Where do they get these ideas from?
Mutations in DNA are relatively common because the copying process is not perfect, despite the mechanisms which have evolved to correct them.
I'll not go into the so-called genetic code here because, with a few clicks on Google, or by opening any of very many books on the subject, this can be easily found by those who wish to know more. Those who don't won't have bothered reading this far.
If anyone can tell me why a mutation which changes the genetic code for a small portion of a given enzyme from, let's say, UUAUAUCAUGUAGAUAACCCCUGA to UUAUCUCAUGUAGAUAACCCCUGA in the short sequence of mRNA, is prohibited by the second law of thermodynamics, I'd be very grateful...
Of course, there is no reason at all why this should be impossible; a proposition which is rendered even more absurd by the observable fact that it happens. There should be a clue here for Creationists. Hint: impossible things don't happen; things that happen are not impossible. (I appreciate the logic there might need more than a few moments thought for the average Creationist.)
So, what would this mutation do? In case you didn't notice, the second group of three letters has changed from UAU to UCU. In terms of the genetic code, this will mean the resulting protein will have the amino acid serine in place of tyrosine. The answer of course depends on what the protein does. If it's an enzyme, the chances are that, unless this section is from the functional part of the enzyme, it will have little or no effect at all.
So what new information has arisen by this mutation? Unless you regard changed information as new information, there is no new information. The information still tells a cell how to build the enzyme.
But what might have changed is the meaning of the information, and, just as with written language, the meaning of the information written down depends entirely on the environmental context. Written language only has meaning in the context of an environment in which literate speakers of the language exist. Otherwise, it has no more meaning than random marks on paper.
I'll illustrate this with a hypothetical, but possible, scenario:
In recent history, humans learned to harvest the natural sap of the rubber tree and use it as a material. A little latter, we learned to process this latex chemically by a process called vulcanisation which makes a much stronger substance suitable for making things like car tyres. Now millions of tons of vulcanized rubber are deposited annually on our roads and washed off into waterways or blown as dust into surrounding countryside to be incorporated into the soil.
Now, rubber is a natural substance, even in it's vulcanized form, so in theory is should be possible for a bacterium or a fungus to evolve the ability to process all this rubber in the environment. Maybe if only for the sake of road safety, we are fortunate that none has yet done so, so far as we know.
But, what if a mutation like the one I mentioned above were to change an enzyme in such a way that made this possible? What if this small change in information, in the context of all this man-made vulcanized rubber, gave a bacterium or a fungus a new source of energy?
What an advantage that would give it compared to forms carrying the non-mutated gene!
But what if this mutation had arisen, say, 500 years ago? It would, of course, have been completely meaningless and would have conveyed no advantage whatsoever. A mutation only has meaning in the context of the environment in which it arises. Out of that context it has no meaning at all.
It is the meaning of the information which changes and that depends on the environment. Environmental change drives evolution by giving meaning to the information in the genetic code. New meaning arises because new environments arise.