Frequency dependent selection is a type of natural selection where the reproductive success, or fitness, of a specific trait is directly tied to how common or rare it is within a population. Unlike simpler forms of selection where a trait’s advantage remains constant, the selective pressure itself changes based on the relative abundance of different phenotypes. It highlights how interactions between individuals, whether through competition, predation, or mating, shape the evolutionary trajectory of a population.
The Mechanism of Frequency Dependent Selection
The core principle behind this mode of selection is that the ecological consequences of possessing a trait shift as its prevalence changes. When a phenotype is rare, it might offer an unexpected advantage, but once it becomes widespread, that advantage can vanish or even become a disadvantage. The mechanism often involves the behavior or perception of other species, such as predators, parasites, or mates.
A common way this mechanism operates is through the formation of a “search image” by a predator. Predators often develop a mental template for the most frequently encountered prey type, allowing them to efficiently locate common food sources. This cognitive bias means that the most abundant prey morphs are disproportionately targeted, suffering higher rates of predation. Conversely, the rare morphs are overlooked and gain a selective advantage simply because they are uncommon. This direct link between a trait’s frequency and the intensity of selection pressure defines frequency dependent selection.
Negative Frequency Dependence and Maintaining Diversity
Negative frequency dependence occurs when the selective advantage of a trait increases as it becomes rarer in the population. This mechanism actively maintains genetic diversity, preventing any single phenotype from dominating the gene pool. The fitness of an individual decreases as its trait becomes more common, creating a selective pressure that constantly favors the minority. This is a form of balancing selection, which preserves polymorphism within a species.
As a rare trait becomes fitter and its frequency increases, its selective advantage begins to diminish until all competing traits have equal fitness. This point represents a stable equilibrium frequency, where the population’s composition will naturally return if it is perturbed. A classic example is the evolution of host-parasite systems, where common host genotypes are readily infected by specialized parasites, giving a selective edge to rare, resistant host genotypes. Once those rare genotypes become common, the parasite population evolves to overcome their resistance, cycling the advantage back to a different rare host type. This perpetual cycle ensures that multiple forms of resistance and susceptibility persist in the population.
This rarity advantage can also manifest in behavioral strategies or resource competition. If a species relies on two different resources, individuals exploiting the less-used resource will face less competition and enjoy higher fitness. As more individuals switch to the now-successful strategy, competition intensifies, and the advantage shifts back to the alternative, less-common strategy. This dynamic prevents any single strategy from becoming fixed, maintaining a stable mix of behaviors or resource-use patterns.
Positive Frequency Dependence and Trait Fixation
In contrast to the negative form, positive frequency dependence is a process where a trait’s fitness increases as it becomes more widespread in the population. The more common a phenotype is, the greater its survival or reproductive success, leading to strong selection against rare variants. This mechanism drives the population toward monomorphism, where a single, successful trait becomes fixed and genetic variation is lost. The selective advantage gained by being common reinforces the dominance of that trait.
This mechanism is often driven by a need for collective signals or shared benefits. For example, if a behavior provides protection only when many individuals perform it, like a collective anti-predator display, rare individuals that deviate from the common signal are selected against. Any new or rare mutation is immediately disadvantaged because it does not benefit from the protective effect of the majority.
This type of selection creates a powerful self-fulfilling prophecy where success breeds more success, quickly eliminating intermediate states and alternative phenotypes. This process explains why certain warning signals or group-specific traits become universally adopted within a species or community. The cost of being different is simply too high when the entire population benefits from conformity.
Classic Examples in Nature
One of the most vivid examples of negative frequency dependence is found in the mating strategies of the male common side-blotched lizard (Uta stansburiana). Males exhibit three distinct throat colors—orange, blue, and yellow—each representing a different reproductive strategy in a “rock-paper-scissors” dynamic. Orange-throated males are highly aggressive and defend large territories, outcompeting the blue-throated males, who defend smaller territories. Yellow-throated males are sneaky non-territorial mimics of females, allowing them to successfully mate with the females of the dominant orange-throated males.
When orange is common, yellow thrives by exploiting them, but as yellow becomes common, blue males, which are better at defending their small harems from sneakers, gain the advantage. When blue becomes common, the aggressive orange males can easily outcompete the territorial blue males. This cycle of relative fitness, where no single morph can permanently dominate, ensures that all three color morphs and their corresponding strategies persist in a fluctuating stable equilibrium, maintaining high levels of genetic variation.
In the realm of positive frequency dependence, Müllerian mimicry in butterflies is the definitive case study. Species like the Heliconius butterflies of South America are all unpalatable or toxic to predators and share the same conspicuous warning coloration. A predator that eats one butterfly species and learns to avoid its pattern will subsequently avoid all other species that share that same pattern. The selective advantage for any individual butterfly increases directly with the number of other toxic individuals that possess the identical warning pattern, reinforcing the common signal. Rare individuals with a mutant color pattern are quickly eliminated because they must individually “teach” the predator to avoid them, paying a high cost that the common morphs avoid due to their collective, shared signal.