One way for the snail species to have complete chiral dimorphism where one form is an identical mirror image of the other form is if one form can't reproduce with the other form and it's predator is a species of snake that also has chiral dimorphism. If the snail species has chiral dimorphism and and one form can't reproduce with the other form but the species of snake that eats it does not, that means each individual snake of that species can eat either form and the evolutionary stable strategy is to have a mixture of both forms of snail in unequal proportions; if one form becomes more common, the snake will evolve to change a tiny bit in shape to be better at eating that form and less good at eating the other form so that form will evolve to be less common again. If the species of snail reproduces asexually and has chiral dimorphism and the species of snake that eats it also does, then the evolutionary stable will also be for both forms of each species to be identical mirror images occurring in equal proportions, even if it's very rare for in individual snail of that species to be eaten by a snake. If an asexually reproducing species of snail has chiral dimorphism and the species of snake that eats it doesn't, then both forms of snake will still occurr in equal proportions but the random fluctuations from that equilibrium will be larger because it will be the snake's brain adaption that makes it better at eating the more common form but its brain will already be so well adapted to eating both forms. According to Remi.b's answer at Does the species of snake that eats Amphidromous inversus have chiral dimorphism? and https://www.ncbi.nlm.nih.gov/pubmed/16217668, Amphidroumous inversus has chiral dimorphism occurring in very nearly equal proportions and one form can reproduce with the other form but the species of snake that it's it doesn't have chiral dimorphism. Maybe it can be treated very much like the case where the snail species reproduces asexually and the snake does not have chiral dimorphism but one form isn't quite an identical mirror image of the other form because the identical mirror image of either form would be less able to reproduce with that form and the species of snake that eats it is very close to symmetrical but isn't quite because the two forms of snail aren't identical mirror images. How can predation cause mutants of the rarer form of snail to arise and evolve into a different species entirely? Is it really because no mutants of the predator species of the mirror image form arise getting it into the evolutionary stable strategy where the predator and prey both have chiral dimorphism occurring in equal propertions?