Apostatic selection is 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 negative frequency-dependent 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.
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.
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'. 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.
Prey switching is the concept that predators sometimes 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.
The concept of a ‘search image’ in predatory birds is that they only look for a single cryptic food even though there are other cryptic alternatives that may be as equally beneficial. A search image defined by Luuk Tinbergen as 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.
Because of this, it makes it more advantageous to be a less common morph of a species, so apostatic selection occurs on these less common morphs. Experiments have been done on hawks to show that because of the fact that they are of high intelligence, formation of a search image is plausible and a reasonable hypothesis is the relationship of apostatic selection and polymorphism.
Hypothesis for Polymorphism
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.
Experiments to Prove Apostatic Selection
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. 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.
It is shown in hawks, since 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. This seems to be true of predators as well. The main cause of apostatic selection is that it allows predators to maximize their fitness because they concentrate on common morphs rather than rare ones. Apostatic selection is the main mechanism that creates polymorphism.
Apostatic selection can also 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 will be favored because the predator has continued reinforcement that the prey is harmful or unpalatable. When the mimic becomes more common than the model, it will switch and be preyed upon much more often. Therefore, the dishonest signals in prey can be selected for or against depending on predation pressure.
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