Apostatic selection is a form of negative frequency-dependent selection. It describes the survival of individual prey animals that are different (through mutation) from their species in a way that makes it more likely for them to be ignored by their predators. It operates on polymorphic species, species which have different forms. In apostatic selection, the common forms of a species are preyed on more than the rarer forms, giving the rare forms a selective advantage in the population. It has also been discussed that apostatic selection acts to stabilize prey polymorphisms.
The term "apostatic selection" was introduced in 1962 by Bryan Clarke in reference to predation on polymorphic grove snails and since then it has been used as a synonym for negative frequency-dependent selection. The behavioural basis of apostatic selection was initially neglected, but was eventually established by A.B Bond.
Apostatic selection can also apply to the predator if the predator has various morphs. There are multiple concepts that are closely linked with apostatic selection. One is the idea of prey switching, which is another term used to look at a different aspect of the same phenomenon, as well as the concept of a search image. Search images are relevant to apostatic selection as it is how a predator is able to detect an organism as a possible prey. Apostatic selection is important in evolution because it can sustain a stable equilibrium of morph frequencies, and hence maintains large amounts of genetic diversity in natural populations.
It is important to note however, that a rare morph being present in a population does not always mean that apostatic selection will occur, as the rare morph could be targeted at a higher rate. From a predator's view, being able to select for rare morphs actually increases the predator's own fitness.
Prey switching
In prey switching, predators switch from primary prey to an alternative food source for various reasons. This is related to apostatic selection because when a rare morph is being selected for, it is going to increase in abundance in a specific population until it becomes recognized by a predator. Prey switching, therefore, seems to be a result of apostatic selection. Prey switching is related to prey preference as well as the abundance of the prey.
Effects on populations
It has also been determined that apostatic selection causes stabilization of prey polymorphisms due to the limitations of predators' behaviour. Since the 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.