-the-less, the Theory of Gravity remains 'just' a theory with parts still waiting to be proven. No sane person would doubt gravity because the current theory has a few small gaps, yet the Theory of Evolution is actually more satisfactory than is the Theory of Gravity.
I came across this article on the superb Friends of Charles Darwin website. I'll let its intelligent designer, Richard Carter do the talking:
As with gravity, the effects of Darwinian evolution are all around us. We see them in the pigeons in our streets, the spiders in our bathtubs, and the opposable thumbs on our hands. Yet most of us hardly give them a second's thought. And, as with gravity, precocious young whippersnappers have amended and improved Darwin's original theory, but its original essence is still very recognisable in the latest thinking.Of course this was written before the recent announcement of the discovery of the Higgs boson at CERN, which has gone some way to solving one of the mysteries in the Theory of Gravity by explaining where mass comes from, but the evidence for gravitons or gravity waves is still awaited. They remain 'just a theory'.
But here the comparison begins to break down. Where Darwinian evolution differs from Newtonian (and, indeed, Einsteinian) gravitation, for the time-being at least, is that we do not yet have an observable mechanism for gravity. What actually makes gravity work? Yes, physicists can hypothesise about gravitons and gravitational waves as much as they like, but they have not yet been able to detect either of them in the laboratory (and I will surely never be able to detect them in my bedroom).
Nature red in beak and claw:
A sparrowhawk eating a sparrow
in my garden last April.
© Richard Carter
Darwinian evolution, on the other hand, certainly does have observable mechanisms driving it: variations amongst the individuals of a species, heredity and—Darwin's big idea—Natural Selection (capital 'N', capital 'S'). We can see all of these mechanisms at work on a daily basis, if we take the trouble to look. We can see them in our streets and gardens, on our bird feeders, and in the cobwebs on our bedroom ceilings. Everywhere we care look, Nature is still red in tooth an claw.
Darwinian evolution does not rely on mysterious, unobservable particles or waves; its mechanisms are in action all around us in our everyday world. Which is why, although Newtonian gravitation has a certain attraction, it seems to me, Darwinian evolution is, in this way at least, a far more satisfactory theory.
Strange then that creationist pseudo-scientists are never found trying to get away with casting doubt on the Theory of Gravity when they don't even need to tell lies; they could just point out the parts which are unproven and to the gaps in the theory which science has not yet closed and then rely on the parochial ignorance of their target audience to conclude that it must have been the locally popular god who did it.
Why don't they do so?
Because they would never get away with it. No sane person would dispute the existence of gravity, so casting doubt on the current scientific theory which explains it would do nothing to convince people that they can fly or jump off high buildings with impunity.
Besides, there is no claim from ancient mythology that gravity was magically created by a magic man who will cause unimaginably nasty things to happen to you if you don't do what the theologians tell you to do, believe what they tell you to believe, vote for those whom they tell you to vote for, make generous donations to those who promulgate these lies and misinformation, and buy their books.