Final answer:
The answer is D. There is no change in shell thickness within the population because without genetic variation or mutation, there is no material for natural selection to select for or against.
Step-by-step explanation:
If there is no variation in shell thickness within a population of snails, and no mutations occur, the correct response to what happens to shell thickness in response to crab predation is D. There is no change in shell thickness within the population. Variation and mutation are essential mechanisms of evolution. In the absence of genetic variation or new mutations, there is no raw material for natural selection to act upon. Hence, the shell thickness would remain the same even in the presence of crab predation, unless other evolutionary mechanisms come into play. This is an example of how environmental conditions can lead to speciation and the evolution of particular traits such as camouflage or beak size in birds, as related adaptations are passed on to successive generations.
If there is no variation in shell thickness within a population of snails and no mutations occur, it means that all snails in the population have the same shell thickness. If crab predation occurs, the snails with thicker shells will have a higher chance of survival because the thicker shells provide better protection against predation. Over several generations, natural selection will favor the snails with thicker shells, leading to an increase in the average shell thickness in the population. Therefore, the correct answer is C. The average shell thickness evolves to be much thicker over several generations.