Final answer:
Without variation in shell thickness and the absence of mutations, there's no trait for natural selection to act on in response to crab predation, potentially leading to a decline or extinction of the snail population.
Step-by-step explanation:
Without variation in shell thickness and the absence of mutations, the shell thickness in a population of snails would not change in response to crab predation. This is because natural selection depends on genetic variability within a population to bring about evolutionary change. Since there's no variation in shell thickness, there's no trait upon which natural selection can act to increase the resistance to crab predation. Over time, if predation pressure continues and favors individuals with thicker shells, the snail population could decrease significantly or possibly become extinct if no mutations arise that could lead to differences in shell thickness.