A species of freshwater snail is introduced to many ponds on an island. In ponds with crayfish (a snail predator), the snails evolved a larger and more globular shell than in ponds without crayfish....

A species of freshwater snail is introduced to many ponds on an island. In ponds with crayfish (a snail predator), the snails evolved a larger and more globular shell than in ponds without crayfish. In an experiment, a grad student puts together snails from both types of ponds and carefully records their mating attempts. She finds that the difference in shell size and shape prevents most mating attempts between snails from the two kinds of ponds. Which statement summarizes what is happening in this example?