Character displacement is an evolutionary phenomenon where differences between similar species become more pronounced in areas where their geographic distributions overlap. This accentuation of traits helps these species coexist in the same environment. When these species live in separate areas, their distinguishing traits may be less obvious or even indistinguishable. This concept highlights how species adapt over generations to reduce direct competition.
The Evolutionary Push: Competition
The primary force driving character displacement is interspecific competition for limited resources like food or habitat. When two species with similar needs inhabit the same area, they directly compete, and natural selection favors individuals better at acquiring resources. This pressure leads to resource partitioning, where species evolve to utilize different aspects of a shared resource.
Natural selection acts on individuals within each species, favoring those with traits that reduce overlap in resource use with the competing species. For instance, if two bird species compete for seeds, individuals with slightly different beak sizes that allow them to exploit different seed types will have a survival advantage. Over many generations, this selective pressure causes a divergence in the traits of coexisting species, minimizing their competitive interactions. This evolutionary response allows both species to persist without one outcompeting the other to extinction.
Identifying Character Displacement
Scientists identify character displacement by comparing populations in two contexts: sympatry and allopatry. Sympatry refers to areas where two similar species live together and their ranges overlap. Allopatry describes areas where only one species exists. In sympatric areas, the traits of the two species show greater divergence.
In allopatric populations, where competitive pressure is absent, a single species’ traits tend to be more similar to its ancestral form. This comparative approach allows researchers to observe evolutionary shifts that minimize competition. Changes can manifest in various ways, including differences in body size, beak shape, foraging behaviors, or reproductive signals. Demonstrating character displacement involves showing that observed differences are greater than expected by chance and linked to differences in resource use.
Notable Examples from the Natural World
A classic illustration of character displacement comes from Darwin’s finches in the Galápagos Islands. The medium ground finch (Geospiza fortis) and the small ground finch (Geospiza fuliginosa) exhibit distinct beak sizes when they inhabit the same islands. On islands where only one species is present, their beak sizes tend to be more similar. However, on islands where both species coexist, the medium ground finch evolves a larger beak to consume larger, tougher seeds, while the small ground finch develops a smaller beak to feed on smaller, softer seeds, reducing direct competition for food.
Another well-documented example involves Anolis lizards in the Caribbean. On islands where multiple Anolis species coexist, they often show distinct differences in body size and limb length, which allows them to utilize different parts of the habitat, such as perching on different sized branches or at varying heights. For instance, if a larger species occupies thick tree trunks, a coexisting smaller species might evolve to use thinner branches or higher perches. This divergence in physical traits and habitat use helps to minimize competition for space and resources.
Threespine stickleback fish (Gasterosteus aculeatus) in coastal British Columbia lakes also provide evidence. In some lakes, there are two distinct forms: a “limnetic” form that feeds on zooplankton in open water and a “benthic” form that consumes invertebrates from the lake bottom. In contrast, lakes with only one stickleback species often contain an intermediate form that exploits both habitats. This specialization in diet and habitat use between the limnetic and benthic forms in two-species lakes results from evolutionary divergence driven by competition for food resources.