common prey type is more abundant, they should be able to produce more offspring and grow exponentially, at a faster rate then those with the rare morph since they are in much smaller numbers. However, due to the fact that the common morph is preyed upon more frequently, it diminishes their expected rate of reproduction, thus maintaining the population in stable amounts of common and rare morphs. Essentially, unless an environmental change or an evolutionary change in predator or prey occurs, a stable equilibrium is produced. Search image A search image is what an individual uses in order to detect their prey. For the predator to detect something as prey, it must fit their criteria. The rare morph of a species may not fit the search image, and thus not be seen as prey. This gives the rare morphs an advantage, as it takes time for the predator to learn a new search image. Search image shift require multiple encounters with the new form of prey, and since a rare morph is typically not encountered multiple times, especially in a row, the prey is left undetected. An example of this is how a Blue tit searches for insect prey using a search image, leaving scarcer types of prey untouched. Predatory birds such as insect-eating tits (Parus) sometimes look only for a single cryptic type of prey even though there are other equally palatable cryptic prey present at lower density. Luuk Tinbergen proposed that this was because the birds formed a search image, a typical image of a prey that a predator can remember and use to spot prey when that image is common. Having a search image can be beneficial because it increases proficiency of a predator in finding a common morph type. Hypothesis for polymorphism Apostatic selection serves as a hypothesis for the persistence of polymorphism in a population because of the variation it maintains in prey. Apostatic selection has been referred to as "selection for variation in its own sake". It has been used as an explanation for many types of polymorphism in various species, including diversity in tropical insects. The selective pressure for tropical insects to look as distinct as possible is high because the insects that appear to have the lowest density in a population are the ones that are preyed on the least. Environmental mechanisms In order for apostatic selection to occur, and for the