Frequency-dependent selection is a form of natural selection where the fitness of a trait or genetic variant within a population is directly influenced by its prevalence. This means that whether a particular characteristic helps an organism survive and reproduce depends on how common or rare that characteristic is compared to others in the same group. This dynamic interaction plays a significant role in shaping the genetic makeup of populations over time.
The Dynamic Balance of Fitness
Frequency-dependent selection operates through two primary mechanisms, each leading to different outcomes for genetic diversity within a population. The fitness of a trait changes dynamically based on its prevalence among other traits.
Negative frequency-dependent selection occurs when rare traits or genotypes gain a fitness advantage. As these rare traits become more successful, their frequency in the population increases. This mechanism often leads to the maintenance of genetic diversity, preventing any single trait from becoming overly dominant. For instance, if a specific defense mechanism in prey is rare, predators might not have learned to counter it, giving those rare individuals a better chance of survival.
In contrast, positive frequency-dependent selection favors common traits or genotypes. This process can lead to a rapid increase in a particular trait’s frequency, potentially resulting in its widespread adoption throughout the population. This can, however, reduce overall genetic diversity as the favored trait spreads and other variations diminish.
Real-World Manifestations
Frequency-dependent selection manifests in various ways across different species. These real-world examples demonstrate the contrasting effects of negative and positive frequency-dependent selection.
Negative frequency-dependent selection is evident in predator-prey dynamics, specifically through apostatic selection. Predators tend to form “search images” for the most common prey variants, making those common forms more vulnerable.
For example, the grove snail exhibits polymorphic shell patterning. Predatory thrushes often concentrate their hunting efforts on common shell patterns. This provides a selective advantage to rarer shell patterns, allowing them to increase in frequency until they too become common, restarting the cycle and maintaining diverse shell patterns within the snail population.
Another instance of negative frequency-dependent selection occurs in host-parasite coevolution. Parasites often adapt to infect the most common genotypes within a host population, giving rare host genotypes a temporary advantage. As these rare host genotypes reproduce, their numbers increase, driving parasites to adapt to the new common host genotypes. This perpetuates an evolutionary “arms race” that maintains genetic diversity in both host and parasite populations.
The male common side-blotched lizard also displays this, with three throat-color morphs—orange, blue, and yellow—each employing a different reproductive strategy. Their populations cycle, with each morph gaining an advantage when the morph that typically outcompetes it is most common, ensuring no single morph dominates.
Positive frequency-dependent selection is illustrated by Müllerian mimicry, where multiple unpalatable or dangerous species evolve similar warning signals. For instance, different species of Heliconius butterflies in Central and South America, all toxic to predators, have converged on similar wing coloration patterns. Predators quickly learn to associate these common patterns with an unpleasant experience, making the warning signal more effective as it becomes more widespread. Any new or rare color pattern is more likely to be attacked by predators because it has not yet been learned, thus being eliminated from the population.
Aposematism, or warning coloration, where animals display bright colors to signal their toxicity or danger, also exemplifies positive frequency-dependent selection. The more common a particular warning pattern is within a population, the more readily predators recognize and avoid it. This reinforces the advantage of the common pattern, as predators are less likely to attack individuals displaying a familiar signal.
Impact on Biodiversity
Frequency-dependent selection significantly shapes biodiversity. Its mechanisms play a powerful role in determining the composition and dynamics of populations and communities.
Negative frequency-dependent selection is a strong force in maintaining genetic diversity within populations. By favoring rare traits, it prevents any single genotype or phenotype from completely dominating, preserving a broader range of genetic variations. This continuous selection for less common forms helps ensure populations retain a diverse genetic toolkit, beneficial for adapting to changing environmental conditions.
The process also drives co-evolutionary dynamics, particularly in interactions between species. For example, in host-parasite or predator-prey relationships, frequency-dependent selection can lead to continuous evolutionary “arms races.” As one species adapts to a common trait in another, rarer traits in the second species gain an advantage, leading to reciprocal evolutionary changes over time.
Frequency-dependent selection also influences the direction and speed of evolutionary change. While negative frequency dependence tends to stabilize polymorphisms, positive frequency dependence can lead to rapid fixation of traits, potentially reducing diversity. The balance between these selective pressures contributes to the complex evolutionary trajectories observed in natural populations, influencing how species respond to their environments and interact with each other.