Character displacement

# Overview
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 (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, 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. The characters involved can be morphological, ecological, behavioral, or physiological; they are assumed to be genetically based.
Brown and Wilson (1956) used the term character displacement to refer to instances of both reproductive character displacement, or reinforcement of reproductive barriers, and ecological character displacement driven by competition. As the term character displacement is commonly used, it generally refers to morphological differences due to competition. Brown and Wilson viewed character displacement as phenomenon involved in speciation, stating, we believe that it is a common aspect of geographical speciation, arising most often as a product of the genetic and ecological interaction of two (or more) newly evolved, cognate species  during their period of first contact (1956). While character displacement is important in various scenarios of speciation, including adaptive radiations like the cichlid fish faunas in the rift lakes of East Africa (Meyer 1993), it also plays an important role in structuring communities. The results of numerous studies contribute evidence that character displacement often influences the evolution of resource acquisition among members of an ecological guild (Dayan and Simberloff 2005).
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.

# Examples
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 in the Galapagos (Brown and Wilson 1956). Lack (1947) found that when the two species Geospiza fortis and G. fuliginosa occurred on large islands together, they could be distinguished unequivocally by beak size. When either one occurred by itself on a smaller island, however, the beak size was intermediate in size relative to when the two co-occurred.
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.
Threespine 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. When only one species inhabits a lake, that fish displays an intermediate morphology. Studies on other fish species have shown similar patterns of selection for benthic and limnetic morphologies (Dayan and Simberloff 2005), which can also lead to sympatric speciation (e.g., Barluenga et al. 2006).
The Appalachian salamanders Plethodon hoffmani and P. cinereus displayed no trophic, morphological or resource use differences among allopatric populations; when the species occurred in sympatry, however, they displayed morphological differentiation that was associated with segregation in prey size (Adams and Rohlf 2000). Where these two species co-occurred, P. hoffmani had a faster closing jaw required for larger prey, and P. cinereus had a slower, stronger jaw for smaller prey. 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.
It is often assumed that closely related species are more likely to compete than are more distantly related species, and hence many researchers investigate character displacement among congeners (Dayan and Simberloff 2005). 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. Finches and bees in the Galapagos may provide an interesting example (Schluter 1986).  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 (Schluter 1986).
Introduced species have also provided recent natural experiments to investigate how rapidly character displacement can effect evolutionary change (Dayan and Simberloff 2005). When American mink (Mustela vison) was introduced in north-eastern Belarus, the native European mink (Mustela lutreola) increased in size, and the introduced mink decreased in size (Sidorovich et al. 1999). This displacement was observed within a ten-year study, demonstrating that competition can drive rapid evolutionary change.

# Conceptual development
Character displacement is the situation in which, when two species of animals overlap geographically, the differences between them are accentuated in the zone of sympatry and weakened or lost entirely in the parts of their ranges outside this zone (Brown and Wilson 1956). While the term ecological character displacement first appeared in the scientific literature in 1956, the idea has earlier roots. For example, Joseph Grinnell, in the classic paper that set forth the concept of the ecological niche (1917), stated,  It is, of course, axiomatic that no two species regularly established in a single fauna have precisely the same niche requirements. The existence of character displacement is evidence that the two species do not completely overlap in their niche requirement.
Following the dissemination of the concept, character displacement was viewed as an important force in structuring ecological communities, and biologists identified numerous examples. During the late 1970s and early 1980s, however, the role of competition and character displacement in structuring communities was questioned and its importance greatly downgraded (Losos 2000). Many found the early examples unconvincing and suggested it to be a rare phenomenon. Criticisms with earlier studies included the lack of rigor in statistical analyses and the use of poorly rationalized characters (Losos 2000, Dayan and Simberloff 2005). Additionally, theory seemed to indicate that the conditions that allowed character displacement to occur were limited (Losos 2000). This scrutiny helped motivate theoretical and methodological advances as well as the development of a more rigorous framework for testing character displacement (Losos 2000).
Six criteria have been developed to establish character displacement as the mechanism for differences between sympatric species (Schluter and McPhail 1992; Taper and Case 1992). These include: (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 (Dayan and Simberloff 2005), 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 (Losos 2000, Dayan and Simberloff 2005).