Character displacement is a phenomenon in evolutionary ecology that helps explain how closely related species manage to coexist in the same environment. It describes a pattern where two species living together evolve to be more different from one another in physical or behavioral traits than they are in areas where they live separately. This evolutionary push toward distinctness is driven by the complex relationships between organisms and their environment, and it is a significant mechanism for generating and maintaining the vast biodiversity observed across the globe.
Defining Character Displacement
Character displacement is defined as the evolutionary change in a species’ traits that occurs when two competing species coexist geographically. This pattern is recognized by comparing populations in two settings: sympatry, where the species’ range overlaps with a competitor, and allopatry, where it lives alone. In sympatry, the species exhibits traits noticeably different from the competitor, while in allopatry, those traits may be indistinguishable.
The divergence can manifest in changes to morphology (physical form), behavior, or physiology. For example, if two bird species compete for seeds, their beak sizes might diverge in the area where they overlap. One species might evolve a slightly larger beak, and the other a slightly smaller one. The final result is a greater dissimilarity between the two species when they are together compared to when they are apart.
The Driving Force: Competition and Selection
The engine behind character displacement is intense interspecific competition, which is the struggle between two different species for a shared, limited resource. When two species with similar needs inhabit the same area, they compete for resources like food or shelter. This ecological pressure aligns with the competitive exclusion principle, which suggests that two species cannot indefinitely occupy the exact same ecological niche.
Natural selection then acts on the variation present in the population, favoring individuals whose traits allow them to use resources differently from their competitor. For instance, if a bird competes for medium seeds, individuals with slightly smaller beaks might successfully eat smaller seeds that the competitor ignores. Over generations, this selective advantage for non-overlapping traits pushes the species’ average phenotype away from the competitor, leading to a partitioning of the shared resource.
Two Forms of Trait Divergence
Character displacement is broadly categorized into two main forms based on the nature of the interaction that drives the divergence.
Ecological Character Displacement
This involves changes in traits related to resource acquisition and consumption. This form is driven by exploitative competition, such as two species vying for the same food supply, which results in a divergence of feeding structures or habitat use. The evolutionary response is a reduction in the overlap of their ecological niches, allowing them to coexist by utilizing different parts of the resource spectrum.
Reproductive Character Displacement
This involves the evolution of traits that minimize costly reproductive interactions between the two species. This is often driven by selection against hybridization, where two species interbreed but produce offspring that are sterile or less fit. Natural selection favors individuals who can better recognize and choose mates from their own species, leading to divergence in traits like courtship signals, mating calls, or color patterns. This process ensures that energy and resources are not wasted on producing non-viable hybrid offspring.
Real-World Examples of Character Displacement
Galápagos Finches (Ecological)
The finches of the Galápagos Islands provide a compelling example of ecological character displacement. On Daphne Major, the medium ground finch (Geospiza fortis) and the large ground finch (Geospiza magnirostris) compete for seeds. When a severe drought reduced the available food, the two species depleted the seed supply, creating intense selection pressure.
This competition drove rapid evolutionary change in the medium ground finch. The average beak size became smaller, allowing the species to specialize on smaller, softer seeds that the large-beaked competitor ignored. Individuals with larger beaks in the G. fortis population were at a strong selective disadvantage, leading to a significant divergence in beak size. Genomic analysis later identified the HMGA2 gene playing a role in this rapid shift, demonstrating the genetic basis for the divergence.
Caribbean Anolis Lizards (Reproductive)
Reproductive character displacement is observed in the Anolis lizards of the Caribbean, specifically the distichus species group on Hispaniola. These lizards use a colorful, retractable throat fan called a dewlap for species recognition and courtship displays. In areas where two closely related Anolis species overlap, the color or pattern of the dewlap becomes noticeably more distinct to prevent interbreeding.
For instance, the Anolis caudalis lizard exhibits different dewlap colors at the northern and southern limits of its range, where it encounters different competing species. This phenotypic divergence ensures that individuals can quickly and accurately identify a suitable mate of their own species. This pattern of enhanced divergence in a mating signal where species co-occur is a classic signature of reproductive character displacement driven by the need for reproductive isolation.