Allopatric speciation

Allopatric speciation ()also called geographic speciation, vicariant speciation, or its earlier name the dumbbell modelis a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

Various geographic changes can arise such as the movement of continents, and the formation of mountains, islands, bodies of water, or glaciers. Human activity such as agriculture or developments can also change the distribution of species populations. These factors can substantially alter a region's geography, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genetic changes as they become subjected to different selective pressures, experience genetic drift, and accumulate different mutations in the separated populations' gene pools. The barriers prevent the exchange of genetic information between the two populations leading to reproductive isolation. If the two populations come into contact they will be unable to reproduce—effectively speciating. Other isolating factors such as population dispersal leading to emigration can cause speciation (for instance, the dispersal and isolation of a species on an oceanic island) and is considered a special case of allopatric speciation called peripatric speciation.

Allopatric speciation is typically subdivided into two major models: vicariance and peripatric. These models differ from one another by virtue of their population sizes and geographic isolating mechanisms. The terms allopatry and vicariance are often used in biogeography to describe the relationship between organisms whose ranges do not significantly overlap, but are immediately adjacent to each other; they do not occur together or only occur within a narrow zone of contact. Historically, the language used to refer to modes of speciation directly reflected biogeographical distributions. As such, allopatry is a geographical distribution opposed to sympatry (speciation within the same area). Furthermore, the terms allopatric, vicariant, and geographical speciation are often used interchangeably in the scientific literature. From this arises a host of issues in defining species, defining isolating barriers, measuring reproductive isolation, among others. Nevertheless, verbal and mathematical models, laboratory experiments, and empirical evidence overwhelmingly supports the occurrence of allopatric speciation in nature. The vicariance theory, which showed coherence along with the acceptance of plate tectonics in the 1960s, was developed by Croizat in the early 1950s as an explanation for the similarity of plants and animals found in South America and Africa by deducing that they had originally been a single population before the two continents drifted apart.

Currently, speciation by vicariance is widely regarded as the most common form of speciation;

Allopatric speciation can be represented as the extreme on a gene flow continuum. As such, the level of gene flow between populations in allopatry would be <math>m=0</math>, where <math>m</math> equals the rate of gene exchange. In sympatry <math>m=0.5</math> (panmixis), while in parapatric speciation, <math>0 < m < 0.5</math> represents the entire continuum, although some scientists argue that a classification scheme based solely on geographic mode does not necessarily reflect the complexity of speciation. Allopatry is often regarded as the default or "null" model of speciation, but this too is debated.

Reproductive isolation

Reproductive isolation acts as the primary mechanism driving genetic divergence in allopatry and can be amplified by divergent selection. Pre-zygotic and post-zygotic isolation are often the most cited mechanisms for allopatric speciation, and as such, it is difficult to determine which form evolved first in an allopatric speciation event. It is more often invoked in sympatric speciation studies, as it requires gene flow between two populations. However, reinforcement may also play a role in allopatric speciation, whereby the reproductive barrier is removed, reuniting the two previously isolated populations. Upon secondary contact, individuals reproduce, creating low-fitness hybrids. Traits of the hybrids drive individuals to discriminate in mate choice, by which pre-zygotic isolation increases between the populations. known as hybrid speciation. Reinforcement can play a role in all geographic modes (and other non-geographic modes) of speciation as long as gene flow is present and viable hybrids can be formed. The production of inviable hybrids is a form of reproductive character displacement, under which most definitions is the completion of a speciation event. Reinforcement in allopatry has been shown to occur in nature (evidence for speciation by reinforcement), albeit with less frequency than a classic allopatric speciation event. Fisherian sexual selection can also lead to reproductive isolation if there are minor variations in selective pressures (such as predation risks or habitat differences) among each population. (See the Further reading section below). Mathematical models concerning reproductive isolation-by distance have shown that populations can experience increasing reproductive isolation that correlates directly with physical, geographical distance. This has been exemplified in models of ring species; The size of the isolated population is important because individuals colonizing a new habitat likely contain only a small sample of the genetic variation of the original population. This promotes divergence due to strong selective pressures, leading to the rapid fixation of an allele within the descendant population. This gives rise to the potential for genetic incompatibilities to evolve. These incompatibilities cause reproductive isolation, giving rise to rapid speciation events. When a population of a species experiences a period of geographic range expansion and contraction, it may leave small, fragmented, peripherally isolated populations behind. These isolated populations will contain samples of the genetic variation from the larger parent population. This variation leads to a higher likelihood of ecological niche specialization and the evolution of reproductive isolation. Centrifugal speciation has been largely ignored in the scientific literature. Nevertheless, a wealth of evidence has been put forth by researchers in support of the model, much of which has not yet been refuted. Examples of microallopatric speciation in nature have been described. Rico and Turner found intralacustrine allopatric divergence of Pseudotropheus callainos (Maylandia callainos) within Lake Malawi separated only by 35 meters. Gustave Paulay found evidence that species in the subfamily Cryptorhynchinae have microallopatrically speciated on Rapa and its surrounding islets. A sympatrically distributed triplet of diving beetle (Paroster) species living in aquifers of Australia's Yilgarn region have likely speciated microallopatrically within a 3.5 km<sup>2</sup> area. The term was originally proposed by Hobart M. Smith to describe a level of geographic resolution. A sympatric population may exist in low resolution, whereas viewed with a higher resolution (i.e. on a small, localized scale within the population) it is "microallopatric". Ben Fitzpatrick and colleagues contend that this original definition, "is misleading because it confuses geographical and ecological concepts". Ecological allopatry is a reverse-ordered form of allopatric speciation in conjunction with reinforcement. The terms allo-parapatric and allo-sympatric have been used to describe speciation scenarios where divergence occurs in allopatry but speciation occurs only upon secondary contact. or "mixed-mode" speciation events.—adding a level of robustness unavailable to early researchers. continents, and even among mountains.

Islands are often home to species endemics—existing only on an island and nowhere else in the world—with nearly all taxa residing on isolated islands sharing common ancestry with a species on the nearest continent. Not without challenge, there is typically a correlation between island endemics and diversity; that is, that the greater the diversity (species richness) of an island, the greater the increase in endemism. Increased diversity effectively drives speciation. Furthermore, the number of endemics on an island is directly correlated with the relative isolation of the island and its area. In some cases, speciation on islands has occurred rapidly.

Dispersal and in situ speciation are the agents that explain the origins of the organisms in Hawaii. Various geographic modes of speciation have been studied extensively in Hawaiian biota, and in particular, angiosperms appear to have speciated predominately in allopatric and parapatric modes. The pattern indicates repeated vicariant speciation events among these groups. Dispersal-mediated allopatric speciation is also thought to be a significant driver of diversification throughout the Neotropics.

left|thumb|upright=1.7|Allopatric speciation can result from mountain topography. Climatic changes can drive species into altitudinal zones—either valleys or peaks. Colored regions indicate distributions. As distributions are modified due to the change in suitable habitats, reproductive isolation can drive the formation of a new species.

Patterns of increased endemism at higher elevations on both islands and continents have been documented on a global level. often constricted to graded zones. The formation of the Himalayan mountains and the Qinghai–Tibetan Plateau for example have driven the speciation and diversification of numerous plants and animals such as Lepisorus ferns; glyptosternoid fishes (Sisoridae); and the Rana chensinensis species complex. Uplift has also driven vicariant speciation in Macowania daisies in South Africa's Drakensberg mountains, along with Dendrocincla woodcreepers in the South American Andes. The Laramide orogeny during the Late Cretaceous even caused vicariant speciation and radiations of dinosaurs in North America.

Adaptive radiation, like the Galapagos finches observed by Charles Darwin, is often a consequence of rapid allopatric speciation among populations. However, in the case of the finches of the Galapagos, among other island radiations such as the honeycreepers of Hawaii represent cases of limited geographic separation and were likely driven by ecological speciation.

Isthmus of Panama

right|thumb|upright=1.3|A conceptual representation of species populations becoming isolated (blue and green) by the closure of the [[Isthmus of Panama (red circle). With the closure, North and South America became connected, allowing the exchange of species (purple). Grey arrows indicate the gradual movement of tectonic plates that resulted in the closure.]]

Geological evidence supports the final closure of the isthmus of Panama approximately 2.7 to 3.5 mya, with some evidence suggesting an earlier transient bridge existing between 13 and 15 mya. Recent evidence increasingly points towards an older and more complex emergence of the Isthmus, with fossil and extant species dispersal (part of the American biotic interchange) occurring in three major pulses, to and from North and South America. Further, the changes in terrestrial biotic distributions of both continents such as with Eciton army ants supports an earlier bridge or a series of bridges. Regardless of the exact timing of the isthmus closer, biologists can study the species on the Pacific and Caribbean sides in what has been called, "one of the greatest natural experiments in evolution". as phylogenetic reconstructions support the relationships of 15 pairs of sister species of Alpheus, each pair divided across the isthmus Recently diverged species live in shallow mangrove waters

Diadema antillarum and Diadema mexicanum
Echinometra lucunter and Echinometra vanbrunti
Echinometra viridis and E. vanbrunti
Bathygobius soporator and Bathygobius ramosus
B. soporator and Bathygobius andrei
Excirolana braziliensis and variant morphs

Refugia

Ice ages have played important roles in facilitating speciation among vertebrate species. This concept of refugia has been applied to numerous groups of species and their biogeographic distributions. or the prairie dogs Cynomys mexicanus and C. ludovicianus.

Superspecies

right|thumb|upright=1.5|The red shading indicates the range of the [[bonobo (Pan paniscus). The blue shading indicates the range of the Common chimpanzee (Pan troglodytes). This is an example of allopatric speciation because they are divided by a natural barrier (the Congo River) and have no habitat in common. Other Pan subspecies are shown as well.]]

Numerous species pairs or species groups show abutting distribution patterns, that is, reside in geographically distinct regions next to each other. They often share borders, many of which contain hybrid zones. Some examples of abutting species and superspecies (an informal rank referring to a complex of closely related allopatrically distributed species, also called allospecies) include:

Western, Chihuahuan, and Eastern meadowlarks in North America reside in dry western and wet eastern geographic regions with rare occurrences of hybridization, most of which results in infertile offspring.
Bonobos and chimpanzees.
Climacteris tree creeper birds in Australia.
Birds-of-paradise in the mountains of New Guinea (genus Astrapia). All of these species pairs connect at zones of hybridization that correspond with major geographic barriers.

In birds, some areas are prone to high rates of superspecies formation such as the 105 superspecies in Melanesia, comprising 66 percent of all bird species in the region. Patagonia is home to 17 superspecies of forest birds, while North America has 127 superspecies of both land and freshwater birds. Sub-Saharan Africa has 486 passerine birds grouped into 169 superspecies. Australia has numerous bird superspecies as well, with 34 percent of all bird species grouped into superspecies. Later investigation found that the populations evolved behavioral isolation as a pleiotropic by-product from this adaptive divergence. or insufficient genetic diversity. Various isolation indices have been developed to measure reproductive isolation (and are often employed in laboratory speciation studies) such as here (index <math>Y</math> and index <math>I</math>):

Here, <math>A</math> and <math>D</math> represent the number of matings in heterogameticity where <math>B</math> and <math>C</math> represent homogametic matings. <math>A</math> and <math>B</math> is one population and <math>D</math> and <math>C</math> is the second population. A negative value of <math>Y</math> denotes negative assortive mating, a positive value denotes positive assortive mating (i. e. expressing reproductive isolation), and a null value (of zero) means the populations are experiencing random mating. Using index Y presented previously, a survey of 25 allopatric speciation experiments (included in the table below) found that reproductive isolation was not as strong as typically maintained and that laboratory environments have not been well-suited for modeling allopatric speciation.

|Locomotion

|112

|Indirect; divergent

|No

|Pre-zygotic

|1974

|Temperature, humidity

|70–130

|Indirect; divergent

|Yes

|Pre-zygotic

|1980

|DDT adaptation

|600 (25 years, +15 years)

|Direct

|No

|Pre-zygotic

|2003

|17, 9, 9, 1, 1, 7, 7, 7, 7

|Direct, divergent

|Pre-zygotic

|1974

|40; 50

|Direct; divergent

|Pre-zygotic

|1974

|Locomotion

|45

|Direct; divergent

|No

|None

|1979

|Direct; divergent

|Pre-zygotic

|1953

|36; 31

|Direct; divergent

|Pre-zygotic

|1956

|EDTA adaptation

|3 experiments, 25 each

|Indirect

|No

|Post-zygotic

|1966

|8 experiments, 25 each

|Direct

|1997

|Abdominal chaeta

number

|21–31

|Direct

|Yes

|None

|1958

|Sternopleural chaeta number

|32

|Direct

|No

|None

|1969

|Phototaxis, geotaxis

|20

|No

|None

|1975 1981

|Yes

|1998

|Yes

|1999

|Direct; divergent

|Pre-zygotic

|1971 1973 1979 1983

|D. simulans

|Scutellar bristles, development speed, wing width;

desiccation resistance, fecundity, ethanol resistance;

courtship display, re-mating speed, lek behavior;

pupation height, clumped egg laying, general activity

|3 years

|Yes

|Post-zygotic

|1985

| rowspan="2" |D. paulistorum

|131; 131

|Direct

|Pre-zygotic

|1976

|5 years

|1966

|D. willistoni

|pH adaptation

|34–122

|Indirect; divergent

|No

|Pre-zygotic

|1980

| rowspan="6" |D. pseudoobscura

|Carbohydrate source

|12

|Indirect

|Yes

|Pre-zygotic

|1989

|Temperature adaptation

|25–60

|Direct

|1964 1969

|Phototaxis, geotaxis

|5–11

|Indirect

|No

|Pre-zygotic

|1966

|Pre-zygotic

|1978 1985

|Yes

|1993

|Temperature photoperiod; food

|37

|Divergent

|Yes

|None

|2003

| rowspan="2" |D.pseudoobscura &

D. persimilis

|22; 16; 9

|Direct; divergent

|Pre-zygotic

|1950

|4 experiments, 18 each

|Direct

|Pre-zygotic

|1966

| rowspan="2" |D. mojavensis

|12

|Direct

|Pre-zygotic

|1987

|Development time

|13

|Divergent

|Yes

|None

|1998

|D. adiastola

|Yes

|Pre-zygotic

|1974

|D. silvestris

|Yes

|1980

| rowspan="3" |Musca domestica

|Geotaxis

|38

|Indirect

|No

|Pre-zygotic

|1974

|Geotaxis

|16

|Direct; divergent

|No

|Pre-zygotic

|1975

|Yes

|1991

|Bactrocera cucurbitae

|Development time

|40–51

|Divergent

|Yes

|Pre-zygotic

|1999

|Zea mays

|6; 6

|Direct; divergent

|Pre-zygotic

|1969

|D. grimshawi

History and research techniques

Early speciation research typically reflected geographic distributions and were thus termed geographic, semi-geographic, and non-geographic. of which he used the term Separationstheorie. His idea was later interpreted by Ernst Mayr as a form of founder effect speciation as it focused primarily on small geographically isolated populations.

Controversy exists as to whether Charles Darwin recognized a true geographical-based model of speciation in his publication of the Origin of Species. F. J. Sulloway contends that Darwin's position on speciation was "misleading" at the least and may have later misinformed Wagner and David Starr Jordan into believing that Darwin viewed sympatric speciation as the most important mode of speciation. Much later, the biologist Ernst Mayr was the first to encapsulate the then contemporary literature in his 1942 publication Systematics and the Origin of Species, from the Viewpoint of a Zoologist and in his subsequent 1963 publication Animal Species and Evolution. Like Jordan's works, they relied on direct observations of nature, documenting the occurrence of allopatric speciation, of which is widely accepted today. however, it is thought that Wagner, Karl Jordan, and David Starr Jordan played a large role in the formation of allopatric speciation as an evolutionary concept; where Mayr and Dobzhansky contributed to the formation of the modern evolutionary synthesis.

The late 20th century saw the development of mathematical models of allopatric speciation, leading to the clear theoretical plausibility that geographic isolation can result in the reproductive isolation of two populations. Today, it is widely regarded as the most common form of speciation taking place in nature. Correlation of geographic distribution with phylogenetic data also spawned a sub-field of biogeography called vicariance biogeography). Other techniques used today have employed measures of gene flow between populations, or the environmentally-mediated speciation taking place among dendrobatid frogs in Ecuador Biotechnological advances have allowed for large scale, multi-locus genome comparisons (such as with the possible allopatric speciation event that occurred between ancestral humans and chimpanzees), linking species' evolutionary history with ecology and clarifying phylogenetic patterns.

Reproductive isolation

Isthmus of Panama

Refugia

Superspecies

History and research techniques

References

Further reading