Character displacement
Encyclopedia
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 evolution
ary 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 William L. Brown and E. O. 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 [derived from the same immediate parental species] during their period of first contact” (1956). While character displacement is important in various scenarios of speciation
, including adaptive radiation
s 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.”
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 affect 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.
, 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).
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary change driven by competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle
Competitive exclusion principle
In ecology, the competitive exclusion principle, sometimes referred to as Gause's law of competitive exclusion or just Gause's law, is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant...
, 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
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
.
Character displacement was first explicitly explained by William L. Brown and E. O. Wilson
E. O. Wilson
Edward Osborne Wilson is an American biologist, researcher , theorist , naturalist and author. His biological specialty is myrmecology, the study of ants....
(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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
, 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 [derived from the same immediate parental species] during their period of first contact” (1956). While character displacement is important in various scenarios of speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
, including adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
s 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
Guild (ecology)
A guild is any group of species that exploit the same resources, often in related ways. As can be seen from the list of examples below, it does not follow that the species within a guild occupy the same, or even similar, ecological niches...
(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, AnolisAnolis
Anolis is a genus of lizards belonging to the family Polychrotidae. With nearly 400 species, Anolis represents the world's most species rich amniote genus. Several species of Anolis are occasionally ascribed to the genus Norops, but the validity of the Norops genus is not widely accepted...
lizards on islands, three-spined stickleback
Three-spined stickleback
The three-spined stickleback, Gasterosteus aculeatus, is a fish native to much of northern Europe, northern Asia and North America. It has been introduced into parts of southern and central Europe.-Distribution and morphological variation:...
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
Bismarck Archipelago
The Bismarck Archipelago is a group of islands off the northeastern coast of New Guinea in the western Pacific Ocean and is part of the Islands Region of Papua New Guinea.-History:...
and notably, Darwin's finches
Darwin's finches
Darwin's finches are a group of 14 or 15 species of passerine birds. It is still not clear which bird family they belong to, but they are not related to the true finches. They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle...
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
Anolis
Anolis is a genus of lizards belonging to the family Polychrotidae. With nearly 400 species, Anolis represents the world's most species rich amniote genus. Several species of Anolis are occasionally ascribed to the genus Norops, but the validity of the Norops genus is not widely accepted...
on the islands in the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
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
Lesser Antilles
The Lesser Antilles are a long, partly volcanic island arc in the Western Hemisphere. Most of its islands form the eastern boundary of the Caribbean Sea with the Atlantic Ocean, with the remainder located in the southern Caribbean just north of South America...
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
Gasterosteus
Sticklebacks is a genus of fishes in the family Gasterosteidae. Consists of six species:* Gasterosteus aculeatus Linnaeus, 1758 - Three-spined stickleback* Gasterosteus crenobiontus Băcescu & Mayer, 1956 - Techirghiol stickleback...
spp.) in post-glacial lakes in western Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
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
Sympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
(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
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
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
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
have also provided recent “natural experiments” to investigate how rapidly character displacement can affect evolutionary change (Dayan and Simberloff 2005). When American mink
Mink
There are two living species referred to as "mink": the European Mink and the American Mink. The extinct Sea Mink is related to the American Mink, but was much larger. All three species are dark-colored, semi-aquatic, carnivorous mammals of the family Mustelidae, which also includes the weasels and...
(Mustela vison) was introduced in north-eastern Belarus
Belarus
Belarus , officially the Republic of Belarus, is a landlocked country in Eastern Europe, bordered clockwise by Russia to the northeast, Ukraine to the south, Poland to the west, and Lithuania and Latvia to the northwest. Its capital is Minsk; other major cities include Brest, Grodno , Gomel ,...
, 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 GrinnellJoseph Grinnell
Joseph Grinnell was a field biologist and zoologist. He made extensive studies of the fauna of California, and is credited with introducing a method of recording precise field observations known as the Grinnell System...
, in the classic paper that set forth the concept of the ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
(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
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
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
Sympatry
In biology, two species or populations are considered sympatric when they exist in the same geographic area and thus regularly encounter one another. An initially-interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation...
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
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
, quantitative genetics
Quantitative genetics
Quantitative genetics is the study of continuous traits and their underlying mechanisms. It is effectively an extension of simple Mendelian inheritance in that the combined effects of one or more genes and the environments in which they are expressed give rise to continuous distributions of...
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).