An Overview
Selection is frequency dependent when the fitness of a phenotype, genotype, or gene (or species) varies with its relative abundance in the population (or community) and hence can be detected only when measured at two or more frequencies.
In “negative frequency-dependent selection,” fitness decreases with frequency and thus rare genotypes or species could be maintained at a stable equilibrium. As a form of balancing selection, this mechanism could be one explanation for the persistence of higher than expected levels of genetic variation in natural populations, whether measured at the level of molecules, morphology or behavior. In “positive frequency-dependent selection,” fitness increases with frequency so that rare genotypes are eliminated and local genetic diversity reduced. The importance of frequency-dependent selection in the evolution of natural populations has yet to be fully substantiated, in part perhaps because of the practical difficulties in detecting it and distinguishing it from other types of selection, whether balancing or non-balancing.
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Negative Frequency-dependent Selection
A rather interesting example of this type of selection has been seen in an unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three colour patterns for their throats: blue, orange and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form solid pair bonds with their mates; and yellow males look a bit like female and are the smallest, allowing them to sneak in copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. The large, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females; the blue males are good at guarding their mates against yellow sneaking males; and the yellow males can sneak copulations from the potential mates of the large, orange males.
A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.
In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes. In one generation, orange might be predominant and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored.
Self incompatibility locus in plant fertilization
Because of the close proximity of the pistil and anther in bisexual flowers, there is a great tendency for pollen to land on the stigma of that same flower. If there were no mechanisms to prevent fertilization by self pollen, inbreeding would result and this would reduce the genetic variability in the species. Luckily, this is not the case. A number of strategies have evolved in flowering plants that prevent self-fertilization. One of the strategies is called self-incompatibility. It was described by Charles Darwin in a book published more than a century ago. He observed that some plant species were completely sterile to their own pollen, but fertile with that of any other individual of the same species. Since Darwin's observation, self-incompatibility has been found to occur in more than half of the flowering plant species. Self-incompatibility allows the pistil of a flower to distinguish between self (genetically related) pollen and non-self (genetically unrelated) pollen. Self pollen is not accepted, whereas non-self pollen are accepted for fertilization. As Darwin observed in the self-incompatible plants, they are completely sterile with respect to self pollen, but perfectly fertile with respect to non-self pollen. Self-incompatibility can be classified into heteromorphic and homomorphic types. In the homomorphic type, flowers of the same species have the same morphological type, whereas in the heteromorphic type, flowers of the same species can have two or three different morphological types, and pollination is compatible only between flowers of different morphological types. The homomorphic type is further classified into gametophytic and sporophytic types based on whether the pollen behavior in self-incompatibility interactions is determined by the genotype of the pollen itself (gametophytic) or by the genotype of the plant from which the pollen is derived (sporophytic). The gametophytic type is more common (found in more than 60 families of flowering plants) than the sporophytic (found in 6 families), and the two types apparently evolved independently. To date, only one of the sporophytic families (Brassicaceae) and five of the gametophytic families (Papaveraceae, Rosaceae, Poaceae, Scrophulariaceae, and Solanaceae) have been studied at the molecular level.
In the solanaceous type of self-incompatibility, a single polymorphic genetic locus called the S locus determines the outcome of pollination. If a plant carries Si and S2 alleles, the pollen produced will carry either the Si or S2 allele. When these pollen grains land on the stigma of the same flower, they will germinate and grow down into the style; however, their growth will be arrested in the upper one-third segment of the style. This is because the pistil recognizes Si and S2 pollen as self pollen through the matching of the S alleles. The tip of a self pollen tube usually swells and bursts open. These self pollen tubes are thus unable to deliver their sperm cells to the ovary for fertilization. If 52 and 53 pollen from another plant land on the stigma of the S152 plant, the result will be different. Again, the S2 pollen will be rejected because of the matching of S alleles. But the S3 pollen, carrying an S allele different from the two carried by the pistil, will be recognized as non-self pollen and will germinate and grow all the way down through the style to the ovary to effect fertilization.
Negative frequency-dependent selection in humans
An example of negative frequency-dependent selection can also be seen in the interaction between the human immune system and multiple infectious microbes like pathogenic bacteria or viruses. When a particular human population is infected by a common strain of microbe, the majority of individuals in the population become immune to it. Then this selects for rarer strains of the microbe which can still infect the population because of genome mutations; these strains have greater evolutionary fitness because they are less common.
The effect of search images on predator–prey interactions
One of the key challenges of both ecology and evolutionary biology is to understand the mechanisms that maintain biodiversity. Frequency-dependent predation is one of the strong stabilizing mechanisms that are essential for stable species coexistence. Predators that feed on a variety of food items in nature can behaviorally switch to abundant prey types in response to temporal and spatial variations in resource availability. This causes frequency-dependent predation that generally promotes the coexistence of competing prey species because the foraging strategies of predators that switch to more common prey types could prevent rare prey types from being eliminated. However, more recent studies that incorporate prey switching as an optimal diet choice have argued that the effect of switching on the stability of predator–prey systems is quite complex, and sometimes destabilizing, depending on prey choice behaviors.
In contrast with the abundance of available theoretical literature showing that frequency-dependent predation affects predator–prey interactions, there is a lack of empirical evidence directly testing the effect of learning and frequency-dependent predation in a multigenerational prey–predator system. Because most behavioral diet choice experiments have been performed over short periods, within one generation, prey densities have been controlled by the experimenter, and the population dynamics of the predator have been ignored. The effect of prey switching on population processes remains unclear.
Foraging is a very important and difficult task for predators when their prey exists in a highly complex and heterogeneous environment. The majority of predators have more than one prey species, and must have the ability to distinguish between them and make decisions related to efficient foraging on those that are available.
In dealing with the spatial and temporal variability of prey species, predators may be able to forage optimally if they have flexible and rapid behavioral plasticity rather than predetermined responses. Some predators learn to focus attention on the cryptic prey type most frequently encountered during recent searching (termed a ‘‘search image’’). Learning provides an adaptive mechanism for behavioral plasticity. Predators detect their prey using visual, olfactory and auditory cues. If their responses to prey-related cues are modified by prior foraging experience, this could improve subsequent foraging efficiency on available hosts.
Rare prey types may be overlooked because of a focus on more common prey. Search imaging reflects biased searching for one of a number of available prey types, and has been studied widely in birds and mammals.
Positive Frequency-dependent Selection
Positive frequency-dependent selection gives an advantage to common phenotypes. A good example is warning coloration in aposematic species. Predators are more likely to remember a common color pattern that they have already encountered frequently than one that is rare. This means that new mutants or migrants that have color patterns other than the common type are eliminated from the population by differential predation. Positive frequency-dependent selection provides the basis for Müllerian mimicry, as described by Fritz Müller
Another, rather complicated example occurs in the Batesian mimicry complex between a harmless mimic, the scarlet kingsnake (Lampropeltis elapsoides), and the model, the eastern coral snake (Micrurus fulvius), in locations where the model and mimic were in deep sympatry, the phenotype of the scarlet kingsnake was quite variable due to relaxed selection. But where the pattern was rare, the predator population was not 'educated', so the pattern brought no benefit. The scarlet kingsnake was much less variable on the allopatry/sympatry border of the model and mimic, most probably due to increased selection since the eastern coral snake is rare, but present, on this border. Therefore, the coloration is only advantageous once it has become common.
Müllerian Mimicry
Müllerian mimicry is a natural phenomenon in which two or more unprofitable (often, distasteful) species, that may or may not be closely related and share one or more common predators, have come to mimic each other's honestwarning signals, to their mutual benefit, since predators can learn to avoid all of them with fewer experiences. It is named after the German naturalist Fritz Müller, who first proposed the concept in 1878, supporting his theory with the first mathematical model of frequency-dependent selection, one of the first such models anywhere in biology.
Müllerian mimicry often occurs in clusters of multiple species called rings. Müllerian mimicry is not limited to butterflies, where rings are common; mimicry rings occur among Hymenoptera, such as bumblebees, and other insects, and among vertebrates including fish and coral snakes. Bumblebees Bombus are all aposematically coloured in combinations, often stripes, of black, white, yellow, and red; and all their females have stings, so they are certainly unprofitable to predators.
There is evidence that several species of bumblebees in each of several areas of the world, namely the American West and East coasts, Western Europe, and Kashmir, have converged or adverged on mutually mimetic coloration patterns. Each of these areas has one to four mimicry rings, with patterns different from those in other areas.
Batesian Mimicry
Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. It is named after the English naturalist Henry Walter Bates, after his work on butterflies in the rainforests of Brazil.
Batesian mimicry is the most well known and widely studied of mimicry complexes, such that the word mimicry is often treated as synonymous with Batesian mimicry. There are many other forms however, some very similar in principle, others far separated. It is often contrasted with Müllerian mimicry, a form of mutually beneficial convergence between two or more harmful species. However, because the mimic may have a degree of protection itself, the distinction is not absolute.
The imitating species is called the mimic, while the imitated species (protected by its toxicity, foul taste or other defenses) is known as the model. The predatory species mediating indirect interactions between the mimic and the model is variously known as the [signal] receiver, dupe or operator. By parasitizing the honest warning signal of the model, the Batesian mimic gains an advantage, without having to go to the expense of arming itself. The model, on the other hand, is disadvantaged, along with the dupe. If impostors appear in high numbers, positive experiences with the mimic may result in the model being treated as harmless. At higher frequency there is also a stronger selective advantage for the predator to distinguish mimic from model.
An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. Warning coloration advertising prey defenses is a textbook example of a trait under positive frequency-dependent selection, and is thought to be responsible for the remarkable convergence among defended prey species. The scarlet kingsnake, a harmless species, mimics the coloration of the eastern coral snake, a venomous species typically found in the same geographical region. Predators learn to avoid both species of snake due to the similar coloration, and as a result the scarlet kingsnake becomes more common, and its coloration phenotype becomes more variable due to relaxed selection. This phenotype is therefore more “fit” as the population of species that possess it (both dangerous and harmless) becomes more numerous. In geographic areas where the coral snake is less common, the pattern becomes less advantageous to the kingsnake, and much less variable in its expression, presumably because predators in these regions are not “educated” to avoid the pattern.
The scarlet kingsnake: The scarlet kingsnake mimics the coloration of the poisonous eastern coral snake. Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration. The eastern coral snake: The eastern coral snake is poisonous. Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.
Conclusion
Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population It’s usually the result of interactions between species (predation, parasitism, or competition), or between genotypes within species (usually competitive or symbiotic), and has been especially frequently discussed with relation to anti-predator adaptations.
In negative frequency-dependent selection, the fitness of a phenotype decreases as it becomes more common. This is an example of balancing selection.
Frequency-dependent selection in side-blotched lizards allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle. For example, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes.
For plants, if there were no mechanisms to prevent fertilization by self pollen, inbreeding would result and this would reduce the genetic variability in the species. Luckily, this is not the case. A number of strategies have evolved in flowering plants that prevent self-fertilization. One of the strategies is called self-incompatibility.
The forming of search images in predators can lead to a destabilizing diet, making the predators only search for specific prey, while overseeing many other, quite potential prey species.
In positive frequency-dependent selection, the fitness of a phenotype increases as it becomes more common. Two main components can be distinguished, namely Müllerian mimicry and Batesian mimicry.
Müllerian mimicry is a natural phenomenon in which two or more unprofitable (often, distasteful) species, that may or may not be closely related and share one or more common predators, have come to mimic each other's honestwarning signals, to their mutual benefit, since predators can learn to avoid all of them with fewer experiences. A good example would be the Bombus. These bumblebees are all aposematically coloured in combinations, often stripes, of black, white, yellow, and red; and all their females have stings, so they are certainly unprofitable to predators, but the change in colouring doesn’t necessary increase competition between the bumblebees.
Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. Batesian mimicry is the most well known and widely studied of mimicry complexes, such that the word mimicry is often treated as synonymous with Batesian mimicry. There are many other forms however, some very similar in principle, others far separated. It is often contrasted with Müllerian mimicry, a form of mutually beneficial convergence between two or more harmful species. However, because the mimic may have a degree of protection itself, the distinction is not absolute.
A Batesian example is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. The scarlet kingsnake mimics the coloration of the poisonous eastern coral snake. Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.
References
- John A. Allen, Frequency Dependent Selection. Oxford Bibliographies.
- Anonymous. Frequency-Dependent Selection. Biology, Libre Texts. 19.3C
- Darwin, C. (1876) The Effect of Cross and Self Fertilization in the Plant Kingdom (John Murray, London). 2. de Nettancourt, D. (1977) Incompatibility in Angiosperms (Springer, Berlin).
- Mathieu Chouteau, Mónica Arias and Mathieu Joron. “Warning signals are under positive frequency-dependent selection in nature”. NCBI, 23-02-2016.
- Mallet, James (July 2001). 'Mimicry: An interface between psychology and evolution'. PNAS. 98 (16): 8928–8930.
- Dixey, F. A. (1909). 'On Müllerian mimicry and diaposematism. A reply to Mr G. A. K. Marshall'. Transactions of the Entomological Society of London.
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