Apostatic selection

Apostatic selection was used in 1962 by Bryan Clarke in reference to predation on polymorphic grove snails and since then it has been used interchangeably with negative frequency-dependent selection. The behavioural basis of apostatic selection was initially ignored, but 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, and the rare morph could be targeted at a higher rate. From a predatory view, being able to select for rare morphs actually increases its own fitness 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. It has also been determined that apostatic selection causes stabilization of prey polymorphisms, and this is caused by limitations of the predators behaviours. Since the common prey type is more abundant, they should be able to produce more offspring and grow exponentially, at a much 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 diminish the exponential rate that they are expected to reproduce in, thus maintaining the population in stable amounts of common and rare morphs. Essentially, unless and environmental change or a species evolves it produces a stable equilibrium. 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 gets 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 supposed 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. Apostatic selection serves as a hypothesis for polymorphism because the variation it causes in prey. It is an explanation for why external polymorphism exists and this theory has been tested many times. Apostatic selection has been referred to as 'selection for variation in its own sake'. Apostatic selection has been used as an explanation for many types of polymorphism, including diversity in tropical insects. Selection on different morphs in tropical insects is high because there is pressure for phenotypes to look as different as possible from all others because the insects that have the lowest density in a population are the ones that are preyed on the least. In order for apostatic selection to occur, and for the rare morph to have the advantage a variety of criteria needs to be met. First, there needs to be polymorphism present. In addition, the prey present can not be in equal proportions, since then there would not be a benefit to be able to detect either one. This is related to frequency dependent predation, where as the predator obtains the greatest advantage from having a search image for the most common type of prey. This causes the most common form of the prey is the most vulnerable. Changes in prey detection of predators occurs, but the speed in which it occurs and the flexibility a predators search image in dependent on the environment.