How do predator search images help preserve different shell colors in European snails?

Predator search images help preserve multiple shell colors in European snails of the genus Cepaea through a process of oscillating selection pressures.1

When a predator encounters a specific color morph, it develops a "search image"—a perceptual set that causes it to focus its hunting efforts on that particular type. Because the predator concentrates on the most common morph, it frequently overlooks individuals with different shell colors. As the targeted morph becomes increasingly rare due to predation, the predator eventually stops focusing on it and begins hunting randomly. This shift leads the predator to encounter a different color morph, at which point it forms a new search image.1

By continuously shifting their focus between different morphs, predators prevent any single color type from being eliminated, thereby maintaining several contemporaneous color variations within the snail population.1