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.
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.
Effects on populationsEdit
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.
Hypothesis for polymorphismEdit
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.
If the frequency of the different prey types is continuously changes, the predator is not able to change their behavior at a rate in which will provide an advantage. In these situations, the predators who show a more flexible behaviour and have a more broad search image are able to survive. In relation to apostatic selection, large changes in prey frequencies decreases the magnitude of the advantage of the rare morph if their predators have a flexible search image. Also, the high changes in polymorphism frequencies can be an advantage to the prey with the rare morph. This is because the predators without the flexibility of their search image would have to have many encounters with the rare morph to change its search image. Predators need multiple consistent encounters with a prey in order to form its search image around it.
Apostatic selection is also dependent on temporal variation. Since long periods of time are required for natural selection to act on predators, their degree of flexibility in their search image  can not be changed over a short time frame. Therefore, quickly arising rare morphs favors apostatic selection since the predators are not able to change their behavior and search image in that time frame. This is yet another biological process that is a victim to evolutionary time delay.
The predators are more quickly to adapt and decrease apostatic selection when a drastic and abrupt change to the prey frequencies occur. This does not change the flexibility of the predators, but elicits a very high speed in the change of the search image.
Apostatic selection is most strong in environments in which the prey with the rare morphism match the background.
Behavioural Basis of Apostatic SelectionEdit
Most of the work done on the behavioural basis of apostatic selection was done by A.B Bond. It has been suggested that for frequency dependent predation, the amount of encounters with the prey aids to shape the predators prey detection. These ideas are based on the assumption that when the predator is learning foraging behaviour, they are going to obtain the common form more frequently. Since the predator is going to learn what is most frequently and commonly captured, the most common morph is what is identified as prey. This concludes that their foraging behaviour is shaped by this learned preference, thus causing apostatic selection and a fitness benefit to the rare morphs. From this, it was concluded that this search image formation and adaption is the mechanism that drives the most common prey type to be more easily distinguished from the environment, and thus be eaten more frequently.
Various types of experiments have been done to look into apostatic selection. Some involve artificial prey because it is a lot easier to control external variables in a simulated environment, though using wild specimens increases the studies external validity Often a computer screen simulation program is used on animals, often birds of prey, to detect for selection. Another type looks into how apostatic selection can focus on the predator as well as the prey because predator plumage polymorphism can be another example of how apostatic selection works in a population. 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 level organisms, and the cognition it uses is applicable to all organisms in which can display learning. Though a lot of this work has been experimental and lab controlled, there are some examples of it happening with both wild specimens and in the natural habitat of the species.
In hawks, almost all of their polymorphism is found on their ventral side it allows for less common coloration to be favored since it will be recognized least. Polymorphism is defined by foraging strategies, one of which is apostatic selection. Because of the different morphs and the varying selection on them, changes in prey detection maintain prey polymorphism due to apostatic selection.
Apostatic selection can be reflected in Batesian mimicry. Aposematism and apostatic selection is used to explain defensive signaling like Batesian mimicry in certain species. A paper by Pfenning et al., 2006 looks into this concept. In allopatric situations, situations where separate species overlap geographically, mimic phenotypes have a really high fitness and are selected for when their model is present but when it is absent, they suffer intense predation. In this article it was suggested that this is caused by apostatic selection because strength of selection is higher on the mimics that are hidden by their original model.
In Batesian mimicry, if the mimic is less common than the model, then the rare mimic phenotype is favored because the predator has continued reinforcement that the prey is harmful or unpalatable. When the mimic becomes more common than the model, it switches and is preyed upon much more often. Therefore, the dishonest signals in prey can be selected for or against depending on predation pressure.
An example in birds is observed within ground dwelling passerines, in which the wild birds were kept in their natural habitat but were presented with dimorphic prey (artificial). The two colors of prey were present in 9:1 ratios, and then the prey were switched so both colors were in the higher and lower ratio. In all four of the passerine species that were observed, the more common morph of the artificial prey were consumed more frequently. regardless of the color of it. This study also had a second component in which they allowed the birds to become familiar with one color of the prey, and then presented the dimorphic prey in equal amounts. In this case, the passerines consumed more of the prey that they were accustomed too. This is consistent with the idea that the search image influences apostatic selection, and the more familiar form is encountered more making it preferred.
Apostatic selection has also been studied in cichlid fish, which presents a rare polymorphism, the gold ('Midas') colour morph. They discussed how apostatic selection a plausible mechanism for the maintenance of this Midas morph, and ruled out various other explanations for this morph. It was concluded that the rare morph is established by a difference in the predators detection probability of the Midas morph. One limitation of this study is that since the morphs in the wild are not able to be manipulated, no definite conclusions can be made, but the evidence predicts apostatic selection.
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