Fthere is no variation in shell thickness within a population of snails, and no mutations occur, what happens to shell thickness in response to crab predation?

shell thickness does not evolve in the population.

Explanation:

As you already know, snails have a shell that protects them from the attack of other predatory animals, ensuring the survival of the snail. The shell thickness of snails should vary in thickness throughout the life of the animal to ensure this protection. This thickness variation occurs by evolution and should occur throughout the snail population. However, if there is no variation in shell thickness within a snail population, and no mutations occur, the shell thickness does not evolve in the population, leaving the snails susceptible to predation.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.