(By Dr. Girish Chandra)
Character displacement refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur but are minimized or lost where the species’ distributions do not overlap. This pattern results from evolutionary change driven by competition among species for a limited resource. The rationale for character displacement stems from the Competitive Exclusion Principle, also called Gause’s Principle, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.
Character displacement was first explicitly explained by Brown and Wilson (1956): “Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the populations of that species are similar to the other species and may even be very difficult to distinguish from it. In the area of overlap, where the two species occur together, the populations are more divergent and easily distinguished, i.e., they “displace” one another in one or more characters.
Character displacement is the term used to describe an evolutionary change that occurs when two similar species inhabit the same environment. Under such conditions, natural selection favors a divergence in the characters–morphology, ecology, behavior, or physiology–of the organism. The idea was only formalized in the middle of the twentieth century by the American entomologists William L. Brown (1922-1997) and Edward O. Wilson (1929-), who compared the characters of a number of species living together with characters in the same species living apart, or in allopatry. They found that sympatric species possessed many different characters although these same species were sometimes indistinguishable when living allopatrically. Brown and Wilson concluded that these situations resulted from competition: because the species were similar, they competed for the same resources and natural selection favored those species that competed less. Thus, the characters diverged.
Two decades later the concept was revisited by the American ornithologist Peter Grant (1936-). Grant studied the finches of the Galapagos, called Darwin‘s finches, in large part to test the idea of character displacement. In the process, he refined understanding of the concept. Grant pointed out that putative cases of character displacement might be, instead, character convergence, as two species that originally evolved in the presence of one another moved into new areas and, without the pressure of competition, broadened the range of their characters, a process that he called character release. Grant also noted that putative cases of character displacement might have nothing to do with competition or reinforcement of reproductive isolation, but might result from other causes altogether. These arguments cleared the way for a new definition of character displacement, decoupling the concept from a simple comparison of sympatric and allopatric populations.
Competitive release (Grant 1972), defined as the expansion of an ecological niche in the absence of a competitor, is essentially the mirror image of character displacement. It too was described by Brown and Wilson (1956): “Two closely related species are distinct where they occur together, but where one member of the pair occurs alone it converges toward the second, even to the extent of being nearly identical with it in some characters.”
While studies on character displacement have been performed in a wide variety of taxa, a few groups have disproportionately contributed our understanding of this principle: mammalian carnivores, Galapagos finches, Anolis lizards on islands, three-spined stickleback fish and snails (Dayan and Simberloff 2005). In the initial explication of character displacement, many of the examples they set forth as potential evidence for character displacement were observations between multiple pairs of birds. These included rock nuthatches in Asia, Australian honey-eaters of the genus Myzantha, Australian parrots, shearwaters in the Cape Verde Islands, flycatchers of the Bismarck Archipelago and notably, Darwin’s finches (Geospiza) in the Galapagos (Brown and Wilson 1956).
The lizard genus Anolis on the islands in the Caribbean has also been the subject of numerous studies investigating the role of competition and character displacement in community structure (e.g., Losos 1990). Lesser Antilles islands can only support Anolis species of different sizes, and the relative importance of character displacement versus size at colonization in determining invasion success has been explored and debated.
Three-spine sticklebacks (Gasterosteus spp.) in post-glacial lakes in western Canada have contributed significantly to recent research of character displacement (e.g., Schluter 1993, Schluter 1995). Both observations of natural populations and manipulative experiments show that when two recently evolved species occur in a single lake, two morphologies are selected for: a limnetic form that feeds in open water and a benthic form that feeds at the lake bottom. They differ in size, shape and the number and length of gill rakers, all of which is related to divergence in their diet. Hybrids between the two forms are selected against.
Other studies have found Plethodon salamander species that demonstrate character displacement from aggressive behavioral interference rather than exploitation (Adams 2004). That is, morphological character displacement between the two species is due to aggressive interaction between them rather than the exploitation of different food resources. While character displacement was originally discussed in the context of very closely related species, evidence suggests that even interactions among distantly related species can result in character displacement. Two finch species (Geospiza fuliginosa and G. difficilis) exploit more flower nectar on islands where the lager carpenter bee (Xylocopa darwini) is absent than on islands with the bees. Individual finches that harvest nectar are smaller than conspecifics that do not.
Six criteria have been developed to establish character displacement as the mechanism for differences between sympatric species.
(1) Differences between sympatric taxa are greater than expected by chance.
(2) Differences in character states are related to differences in resource use.
(3) Resources are limiting and interspecific competition for these resources is a function of character similarity.
(4) Resource distribution are the same in sympatry and allopatry such that differences in character states are not due to differences in resource availability.
(5) Differences must have evolved in situ.
(6) Differences must be genetically based.
Rigorously testing these criteria necessitates a synthetic approach, combining areas of research like community ecology, functional morphology, adaptation, quantitative genetics and phylogenetic systematics. While satisfying all six criteria in a single study of character displacement is not often feasible, they provide the necessary context for research of character displacement.
The introduction of character displacement turned ecologists’ attention away from the principle of competitive exclusion–in which one species drives a competitor to extinction within their zone of sympatry–and focused them instead on learning how organisms survived together. The American ecologist G. Evelyn Hutchinson (1903-1991), for example, used character displacement to partially explain how the earth supports so many organisms: because of character displacement, even similar organisms can live together. Other ecologists and evolutionary biologists noted that character displacement may be the engine driving adaptive radiations, in which a single species gives rise to many others, filling a variety of ecological niches. The process is not as ubiquitous as once believed but, according to E. O. Wilson, character displacement still “represents one process by which communities can be organized, mediating a rise in general biological diversity.”