[Richard Dawkins] points out (In 'Devil's Chaplain') that all species experience freak mutations from time to time. This is only natural. And these disfigured creatures don't die automatically. If they live long enough to breed, their mutation becomes part of the next generation, and can even become an asset down the line.Well, nobody is arguing with the proposed general model of evolution - we all know that mutations are supposed to be the engines of evolution. No big deal there, if this is what Dawkins is suggesting.
Slug -> Slug with hardnened calcium [sic] on back ->->->-> slugs with most calcium don't get eaten as much ->->->->->->->->->->->->->->->-> Snail
I'm not saying that example is necassarily true (I'm not an expert on slugs...) but it explains the general theory, and fits in with scientific fact.
But let's look at this example. Set aside also for now the issue of whether or not this process is observed in evolutionary history - whether we see intermediates between slug and snail. This "story" doesn't actually explain anything, in scientific terms. All it does is repeat the proposed mechanism, in the context of slugs and snails, whilst adding no further detail.
What "mutation" causes a slug to have "hardened calcium" on its back? It won't be just the substitution of one DNA base. I seriously doubt that it would be the addition of just one protein. The slug would have to evolve a mechanism for synthesising the calcium based compounds, and a mechanism for secreting these compounds onto its back, and a mechanism for regulating this secretion and synthesis. There would have to be additional differentiation in the secretory cells. In fact, I would suggest that a single mutation which led to such an evolutionary step becoming apparent would have to have been preceded by substantial developments, which yielded no substantive evolutionary advantage.
As it happens, once the means of getting layers of calcite onto the back of the slug/snail intermediate are there, the means of getting the curly shell could be more straightforward - they could be a function of continued differential secretion. But the real evolutionary challenge is the one represented in this extract by the single arrow.
That's one of the issues that I have with darwinism. It accepts the "just-so stories" as being sufficient answers - at least for the proles who don't really understand. Certainly there are scientists who go on looking for answers, and I'm not saying "God did it" - it is thoroughly appropriate to continue to explore exactly how such processes occurred. But the published answers that everybody is told aren't based on evidence; they are based on a prior commitment to a naturalistic explanation. I would suggest that, although examples of small-scale evolutionary changes are readily available, the number of detailed explanation of even apparently slight evolutionary physiological changes is very small.