Natural selection is a fundamental process that shapes life on Earth, leading to changes in populations over generations. It favors traits that enhance an organism’s ability to survive and reproduce, ensuring advantageous characteristics are passed on. This process results in populations becoming better suited to their surroundings. However, a trait’s advantage can shift depending on various factors within a population.
The Basics of Frequency Dependence
Evolutionary processes are influenced by how common or rare a particular trait is within a population, a phenomenon known as frequency-dependent selection. This means the fitness, or reproductive success, of a phenotype or genotype changes based on its prevalence. In positive frequency-dependent selection, a trait becomes more advantageous as it becomes more common. Conversely, negative frequency-dependent selection (NFDS) occurs when a trait’s fitness decreases as it becomes more widespread, and increases as it becomes rarer.
Under NFDS, rarity confers an advantage. As a rare trait becomes more prevalent, its benefit diminishes, potentially becoming a disadvantage. This dynamic creates a stable equilibrium where different traits can persist rather than one becoming dominant. This selection often results from interactions within or between species, such as in predator-prey relationships or resource competition.
How Negative Frequency Dependence Works
The advantage of rarity in negative frequency-dependent selection stems from various ecological and biological interactions. One mechanism involves predator behavior and “search images.” Predators often develop a mental image of common prey types, making them efficient at finding them. This focused predation puts rare prey at an advantage, as they are less likely to be targeted, allowing their numbers to increase. As these previously rare prey become more common, predators may switch their search image, initiating a new cycle.
Rarity also provides an advantage in host-parasite interactions. Parasites typically evolve to infect common host genotypes, representing the largest resource. Rare host genotypes, less recognized by prevalent parasites, experience lower infection rates and higher survival. As these rare, resistant host genotypes increase, new parasite strains capable of infecting them will be favored, leading to a continuous evolutionary arms race.
Competition for limited resources can also drive NFDS. When a strategy or trait is common, individuals with that trait face intense competition. Rare alternative strategies, however, might encounter less competition, gaining an advantage in acquiring resources. This allows rare strategies to increase in frequency until they become common, facing similar competitive pressures and favoring other rare alternatives.
Where Negative Frequency Dependence is Observed
Negative frequency-dependent selection manifests in various natural systems, influencing biological diversity. One example is host-parasite coevolution, where host genotype fitness depends on its frequency relative to the parasite population. In freshwater snails (Potamopyrgus antipodarum) and their trematode parasites, common host genotypes are more susceptible to infection, while rare genotypes gain a selective advantage due to reduced parasitic pressure. This dynamic ensures no single host genotype dominates, fostering genetic variation.
Sex ratios within populations also illustrate NFDS, often explained by Fisher’s principle. If one sex becomes rarer, individuals of that sex gain a reproductive advantage due to more mating opportunities. For instance, if males are rare, each male likely produces more offspring than a female, favoring parents who produce more of the rarer sex until the ratio approaches equality. This feedback loop prevents extreme imbalances, maintaining a roughly 1:1 ratio in many species.
Batesian mimicry provides another illustration, particularly in non-poisonous species mimicking dangerous ones. When mimics are rare, predators avoiding dangerous models also avoid them, granting protection. However, if mimics become too common, predators are more likely to consume them without negative consequences, learning the warning signal is unreliable. This increased predation reduces the mimic’s fitness, making rarity advantageous again.
In some plant species, NFDS influences floral traits like color. For example, in nectarless orchids, pollinators may associate common flower colors with a lack of reward and avoid them. Rare flower color morphs, being less frequently encountered, may receive more pollinator visits, providing a reproductive advantage. This preference for rare morphs helps maintain flower color diversity, preventing any single color from becoming too common and losing pollinator appeal.
Its Role in Maintaining Diversity
Negative frequency-dependent selection preserves genetic and phenotypic diversity within populations. Rare variants are continuously favored, so this selection acts as a balancing force, preventing any single trait from becoming fixed or dominant. It actively promotes the survival and reproduction of less common forms, ensuring a range of variations persists. This mechanism is a powerful selective force for maintaining polymorphisms, which are multiple forms of a trait or gene.
Maintaining diversity through NFDS is important for population resilience and adaptability. A diverse population has a greater capacity to respond to environmental changes, new diseases, or shifts in resource availability. For instance, if a new pathogen emerges, some rare genotypes might possess resistance, enabling the population to survive and adapt. This ongoing preservation of variation helps populations navigate dynamic environments.