Character displacement is a phenomenon in evolutionary biology that reveals the profound influence of interspecies competition on the diversity of life. When two species with similar needs occupy the same geographic area, the resulting struggle for survival can drive a rapid evolutionary change in one or both populations. This process involves the modification of a “character,” which in this context is any measurable, heritable trait, such as body size, feeding structure, or coloration. Character displacement is a powerful mechanism that allows similar species to coexist by evolving differences, rather than one competitor excluding the other entirely.
Defining Character Displacement
Character displacement is recognized by a distinct geographical pattern of trait divergence between two species. This pattern emerges when populations of two similar species living together, a state called sympatry, show more pronounced differences in a specific trait than populations of the same species living apart, a state known as allopatry. The divergence is a direct evolutionary response to a costly interaction, most often the intense competition for resources or the interference of reproduction.
For two species to display character displacement, they must have initially been similar enough to compete for the same limited resource or to risk interbreeding. Populations of the two species in sympatry will be easily distinguishable by the specific trait under selective pressure. Conversely, the populations of those same species found in allopatry often look remarkably alike. This observation confirms that the evolution of difference is specifically tied to the pressure experienced in the shared environment.
The Evolutionary Mechanism
The driving force behind character displacement is natural selection, which minimizes the detrimental effects of species interaction. When two species overlap in resource use—such as both preferring medium-sized seeds—individuals with traits closest to the average resource use suffer the lowest fitness. These intermediate individuals face intense competition from both species and are less successful at acquiring necessary resources. The evolutionary outcome is a selective pressure that favors individuals at the extremes of the trait spectrum, a process known as divergent selection.
For example, among seed-eating birds, those with slightly smaller beaks shift to eating smaller seeds, while those with slightly larger beaks focus on larger, tougher seeds. This differentiation creates a “fitness valley” for individuals whose beak size falls in the middle, as they are outcompeted by both specialists. Over generations, the average trait value in each competing species shifts away from the other, resulting in two distinct resource-use strategies that enable long-term coexistence.
Ecological Versus Reproductive Displacement
Character displacement is categorized into two major forms based on the source of selective pressure. The first is ecological character displacement, involving the evolution of traits directly related to resource exploitation. This divergence is driven by competition for shared resources like food, nesting sites, or physical space. The traits that change are often morphological, such as the size of a bird’s beak, the length of a lizard’s limb, or the structure of a fish’s jaw.
The second category is reproductive character displacement, driven by the need to avoid costly reproductive mistakes. Coexisting similar species risk hybridization, producing offspring that are sterile or have low fitness. This waste of reproductive effort creates selection pressure for traits that enhance species recognition and prevent interbreeding. This divergence, sometimes called reinforcement, acts on traits like mating calls, courtship displays, or unique color patterns, making them more distinct in the area of overlap.
Ecological Displacement
Ecological displacement focuses on partitioning the shared environment to reduce resource competition. Trait divergence allows both species to occupy a slightly different niche, enabling them to share the ecosystem without one driving the other to extinction. This evolutionary separation is often seen in feeding structures, where small changes in morphology can open up an entirely new food source for a population. The result is that the two species utilize different parts of the resource gradient, minimizing competitive interaction.
Reproductive Displacement
Reproductive displacement ensures individuals mate only with members of their own species, which is crucial where hybridization is possible. Selection favors individuals better at identifying and choosing conspecific mates, leading to the rapid evolution of species-specific mating signals. For instance, the mating calls of two frog species might be very similar in allopatry, but in sympatry, they evolve drastically different call frequencies or pulse rates. This divergence in signaling acts as a pre-mating barrier, reinforcing reproductive isolation.
Documented Examples
One well-studied example involves the medium ground finch (Geospiza fortis) and the large ground finch (G. magnirostris) on the Galápagos Islands. Where both species coexist, G. fortis evolved a significantly smaller beak size, while G. magnirostris possesses a much larger beak. This divergence allows the medium finch to specialize on smaller, softer seeds and the large finch to focus on cracking large, hard seeds, minimizing competition for intermediate sizes.
Another classic case is found in the Appalachian salamanders, Plethodon hoffmani and P. cinereus. In areas where these two species live in sympatry, they show distinct differences in their jaw morphology and overall body size compared to when they live separately. This morphological differentiation is directly associated with a segregation in the size of the prey they consume, demonstrating how competition for food resources drives changes in body structure.