Sympatry
Encyclopedia
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
. Such speciation may be a product of reproductive isolation
— which prevents hybrid offspring from being viable or able to reproduce, thereby reducing gene flow
— that results in genetic divergence. Sympatric speciation can also imply partially-divergent species that are not yet genetically isolated, and it may be the result of secondary contact after divergence has begun. Sympatric species may or may not interbreed.
, which is arguably the dominant mode of speciation.
As such, researchers have long debated the conditions under which sympatry truly applies, especially with respect to parasitism
. Because parasitic organisms often inhabit multiple hosts during a life cycle, evolutionary biologist Ernst Mayr
stated that internal parasites existing within different hosts demonstrate allopatry, not sympatry. Today, however, many biologists consider parasites and their hosts to be sympatric (see examples below). Conversely, zoologist Michael JD White
considered two populations sympatric if genetic interbreeding was viable within the habitat overlap. This may be further specified as sympatry occurring within one deme
; that is, reproductive individuals must be able to locate one another in the same population in order to be sympatric.
Others question the ability of sympatry to result in complete speciation: until recently, many researchers considered it nonexistent, doubting that selection alone could create disparate, but not geographically separated, species. In 2003, biologist Karen McCoy suggested that sympatry can act as a mode of speciation only when “the probability of mating between two individuals depend[s] [solely] on their genotypes, [and the genes are] dispersed throughout the range of the population during the period of reproduction.”. In essence, sympatric speciation does require very strong forces of natural selection to be acting on heritable traits, as there is no geographic isolation to aid in the splitting process. Yet, recent research has begun to indicate that sympatric speciation is not as uncommon as was once assumed.
The lack of geographic constraint in isolating sympatric populations implies that the emerging species avoid interbreeding via other mechanisms. Before speciation is complete, two diverging populations may still produce viable offspring. As speciation progresses, isolating mechanisms
- such as gametic incompatibility that renders fertilization of the egg impossible - are selected for in order to increase the reproductive divide between the two populations.
. It is also apparent in the differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote
) in both sympatric and allopatric populations. There are two main theories regarding this process: 1) differential fusion, which suggests that only populations with a keen ability to discriminate between species will persist in sympatry, and 2) character displacement
, which implies that distinguishing characteristics will be heightened in areas where the species co-occur in order to facilitate discrimination.
decreases the probability of such hybridization by selecting for the ability to identify mates of one’s own species from those of another species.
between individuals of the two species. For example, divergence in the mating signals of two species will limit hybridization by reducing one's ability to identify an individual of the second species as a potential mate. Support for the reproductive character displacement hypothesis comes from observations of sympatric species in overlapping habitats in nature. Increased prezygotic isolation, which is associated with reproductive character displacement, has been observed in cicadas of genus Magicicada
, stickleback
fish, and the flowering plants of the genus Phlox
.
Differential fusion is less widely-recognized than character displacement, and several of its implications are refuted by experimental evidence. For example, differential fusion implies greater postzygotic isolation among sympatric species, as this functions to prevent fusion between the species. However, Coyne and Orr found equal levels of postzygotic isolation among sympatric and allopatric species pairs in closely related Drosophila
. Nevertheless, differential fusion remains a possible, though not complete, contributor to species discrimination.
- which was once highly debated among researchers - is progressively gaining credibility as a viable form of speciation.
- called “transient,” “resident,” and “offshore” - demonstrate partial sympatry, crossing paths with relative frequency. The results of recent genetic analyses using mtDNA
indicate that this is due to secondary contact, in which the three types encountered one another following the bidirectional migration of “offshore” and “resident” whales between the North Atlantic and North Pacific. Partial sympatry in these whales is not, therefore, not the result of speciation. Furthermore, Killer whale populations that consist of all three types have been documented in the Atlantic, evidencing that interbreeding occurs among them. Thus, secondary contact does not always result in total reproductive isolation
, as has often been predicted.
(Clamator glandarius) and its magpie
host, both native to Southern Europe, are completely sympatric species. However, the duration of their sympatry varies with location. For example, Great Spotted Cuckoos and their magpie hosts in Hoya de Gaudix, southern Spain, have lived in sympatry since the early 1960s, while species in other locations have more recently become sympatric.
The Great Spotted Cuckoo exhibits brood parasitism
by laying a mimicked version of the magpie egg in the magpie’s nest. Since cuckoo eggs hatch before magpie eggs, magpie hatchlings must compete with cuckoo hatchlings for resources provided by the magpie mother. This relationship between the cuckoo and the magpie in various locations can be characterized as either recently sympatric or anciently sympatric. The results of an experiment by Solar & Moller (1990) showed that in areas of ancient sympatry (species in cohabitation for many generations), magpies were more likely to reject most of the cuckoo eggs, as these magpies had developed counter-adaptations that aid in identification of egg type. In areas of recent sympatry, magpies rejected comparatively fewer cuckoo eggs. Thus, sympatry can cause coevolution
, by which both species undergo genetic changes due to the selective pressures that one species exerts on the other.
protect and nourish various species of fungus as a source of food in a system known as ant-fungus mutualism
. Leafcutter ants belonging to the genus Acromyrmex
are known for their mutualistic relationship with Basidiomycete fungi. Ant colonies are closely associated with their fungus colonies, and may have co-evolved with a consistent vertical lineage of fungi in individual colonies. Ant populations defend against the horizontal transmission
of foreign fungi to their fungal colony, as this transmission may lead to competitive stress on the local fungal garden. Invaders are identified and removed by the ant colony, inhibiting competition and fungal interbreeding. This active isolation of individual populations helps maintain the genetic purity of the fungal colony, and this mechanism may lead to sympatric speciation
within a shared habitat.
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...
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
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...
. Such speciation may be a product of reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
— which prevents hybrid offspring from being viable or able to reproduce, thereby reducing gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
— that results in genetic divergence. Sympatric speciation can also imply partially-divergent species that are not yet genetically isolated, and it may be the result of secondary contact after divergence has begun. Sympatric species may or may not interbreed.
Types of species
Four main types of species exist in nature. Sympatric species contrast with parapatric species, which contact one another in adjacent but not shared ranges and do not interbreed; peripatric species, which are separated only by areas in which neither organism occurs; and allopatric species, which occur in entirely distinct ranges that are neither adjacent nor overlapping. Allopatric species isolated from one another by geographical factors (e.g., mountain ranges or bodies of water) may experience genetic - and, ultimately, phenotypic - changes in response to their varying environments. These may drive allopatric speciationAllopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
, which is arguably the dominant mode of speciation.
Evolving definitions and controversy
The lack of geographic isolation as a definitive barrier between sympatric species has yielded controversy among ecologists, biologists, and zoologists regarding the validity of the term.As such, researchers have long debated the conditions under which sympatry truly applies, especially with respect to parasitism
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
. Because parasitic organisms often inhabit multiple hosts during a life cycle, evolutionary biologist Ernst Mayr
Ernst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
stated that internal parasites existing within different hosts demonstrate allopatry, not sympatry. Today, however, many biologists consider parasites and their hosts to be sympatric (see examples below). Conversely, zoologist Michael JD White
Michael JD White
*White M.J.D. and Webb G.C. Blattodea, Mantodea, Isoptera, Grylloblattodea, Phasmatodea, Dermaptera, and Embioptera. Borntraeger. ISBN 3443260055...
considered two populations sympatric if genetic interbreeding was viable within the habitat overlap. This may be further specified as sympatry occurring within one deme
Deme (biology)
In biology, a deme is a term for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool...
; that is, reproductive individuals must be able to locate one another in the same population in order to be sympatric.
Others question the ability of sympatry to result in complete speciation: until recently, many researchers considered it nonexistent, doubting that selection alone could create disparate, but not geographically separated, species. In 2003, biologist Karen McCoy suggested that sympatry can act as a mode of speciation only when “the probability of mating between two individuals depend[s] [solely] on their genotypes, [and the genes are] dispersed throughout the range of the population during the period of reproduction.”. In essence, sympatric speciation does require very strong forces of natural selection to be acting on heritable traits, as there is no geographic isolation to aid in the splitting process. Yet, recent research has begun to indicate that sympatric speciation is not as uncommon as was once assumed.
Sympatric speciation
The lack of geographic constraint in isolating sympatric populations implies that the emerging species avoid interbreeding via other mechanisms. Before speciation is complete, two diverging populations may still produce viable offspring. As speciation progresses, isolating mechanisms
Isolating mechanisms
Isolating Mechanisms are features of behavior, morphology, or genetics which serve to prevent breeding between species. Reproductive isolation of populations is established. It is particularly important to the biological species concept, as species are defined by reproductive isolation...
- such as gametic incompatibility that renders fertilization of the egg impossible - are selected for in order to increase the reproductive divide between the two populations.
Species discrimination
Sympatric groups frequently show a greater ability to discriminate between their own species and other closely related species than do allopatric groups. This is shown in the study of hybrid zonesHybrid zone
A hybrid zone exists where the ranges of two interbreeding species meet. For a hybrid zone to be stable, the offspring produced by the cross have to be less fit than members of the parent species, although this condition does not need to be met in the very first hybrid generation...
. It is also apparent in the differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
) in both sympatric and allopatric populations. There are two main theories regarding this process: 1) differential fusion, which suggests that only populations with a keen ability to discriminate between species will persist in sympatry, and 2) character displacement
Character displacement
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...
, which implies that distinguishing characteristics will be heightened in areas where the species co-occur in order to facilitate discrimination.
Reinforcement
Reinforcement is the process by which natural selection reinforces reproductive isolation. In sympatry, reinforcement increases species discrimination and sexual adaptation in order to avoid maladaptive hybridization and encourage speciation. If hybrid offspring are either sterile or less-fit than non-hybrid offspring, mating between members of two different species will be selected against. Natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
decreases the probability of such hybridization by selecting for the ability to identify mates of one’s own species from those of another species.
Reproductive character displacement
Reproductive character displacement strengthens the reproductive barriers between sympatric species by encouraging the divergence of traits that are crucial to reproduction. Divergence is frequently distinguished by assortative matingAssortative mating
Assortative mating , and the related concept Disassortative mating, is the phenomenon where a sexually reproducing organism chooses to mate with individuals that are similar or dissimilar to itself in some specific manner...
between individuals of the two species. For example, divergence in the mating signals of two species will limit hybridization by reducing one's ability to identify an individual of the second species as a potential mate. Support for the reproductive character displacement hypothesis comes from observations of sympatric species in overlapping habitats in nature. Increased prezygotic isolation, which is associated with reproductive character displacement, has been observed in cicadas of genus Magicicada
Magicicada
Magicicada is the genus of the 13- and 17-year periodical cicadas of eastern North America. They are sometimes called "17-year locusts", although cicadas belong to order Hemiptera, while locusts are Orthoptera....
, stickleback
Stickleback
The Gasterosteidae are a family of fish including the sticklebacks. FishBase currently recognises sixteen species in the family, grouped in five genera. However several of the species have a number of recognised subspecies, and the taxonomy of the family is thought to be in need of revision...
fish, and the flowering plants of the genus Phlox
Phlox
Phlox is a genus of 67 species of perennial and annual plants found mostly in North America in diverse habitats from alpine tundra to open woodland and prairie. Some flower in spring, others in summer and autumn....
.
Differential fusion
An alternative explanation for species discrimination in sympatry is differential fusion. This hypothesis states that of the many species have historically come into contact with one another, the only ones that persist in sympatry (and thus are seen today) are species with strong mating discrimination. On the other hand, species lacking strong mating discrimination are assumed to have fused while in contact, forming one distinct species.Differential fusion is less widely-recognized than character displacement, and several of its implications are refuted by experimental evidence. For example, differential fusion implies greater postzygotic isolation among sympatric species, as this functions to prevent fusion between the species. However, Coyne and Orr found equal levels of postzygotic isolation among sympatric and allopatric species pairs in closely related Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
. Nevertheless, differential fusion remains a possible, though not complete, contributor to species discrimination.
Examples
Sympatry has been increasingly evidenced in current research. Because of this, sympatric speciationSympatric 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...
- which was once highly debated among researchers - is progressively gaining credibility as a viable form of speciation.
Orca: partial sympatry
Several distinct types of Killer whale (Orcinus orca), which are characterized by an array of morphological and behavioral differences, live in sympatry throughout the North Atlantic, North Pacific and Antarctic oceans. In the North Pacific, three whale lineagesLineage
Lineage may refer to:- Science :* Lineage or kinship, descent group that can demonstrate their common descent from an apical ancestor or a direct line of decent from an ancestor....
- called “transient,” “resident,” and “offshore” - demonstrate partial sympatry, crossing paths with relative frequency. The results of recent genetic analyses using mtDNA
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
indicate that this is due to secondary contact, in which the three types encountered one another following the bidirectional migration of “offshore” and “resident” whales between the North Atlantic and North Pacific. Partial sympatry in these whales is not, therefore, not the result of speciation. Furthermore, Killer whale populations that consist of all three types have been documented in the Atlantic, evidencing that interbreeding occurs among them. Thus, secondary contact does not always result in total reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
, as has often been predicted.
Great Spotted Cuckoo and magpie: brood parasitism
The parasitic Great Spotted CuckooGreat Spotted Cuckoo
The Great Spotted Cuckoo is a member of the cuckoo order of birds, the Cuculiformes, which also includes the roadrunners, the anis and the coucals....
(Clamator glandarius) and its magpie
Magpie
Magpies are passerine birds of the crow family, Corvidae.In Europe, "magpie" is often used by English speakers as a synonym for the European Magpie, as there are no other magpies in Europe outside Iberia...
host, both native to Southern Europe, are completely sympatric species. However, the duration of their sympatry varies with location. For example, Great Spotted Cuckoos and their magpie hosts in Hoya de Gaudix, southern Spain, have lived in sympatry since the early 1960s, while species in other locations have more recently become sympatric.
The Great Spotted Cuckoo exhibits brood parasitism
Brood parasite
Brood parasites are organisms that use the strategy of brood parasitism, a kind of kleptoparasitism found among birds, fish or insects, involving the manipulation and use of host individuals either of the same or different species to raise the young of the brood-parasite...
by laying a mimicked version of the magpie egg in the magpie’s nest. Since cuckoo eggs hatch before magpie eggs, magpie hatchlings must compete with cuckoo hatchlings for resources provided by the magpie mother. This relationship between the cuckoo and the magpie in various locations can be characterized as either recently sympatric or anciently sympatric. The results of an experiment by Solar & Moller (1990) showed that in areas of ancient sympatry (species in cohabitation for many generations), magpies were more likely to reject most of the cuckoo eggs, as these magpies had developed counter-adaptations that aid in identification of egg type. In areas of recent sympatry, magpies rejected comparatively fewer cuckoo eggs. Thus, sympatry can cause coevolution
Host-parasite coevolution
Host–parasite coevolution is a special case of coevolution, which is defined as the reciprocal adaptive genetic change of two antagonists through reciprocal selective pressures...
, by which both species undergo genetic changes due to the selective pressures that one species exerts on the other.
Acromyrmex ant: isolation of fungal gardens
Leafcutter antsLeafcutter ant
Leafcutter ants, a non-generic name, are any of 47 species of leaf-chewing ants belonging to the two genera Atta and Acromyrmex.These species of tropical, fungus-growing ants are all endemic to South, Central America, Mexico and parts of the southern United States.The Acromyrmex and Atta ants have...
protect and nourish various species of fungus as a source of food in a system known as ant-fungus mutualism
Ant-fungus mutualism
Ant-fungus mutualism is a symbiosis seen in certain ant and fungal species, where ants actively cultivate fungus much like humans farm crops as a food source. In some species, the ants and fungi are dependent on each other for survival. The leafcutter ant is a well known example of this symbiosis...
. Leafcutter ants belonging to the genus Acromyrmex
Acromyrmex
Acromyrmex is a genus of New World ants of the subfamily Myrmicinae. It contains 31 known species.-Distribution:This genera is found in South America and parts of Central America and the Caribbean islands.-Overview:...
are known for their mutualistic relationship with Basidiomycete fungi. Ant colonies are closely associated with their fungus colonies, and may have co-evolved with a consistent vertical lineage of fungi in individual colonies. Ant populations defend against the horizontal transmission
Horizontal transmission
Horizontal transmission is the transmission of a bacterial, fungal, or viral infection between members of the same species that are not in a parent-child relationship....
of foreign fungi to their fungal colony, as this transmission may lead to competitive stress on the local fungal garden. Invaders are identified and removed by the ant colony, inhibiting competition and fungal interbreeding. This active isolation of individual populations helps maintain the genetic purity of the fungal colony, and this mechanism may 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...
within a shared habitat.
See also
- SpeciesSpeciesIn 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...
- Ring speciesRing speciesIn biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene...
- SpeciationSpeciationSpeciation 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...
- ParasitismParasitismParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
- SelectionSelectionIn the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
- Evolutionary pressureEvolutionary pressureAny cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or selection pressure. With sufficient pressure, inherited traits that mitigate its effects - even if they would be deleterious in other circumstances - can become widely spread...