Answer:
The average of the snails having less thick shell will disappear.
Explanation:
When a population of snails consists of both less thick and more thick snails. When an invasive species of crab enters this population.
This crab is capable of eating the snails having thin shell. Sooner or later the crabs having thin shells will disappear and the snails having thick shells will survive more.
Over a period of time there will be no snails left with thin shells only the shells having thick shells will be naturally selected and will reproduce to have a population.