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.
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. 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. 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".
Environmental mechanisms
In order for apostatic selection to occur, and for the rare morph to have an advantage, a variety of criteria need to be met. First, there needs to be polymorphism present. In addition, the prey cannot be present in equal proportions, since then there would not be a benefit to being 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 to be the most vulnerable. Changes in prey detection by predators do occur, but the speed in which they occur and the flexibility of a predator's search image depend on the environment.
If the frequency of the different prey types continuously changes, the predator may not be able to change its behavior at a rate that will provide an advantage. Another type looks into how apostatic selection can act on the predator as well as the prey, as predator plumage polymorphism can also be influenced by apostatic selection. They hypothesized that a mutant predator morph will become more abundant in a population due to apostatic selection because the prey will not be able to recognize it as often as the common predator morph. Apostatic selection has been observed in both humans and animals, proving that it is not exclusive to lower organisms, and the cognition it requires is applicable to all organisms which display learning. Though a lot of this work has been experimental and lab controlled, there are some examples of it happening in both wild specimens and in the natural habitat of the species.
In hawks, almost all polymorphism is found on their ventral side. It allows for less common coloration to be favored since it will be recognized the least. A paper by Pfenning et al., 2006 looks into this concept. In allopatric situations, situations where separate species overlap geographically, mimic phenotypes have high fitness and are selected for when their model is present, but when it is absent, they suffer intense predation. In Pfenning's article it was suggested that this is caused by apostatic selection because strength of selection is higher on the mimics that have their original model present.
In Batesian mimicry, if the mimic is less common than the model, then the rare mimic phenotype is selected for because the predator has continued reinforcement that the prey is harmful or unpalatable. As the mimic becomes more common than the model, the situation reverses and the mimic is preyed upon more often. Therefore, dishonest signals in prey can be selected for or against depending on predation pressure. The two colors of prey were presented in 9:1 ratios, and then the prey were switched so both colors had an opportunity to be over or underrepresented. One limitation of this study was that the morphs in the wild were not able to be manipulated.
See also
- Frequency-dependent selection
- Polymorphism (biology)