Allopatric speciation (from Ancient Greek ἄλλος, allos, meaning "other", and πατρίς, patris, "fatherland"), also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model,:86 is a mode of speciation that occurs when biological populations of the same species become 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. Both 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 in 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. This article will follow a similar theme, with the exception of special cases such as peripatric, centrifugal, among others.
Observation of nature creates difficulties in witnessing allopatric speciation from "start-to-finish" as it operates as a dynamic process. From this arises a host of various 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.:87–105 Mathematical modeling of the genetic basis of reproductive isolation supports the plausibility of allopatric speciation; whereas laboratory experiments of Drosophila and other animal and plant species have confirmed that reproductive isolation evolves as a byproduct of natural selection.:87
Speciation by vicariance is widely regarded as the most common form of speciation; and is the primary model of allopatric speciation. Vicariance is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous populations (disjunct distributions) by the formation of an extrinsic barrier to the exchange of genes: that is, a barrier arising externally to a species. These extrinsic barriers often arise from various geologic-caused, topographic changes such as: the formation of mountains (orogeny); the formation of rivers or bodies of water; glaciation; the formation or elimination of land bridges; the movement of continents over time (by tectonic plates); or island formation, including sky islands. These can change the distribution of species populations. The emergence of suitable or unsuitable habitat configurations may arise from these changes and can originate by changes in climate or even large scale human activities (for example, agricultural, civil engineering developments, and habitat fragmentation). Among others, these many factors can alter a regions geography in substantial ways, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to different selective pressures, (b) they independently undergo genetic drift, and (c) different mutations arise in the gene pools of the populations. The extrinsic barriers prevent the exchange of genetic information between the two populations, inevitably leading to differentiation due to the ecologically different habitats they experience; selective pressure then invariably leads to complete reproductive isolation.:86 Furthermore, a species' proclivity to remain in its ecological niche (see phylogenetic niche conservatism) through changing environmental conditions may also play a role in isolating populations from one another, driving the evolution of new lineages.
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 , where equals the rate of gene exchange. In sympatry , while in parapatric speciation, represents the entire continuum, though not all scientists accept this geographic mode classification scheme, which 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 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. Pre-zygotic simply implies the presence of a barrier prior to any act of fertilization (such as an environmental barrier dividing two populations), while post-zygotic implies the prevention of successful inter-population crossing after fertilization (such as the production of an infertile hybrid). Since species pairs who diverged in allopatry often exhibit pre- and post-zygotic isolation mechanisms, investigation of the earliest stages in the life cycle of the species can indicate whether or not divergence occurred due to a pre-zygotic or post-zygotic factor. However, establishing the specific mechanism may not be accurate, as a species pair continually diverges over time. For example, if a plant experiences a chromosome duplication event, reproduction will occur, but sterile hybrids will result—functioning as a form of post-zygotic isolation. Subsequently, the newly formed species pair may experience pre-zygotic barriers to reproduction as selection, acting on each species independently, will ultimately lead to genetic changes making hybrids impossible. From the researchers perspective, the current isolating mechanism may not reflect the past isolating mechanism.
Reinforcement has been a contentious factor in speciation. 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. Some arguments have been put forth that suggest the hybrids themselves can possibly become their own species: 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.
Research has well established the fact that interspecific mate discrimination occurs to a greater extent between sympatric populations than it does in purely allopatric populations; however, other factors have been proposed to account for the observed patterns. 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. A major difficulty arises when interpreting reinforcement's role in allopatric speciation, as current phylogenetic patterns may suggest past gene flow. This masks possible initial divergence in allopatry and can indicate a "mixed-mode" speciation event—exhibiting both allopatric and sympatric speciation processes.
Developed in the context of the genetic basis of reproductive isolation, mathematical scenarios model both prezygotic and postzygotic isolation with respect to the effects of genetic drift, selection, sexual selection, or various combinations of the three. Masatoshi Nei and colleagues were the first to develop a neutral, stochastic model of speciation by genetic drift alone. Both selection and drift can lead to postzygotic isolation, supporting the fact that two geographically separated populations can evolve reproductive isolation:87—sometimes occurring rapidly. 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; however, it has been argued that ring species are a special case, representing reproductive isolation-by distance, and demonstrate parapatric speciation instead:102—as parapatric speciation represents speciation occurring along a cline.
Various alternative models have been developed concerning allopatric speciation. Special cases of vicariant speciation have been studied in great detail, one of which is peripatric speciation, whereby a small subset of a species population becomes isolated geographically; and centrifugal speciation, an alternative model of peripatric speciation concerning expansion and contraction of a species range. Other minor allopatric models have also been developed are discussed below.
Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population.:105 If a small population of a species becomes isolated (e.g. a population of birds on an oceanic island), selection can act on the population independent of the parent population. Given both geographic separation and enough time, speciation can result as a byproduct. It can be distinguished from allopatric speciation by three important features: 1) the size of the isolated population, 2) the strong selection imposed by the dispersal and colonization into novel environments, and 3) the potential effects of genetic drift on small populations.:105 However, it can often be difficult for researchers to determine if peripatric speciation occurred as vicariant explanations can be invoked due to the fact that both models posit the absence of gene flow between the populations. 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.:105–106 Models of peripatry are supported mostly by species distribution patterns in nature. Oceanic islands and archipelagos provide the strongest empirical evidence that peripatric speciation occurs.:106–110
Centrifugal speciation is a variant, alternative model of peripatric speciation. This model contrasts with peripatric speciation by virtue of the origin of the genetic novelty that leads to reproductive isolation. 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. One example is the possible center of origin in the Indo-West Pacific.
Microallopatry refers to allopatric speciation occurring on a small geographic scale. 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 km2 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 speciation can occur allopatrically, sympatrically, or parapatrically; the only requirement being that it occurs as a result of adaptation to different ecological or micro-ecological conditions. Ecological allopatry is a reverse-ordered form of allopatric speciation in conjunction with reinforcement. First, divergent selection separates a non-allopatric population emerging from pre-zygotic barriers, from which genetic differences evolve due to the obstruction of complete gene flow. 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.:112 These are effectively models of reinforcement or "mixed-mode" speciation events.
As allopatric speciation is widely accepted as a common mode of speciation, the scientific literature is abundant with studies documenting its existence. The biologist Ernst Mayr was the first to summarize the contemporary literature of the time in 1942 and 1963.:91 Many of the examples he set forth remain conclusive; however, modern research supports geographic speciation with molecular phylogenetics—adding a level of robustness unavailable to early researchers.:91 The most recent thorough treatment of allopatric speciation (and speciation research in general is Jerry Coyne and H. Allen Orr's 2004 publication Speciation. They list six mainstream arguments that lend support to the concept of vicariant speciation:
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.
Islands are not the only geographic locations that have endemic species. South America has been studied extensively with its areas of endemism representing assemblages of allopatrically distributed species groups. Charis butterflies are a primary example, confined to specific regions corresponding to phylogenies of other species of butterflies, amphibians, birds, marsupials, primates, reptiles, and rodents. The pattern indicates repeated vicariant speciation events among these groups. It is thought that rivers may play a role as the geographic barriers to Charis,:97 not unlike the river barrier hypothesis used to explain the high rates of diversity in the Amazon basin—though this hypothesis has been disputed. Dispersal-mediated allopatric speciation is also thought to be a significant driver of diversification throughout the Neotropics.
Patterns of increased endemism at higher elevations on both islands and continents have been documented on a global level. As topographical elevation increases, species become isolated from one another; often constricted to graded zones. This isolation on "mountain top islands" creates barriers to gene flow, encouraging allopatric speciation, and generating the formation of endemic species. Mountain building (orogeny) is directly correlated with—and directly affects biodiversity. 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 radiations, 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.
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 to 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". Additionally, as with most geologic events, the closure was unlikely to have occurred rapidly, but instead dynamically—a gradual shallowing of sea water over millions of years.:93
Studies of snapping shrimp in the genus Alpheus have provided direct evidence of an allopatric speciation event, as phylogenetic reconstructions support the relationships of 15 pairs of sister species pairs of Alpheus on each side of the isthmus and molecular clock dating supports their separation between 3 and 15 million years ago. Recently diverged species reside in shallow mangrove waters while older diverged species live in deeper water, correlating with a gradual closure of the isthmus.:93 Support for an allopatric divergence also comes from laboratory experiments on the species pairs showing nearly complete reproductive isolation.:93
Similar patterns of relatedness and distribution across the Pacific and Atlantic sides have been found in other species pairs such as:
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.:97
Glaciation and subsequent retreat caused speciation in many boreal forest birds, such as with North American sapsuckers (Yellow-bellied, Red-naped, and Red-breasted); the warbler's in the genus Setophaga (S. townsendii, S. occidentalis, and S. virens), Oreothlypis (O. virginiae, O. ridgwayi, and O. ruficapilla), and Oporornis (O. tolmiei and O. philadelphia now classified in the genus Geothlypis); Fox sparrow's (sub species P. (i.) unalaschensis, P. (i.) megarhyncha, and P. (i.) schistacea); Vireo (V. plumbeus, V. cassinii, and V. solitarius); tyrant flycatcher's (E. occidentalis and E. difficilis); chickadee's (P. rufescens and P. hudsonicus); and thrush's (C. bicknelli and C. minimus).
As a special case of allopatric speciation, peripatric speciation is often invoked for instances of isolation in glaciation refugia as small populations become isolated due to habitat fragmentation such as with North American red (Picea rubens) and black (Picea mariana) spruce or the prairie dogs Cynomys mexicanus and C. ludovicianus.
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:
In birds, some areas are prone to high rates of superspecies formation (see speciation in birds) 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.
Experiments on allopatric speciation are often complex and do not simply divide a species population into two. This is due to a host of defining parameters: measuring reproductive isolation, sample sizes (the number of matings conducted in reproductive isolation tests), bottlenecks, length of experiments, number of generations allowed, 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  and index ):
Here, and represent the number of matings in heterogameticity where and represent homogametic matings. and is one population and and is the second population. A negative value of 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.
The experimental evidence has solidly established the fact that reproductive isolation evolves as a by-product of selection.:90 Reproductive isolation has been shown to arise from pleiotropy (i.e. indirect selection acting on genes that code for more than one trait)—what has been referred to as genetic hitchhiking. Limitations and controversies exist relating to whether laboratory experiments can accurately reflect the long-scale process of allopatric speciation that occurs in nature. Experiments often fall beneath 100 generations, far less than expected, as rates of speciation in nature are thought to be much larger.:87 Furthermore, rates specifically concerning the evolution of reproductive isolation in Drosophila are significantly higher than what is practiced in laboratory settings. 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. Nevertheless, numerous experiments have shown pre-zygotic and post-zygotic isolation in vicariance, some in less than 100 generations.:87
Below is a non-exhaustive table of the laboratory experiments conducted on allopatric speciation. The first column indicates the species used in the referenced study, where the "Trait" column refers to the specific characteristic selected for or against in that species. The "Generations" column refers to the number of generations in each experiment performed. If more than one experiment was formed generations are separated by semicolons or dashes (given as a range). Some studies provide a duration in which the experiment was conducted. The "Mode" column indicates if the study modeled vicariant or peripatric speciation (this may not be explicitly. Direct selection refers to selection imposed to promote reproductive isolation whereas indirect selection implies isolation occurring as a pleiotropic byproduct of natural selection; whereas divergent selection implies deliberate selection of each allopatric population in opposite directions (e.g. one line with more bristles and the other line with less). Some studies performed experiments modeling or controlling for genetic drift. Reproductive isolation occurred pre-zygotically, post-zygotically, both, or not at all). It is important to note that many of the studies conducted contain multiple experiments within—a resolution of which this table does not reflect.
|Species||Trait||~Generations (duration)||Selection type||Studied Drift||Reproductive isolation||Year & Reference|
|Escape response||18||Indirect; divergent||Yes||Pre-zygotic||1969|
|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|
|36; 31||Direct; divergent||Pre-zygotic||1956|
|EDTA adaptation||3 experiments, 25 each||Indirect||No||Post-zygotic||1966|
|8 experiments, 25 each||Direct||1997|
|Sternopleural chaeta number||32||Direct||No||None||1969|
|Phototaxis, geotaxis||20||No||None||1975 1981|
|Direct; divergent||Pre-zygotic||1971 1973 1979 1983|
|D. simulans||Scutellar bristles, development speed, wing width;
pupation height, clumped egg laying, general activity
|D. paulistorum||131; 131||Direct||Pre-zygotic||1976|
|D. willistoni||pH adaptation||34–122||Indirect; divergent||No||Pre-zygotic||1980|
|D. pseudoobscura||Carbohydrate source||12||Indirect||Yes||Pre-zygotic||1989|
|Temperature adaptation||25–60||Direct||1964 1969|
|Temperature photoperiod; food||37||Divergent||Yes||None||2003|
|D.pseudoobscura &||22; 16; 9||Direct; divergent||Pre-zygotic||1950|
|4 experiments, 18 each||Direct||Pre-zygotic||1966|
|Bactrocera cucurbitae||Development time||40–51||Divergent||Yes||Pre-zygotic||1999|
|Zea mays||6; 6||Direct; divergent||Pre-zygotic||1969|
Early speciation research typically reflected geographic distributions and were thus termed geographic, semi-geographic, and non-geographic. Geographic speciation corresponds to today's usage of the term allopatric speciation, and in 1868, Moritz Wagner was the first to propose the concept 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.
Edward Bagnall Poulton, an evolutionary biologist and a strong proponent of the importance of natural selection, highlighted the role of geographic isolation in promoting speciation, in the process coining the term "sympatric speciation" in 1903.
Controversy exists as to whether Charles Darwin recognized a true geographical-based model of speciation in his publication of the Origin of Species. In chapter 11, "Geographical Distribution", Darwin discusses geographic barriers to migration, stating for example that "barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions [of the world]". 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.:83 Nevertheless, Darwin never fully accepted Wagner's concept of geographical speciation.
David Starr Jordan played a significant role in promoting allopatric speciation in the early 20th century, providing a wealth of evidence from nature to support the theory.:86 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.:83–84 Prior to this research, Theodosius Dobzhansky published Genetics and the Origin of Species in 1937 where he formulated the genetic framework for how speciation could occur.:2
Other scientists noted the existence of allopatrically distributed pairs of species in nature such as Joel Asaph Allen (who coined the term "Jordan's Law", whereby closely related, geographically isolated species are often found divided by a physical barrier:91) and Robert Greenleaf Leavitt; 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.:87
Since the 1940s, allopatric speciation has been accepted. Today, it is widely regarded as the most common form of speciation taking place in nature.:84 However, this is not without controversy, as both parapatric and sympatric speciation are both considered tenable modes of speciation that occur in nature. Some researchers even consider there to be a bias in reporting of positive allopatric speciation events, and in one study reviewing 73 speciation papers published in 2009, only 30 percent that suggested allopatric speciation as the primary explanation for the patterns observed considered other modes of speciation as possible.
Contemporary research relies largely on multiple lines of evidence to determine the mode of a speciation event; that is, determining patterns of geographic distribution in conjunction with phylogenetic relatedness based on molecular techniques.:123–124 This method was effectively introduced by John D. Lynch in 1986 and numerous researchers have employed it and similar methods, yielding enlightening results. Correlation of geographic distribution with phylogenetic data also spawned a sub-field of biogeography called vicariance biogeography:92 developed by Joel Cracraft, James Brown, Mark V. Lomolino, among other biologists specializing in ecology and biogeography. Similarly, full analytical approaches have been proposed and applied to determine which speciation mode a species underwent in the past using various approaches or combinations thereof: species-level phylogenies, range overlaps, symmetry in range sizes between sister species pairs, and species movements within geographic ranges. Molecular clock dating methods are also often employed to accurately gauge divergence times that reflect the fossil or geological record:93 (such as with the snapping shrimp separated by the closure of the Isthmus of Panama or speciation events within the genus Cyclamen). Other techniques used today have employed measures of gene flow between populations, ecological niche modelling (such as in the case of the Myrtle and Audubon's warblers or the environmentally-mediated speciation taking place among dendrobatid frogs in Ecuador), and statistical testing of monophyletic groups. 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.
Mathematical models of reproductive isolation
The Atrato River (Spanish: Río Atrato) is a river of northwestern Colombia. It rises in the slopes of the Western Cordillera and flows almost due north to the Gulf of Urabá (or Gulf of Darién), where it forms a large, swampy delta.
Its course crosses the department of Chocó, forming that department's border with neighbouring Antioquia on two occasions.
Its total length is about 650 km (400 mi), and it is navigable as far as Quibdó (400 km / 250 mi).
Flowing through a narrow valley between the Cordillera and coastal range, it has only short tributaries, the principal ones being the Truando River, the Sucio River, and the Murrí River. The gold and platinum mines of Chocó line some of its confluents, and the river sands are auriferous.Northwestern Colombia encompasses an area of great diversity in wildlife. During the Pleistocene era at the height of the Atrato river, where it intersected the Cauca-Magdalena, the area was covered by a sea. It is proposed that this created a geographic barrier that may have caused many species to diverge through the process of allopatric speciation. For example, Philip Hershkovitz proposed that the cotton-top tamarin (Saguinus oedipus) and the white-footed tamarin (Saguinus leocopus) diverged by the Atrato, and are today principally separated by the river.Clarkia biloba
Clarkia biloba is a species of flowering plant in the evening primrose family known by the common name twolobe clarkia and two lobed clarkia.Clarkia biloba is endemic to California, where it is known from the Sierra Nevada foothills; one subspecies can also be found in the San Francisco Bay Area. It grows in chaparral, oak woodlands, and yellow pine forest habitats.As the putative progenitor species of Clarkia lingulata it is often used in examples of evolution outside the usual model of allopatric speciation. As such this plant is one of the best plant examples when considering "quantum speciation," a concept closely aligned with peripatric speciation, parapatric speciation and sympatric speciation.Clarkia lingulata
Clarkia lingulata is a rare species of wildflower known by the common name Merced clarkia. This plant is endemic to Mariposa County, California, where it is known from only two sites near the Merced River.
It is thought to have evolved very rapidly, outside the usual model of allopatric speciation, from its parental species Clarkia biloba.It is a state-listed endangered species in California.Curve-billed thrasher
The curve-billed thrasher (Toxostoma curvirostre) is a medium-sized mimid that is a member of the genus Toxostoma, native to the southwestern United States and much of Mexico. Referred to as the default desert bird, it is a non-migratory species. Several subspecies have been classified since 1827, though there is no consensus on the number. Allopatric speciation is believed to have played a major role in the variations of the curve-billed. It shares striking similarities in appearance with another Toxostoma member, Bendire's thrasher. Nevertheless, it is recognized for its grey color and sickle-shaped bill. It generally resides in desert regions of the United States and Mexico, but can inhabit areas predominately populated by humans.
The demeanor of the curve-billed has been described as "shy and rather wild", but it allows humans to view it closely. It is very aggressive in driving out potential threats, whether competitors for food or predators of its chicks. The curve-billed thrasher sometimes mimicks several other species, though not to the extent of other mimids. It has a variety of distinctive songs, and this extensive repertoire of melodies has led it to be known as cuitiacoache (songbird) in Mexico.Drosophila pseudoobscura
Drosophila pseudoobscura is a species of fruit fly, used extensively in lab studies of speciation. It is native to western North America.
In 2005, D. pseudoobscura was the second Drosophila species to have its genome sequenced, after the model organism Drosophila melanogaster.Allopatric speciation has been induced by reproductive isolation in D. pseudoobscura after only eight generations using different food types, starch and maltose.Dwarf elephant
Dwarf elephants are prehistoric members of the order Proboscidea which, through the process of allopatric speciation on islands, evolved much smaller body sizes (around 1.5-2.3 metres) in comparison with their immediate ancestors. Dwarf elephants are an example of insular dwarfism, the phenomenon whereby large terrestrial vertebrates (usually mammals) that colonize islands evolve dwarf forms, a phenomenon attributed to adaptation to resource-poor environments and selection for early maturation and reproduction. Some modern populations of Asian elephants have also undergone size reduction on islands to a lesser degree, resulting in populations of pygmy elephants.
Fossil remains of dwarf elephants have been found on the Mediterranean islands of Cyprus, Malta (at Għar Dalam), Crete (in Chania at Vamos, Stylos and in a now underwater cave on the coast), Sicily, Sardinia, the Cyclades Islands and the Dodecanese Islands. Other islands where dwarf stegodon have been found are Sulawesi, Flores, Timor, other islands of the Lesser Sundas and Central Java, all islands are in Indonesia. The Channel Islands of California once supported a dwarf species descended from Columbian mammoths, while populations of small woolly mammoths were once found on Saint Paul Island; the mammoths on Wrangel Island are no longer considered dwarfs.Emil Selenka
Emil Selenka (27 February, 1842, Braunschweig – 20 February, 1902, Munich) was a German zoologist. He is known for his research on invertebrates and apes and the scientific expeditions he organized to Southeast Asia and South America.
Selenka was the son of bookbinder Johannes Selenka (1801–1871). He studied natural history at the University of Göttingen, and following a graduate dissertation on Holothuroidea, he remained in Göttingen as an assistant to Wilhelm Moritz Keferstein (1833-1870). His research was in this period mainly on the anatomy, taxonomy and embryology of marine invertebrates, especially organisms from the phylum Echinodermata. In 1868 he became a professor of zoology and comparative anatomy at the University of Leiden, followed by a professorship at the University of Erlangen in 1874. In 1895 he was given an honorary professorship at the University of Munich. He was co-founder of the journal Biologisches Zentralblatt.
His later research was on mammals. He studied the early development of the embryo and the development of the germ layer in mammals, and did comparative anatomic research on apes, especially gibbons and orangutans. He found evidence that the lateral distribution of orangutan races was caused by geographic isolation (a process called allopatric speciation). Selenka also examined the evolution of marsupials and their morphologic relation with reptiles. One problem he was interested in, was the evolutionary relation between Australian and South American marsupials.
In order to collect material, Selenka organized expeditions to tropical countries. In 1877 he undertook an expedition to Brasil. From 1892 he led an expedition that lasted two years to Southeast Asia, it visited Ceylon, the Dutch East Indies, Japan, China and Australia. Among the participants was his second wife, the zoologist and feminist Margarethe Selenka (1860-1922), with whom he was married in 1893. When Selenka became severely ill during his stay in the Dutch East Indies and had to return to Germany, his wife continued exploring the jungles of Borneo by herself. The couple wrote a report of their journeys together, titled "Sonnige Welten- Ostasiatische Reiseskizzen". Other publications by Emil Selenka are:
Beiträge zur Anatomie und Systematik der Holothurien, (1867).
Zoologische Studien, (1878).
Studien über Entwickelungsgeschichte der Thiere, (12 volumes, 1883–1913, with Ambrosius Hubrecht).
"Report on the Gephyrea, collected by H.M.S. Challenger during the years 1873-1876"; (published in English, 1885).
Zoologisches Taschenbuch für Studierende zum Gebrauch bei Vorlesungen und praktischen Übungen zusammengestellt, (1897).Between 1873 and 1874 and again from 1889 Selenka was a member of the Royal Netherlands Academy of Arts and Sciences.Gene flow
In population genetics, gene flow (also known as gene migration or allele flow) is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population. It has been shown that it takes only "One migrant per generation" to prevent populations from diverging due to drift. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity within the populations, by modifying the allele frequencies (the proportion of members carrying a particular variant of a gene). High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason, gene flow has been thought to constrain speciation by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to full speciation. In some cases migration may also result in the addition of novel genetic variants to the gene pool of a species or population.
There are a number of factors that affect the rate of gene flow between different populations. Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, and when there are small population sizes. Mobility plays an important role in the migration rate, as highly mobile individuals tend to have greater migratory prospects. Although animals are thought to be more mobile than plants, pollen and seeds may be carried great distances by animals or wind. When gene flow is impeded, there can be an increase in inbreeding, measured by the inbreeding coefficient (F) within a population. For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes. The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so strongly isolated that lack of gene flow has led to high rates of inbreeding.Genetic isolate
A genetic isolate is population of organisms that has little genetic mixing with other organisms within the same species. This may result in speciation, but this is not necessarily the case. Genetic isolates may form new species in several ways:
allopatric speciation, in which two populations of the same species are geographically isolated from one another by an extrinsic barrier, and evolve intrinsic (genetic) reproductive isolation
peripatric speciation, in which a small group of a population is separated from the main population, and experiences genetic drift
parapatric speciation, in which zones of two diverging populations are separate, but do overlap somewhat; partial separation is afforded by geography, so individuals of each species may come in contact from time to time, but selection for specific behaviours or mechanisms may prevent breeding between the two groups.
sympatric speciation, a contentious method of speciation in which species diverge while inhabiting the same place.Human influences on genetic isolates include restricted breeding of dogs, or a community living secluded away from others (such as Tristan da Cunha or Pitcairn Islands). Far larger and less secluded human genetic isolates are peoples like Sardinians or also the Finns, natives of Finland.Glossary of speciation
This is a glossary of terms used in speciation research and related evolutionary disciplines. It is intended as introductory material and a structured organization of the often complex language used in the literature. Related glossaries in biology are: the glossary of biology, glossary of genetics, glossary of ecology, and the glossary of botany.Mountain pigeon
Gymnophaps is a genus of bird in the pigeon family Columbidae; it includes four species, all called mountain pigeons. Mountain pigeons live in the forested hills and mountains of certain islands in eastern Indonesia and in the region of Melanesia. They are tree-dwellers and eat fruit. A remarkable aspect of these pigeons is that they have bright red skin around the eyes. Otherwise, they are dark gray on the back and wings, with paler heads and undersides. Males and females look mostly alike, though the female Papuan mountain pigeon has a grayer belly than the male. Mountain pigeons move around a lot and are often seen flying, usually in flocks of at least ten or as many as a hundred or more birds. They are not known to do a lot of loud cooing, rather their vocalizations are muted or wheezy when they make any noise at all.
The ranges of the four species do not overlap, so this genus is an example of probable allopatric speciation. Two species are island endemics: the Buru mountain pigeon is endemic to the Maluku Islands, and the pale mountain pigeon is endemic to the Solomon Islands. The Papuan mountain pigeon has a range in between the others; it lives on the island of New Guinea and on nearby smaller islands.Parapatric speciation
In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes. This mode of speciation has three distinguishing characteristics: 1) mating occurs non-randomly, 2) gene flow occurs unequally, and 3) populations exist in either continuous or discontinuous geographic ranges. This distribution pattern may be the result of unequal dispersal, incomplete geographical barriers, or divergent expressions of behavior, among other things. Parapatric speciation predicts that hybrid zones will often exist at the junction between the two populations.
In biogeography, the terms parapatric and parapatry are often used to describe the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they do not occur together except in a narrow contact zone. Parapatry is a geographical distribution opposed to sympatry (same area) and allopatry or peripatry (two similar cases of distinct areas).
Various "forms" of parapatry have been proposed and are discussed below. Coyne and Orr in Speciation categorise these forms into three groups: clinal (environmental gradients), "stepping-stone" (discrete populations), and stasipatric speciation in concordance with most of the parapatric speciation literature. Henceforth, the models are subdivided following a similar format.
Charles Darwin was the first to propose this mode of speciation. It was not until 1930 when Ronald Fisher published The Genetical Theory of Natural Selection where he outlined a verbal theoretical model of clinal speciation. In 1981, Joseph Felsenstein proposed an alternative, "discrete population" model (the "stepping-stone model). Since Darwin, a great deal of research has been conducted on parapatric speciation—concluding that its mechanisms are theoretically plausible, "and has most certainly occurred in nature".Peripatric speciation
Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging genes, it can often be difficult to distinguish between them. Nevertheless, the primary characteristic of peripatric speciation proposes that one of the populations is much smaller than the other. The terms peripatric and peripatry are often used in biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur—for example on an oceanic island compared to the mainland. Such organisms are usually closely related (e.g. sister species); their distribution being the result of peripatric speciation.
The concept of peripatric speciation was first outlined by the evolutionary biologist Ernst Mayr in 1954. Since then, other alternative models have been developed such as centrifugal speciation, that posits that a species' population experiences periods of geographic range expansion followed by shrinking periods, leaving behind small isolated populations on the periphery of the main population. Other models have involved the effects of sexual selection on limited population sizes. Other related models of peripherally isolated populations based on chromosomal rearrangements have been developed such as budding speciation and quantum speciation.
The existence of peripatric speciation is supported by observational evidence and laboratory experiments. Scientists observing the patterns of a species biogeographic distribution and its phylogenetic relationships are able to reconstruct the historical process by which they diverged. Further, oceanic islands are often the subject of peripatric speciation research due to their isolated habitats—with the Hawaiian Islands widely represented in much of the scientific literature.Polyphemus (crustacean)
Polyphemus is a genus of water fleas, and the only genus in the family Polyphemidae. There are two species, P. exiguus and P. pediculus, although allopatric speciation has resulted in a number of cryptic species of P. pediculus. Polyphemus exiguus inhabits open zones in the Caspian Sea, while Polyphemus pediculus exists throughout the Holarctic. It lives in diverse conditions, from small ponds to lakes and estuaries such as the Gulf of Saint Lawrence and the Gulf of Finland. It can be found quite far offshore.
Polyphemus are a predatory genus of water flea. The two species have four pairs of legs with exopodites, or outer branches. The legs are adapted for catching mobile prey, generally smaller species of water flea such as young Daphnia and Bosmina.Polyphemus has two compound eyes that are fused to form a single unit, with a zoned set of receptors. This zoned structure is paired with an eye-control system that allows the Polyphemus to visually distinguish target size to avoid predators and track prey. P. pediculus is approximately 1 millimetre (0.04 in) in length. P. exiguus, while similar in morphology, is smaller.Punctuated equilibrium
Punctuated equilibrium (also called punctuated equilibria) is a theory in evolutionary biology which proposes that once a species appears in the fossil record the population will become stable, showing little evolutionary change for most of its geological history. This state of little or no morphological change is called stasis. When significant evolutionary change occurs, the theory proposes that it is generally restricted to rare and geologically rapid events of branching speciation called cladogenesis. Cladogenesis is the process by which a species splits into two distinct species, rather than one species gradually transforming into another.Punctuated equilibrium is commonly contrasted against phyletic gradualism, the idea that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages (called anagenesis). In this view, evolution is seen as generally smooth and continuous.In 1972, paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing their theory and called it punctuated equilibria. Their paper built upon Ernst Mayr's model of geographic speciation, I. Michael Lerner's theories of developmental and genetic homeostasis, and their own empirical research. Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species.Reciprocal silencing
Reciprocal silencing, a genetic phenomenon that primarily occurs in plants, refers to the pattern of redundant genes being silenced following a polyploid event. Polyploidy (wholesale genome duplication) is common in plants and constitutes an important method of speciation. When a polyploid species arises, its genome contains homeologs, duplicated chromosomes with equivalent genetic information. However silencing of redundant genes occurs rapidly in new polyploids through genetic and epigenetic means. This primarily occurs because redundancy allows one of the two genes present for each locus to be silenced without affecting the phenotype of the organism, and thus mutations that eliminate gene expression are much less likely to be deleterious or lethal. This allows mutations that would be lethal in diploid populations to accumulate in polyploids. Reciprocal silencing refers to the specific pattern of silencing where equivalent loci in are both silenced and expressed in a reciprocal manner. This phenomenon is observed on two distinct scales.River barrier hypothesis
The river barrier hypothesis is a hypothesis seeking to partially explain the high species diversity in the Amazon Basin, first presented by Alfred Russel Wallace in his 1852 paper On Monkeys of the Amazon. It argues that the formation and movement of the Amazon and some of its tributaries presented a significant enough barrier to movement for wildlife populations to precipitate allopatric speciation. Facing different selection pressures and genetic drift, the divided populations diverged into separate species.
There are several observable qualities that should be present if speciation has resulted from a river barrier. Divergence of species on either side of the river should increase with the size of the river, expressing weakly or not at all in the headwaters and more strongly in the wider, deeper channels further downriver. Organisms endemic to terra firme forest should be more affected than those that live in alluvial forests alongside the river, as they have a longer distance to cross before reaching appropriate habitat and lowland populations can rejoin relatively frequently when a river shifts or narrows in the early stages of oxbow lake formation. Finally, if a river barrier is the cause of speciation, sister species should exist on opposing shores more frequently than expected by random chance.Rudolf Kaufmann
Rudolf Kaufmann (3 April 1909 – c. 1941), son of the physicist Walter Kaufmann, was a palaeontologist and geologist, and is best remembered for his work on allopatric speciation and punctuated equilibrium in the trilobite genus Olenus in the Upper Cambrian of Sweden and on the island of Bornholm. He was a brother-in-law of Curt Teichert, the well-known German-American palaeontologist and geologist. Studying the Upper Cambrian alum shales in Sweden, Kaufmann found that the trilobite genus Olenus occurred in an unbroken sequence of sediments covering a considerable period of geological time. He was thereby in a position to track the phylogenetic evolution of Olenus, that is, the rise and fall of species within the genus and the changes in their morphology. He coined the idea of Artabwandlung, which is the tendency of clade elements in the same environment to show the same morphological trends. With the 1930s onset of the Nazi regime in Germany, Kaufmann, who had Jewish roots, but had been baptised an evangelical Christian, was dismissed from his position at Greifswald University. He had studied at Königsberg and at Greifswald University under Serge von Bubnoff (1888–1957). His 1933 dissertation dealt with speciation and punctuated equilibrium in Cambrian trilobites in Skåne.
He left Germany for Copenhagen in 1933. Here he was denied employment as a geologist, and worked as a photographer and gave instruction in athletics. He found himself ill at ease in Denmark, and after a spell in Italy, returned to Germany where in October 1935 he started teaching at a Jewish school in Coburg, and was arrested on a charge of 'miscegenation' (he was being treated for a sexually transmitted disease acquired from a prostitute) and in 1936 sentenced to 3 years of hard labour. On his release in 1939 Kaufmann fled to Lithuania, where he was allowed to settle and resume geological work. When Lithuania became part of the U.S.S.R. in 1940 he joined the staff of the Geological Survey at Kaunas, immersing himself in the problems of Pleistocene drift, and marrying a fellow refugee. After the German occupation of Lithuania in 1941, and now an undesirable Jew, he was identified while cycling on a country road and executed by two German soldiers.
Earlier, in his search for work, he had moved to Italy and found work in a photographic shop in Bologna. There, in the summer of 1935 he met Ingeborg Magnusson, a 28-year-old Swedish woman on holiday. This relationship was to last five years, during which period they spent 13 days together - he visited her once in Stockholm, and she saw him once in Germany. These details came to light in 1991 when a German stamp collector bought a package of 30 letters, written by Kaufmann to Ingeborg, at a Frankfurt stamp auction.Stephanomeria malheurensis
Stephanomeria malheurensis, the Malheur wirelettuce, is a species of flowering plant in the family Asteraceae. It is endemic to Oregon in the United States. It is a federally listed endangered species.
It was discovered in 1966 and the population at the type locality in Harney County is the only one ever known. For several years in the 1980s it disappeared. Scattered individuals have been noted over the years. Plants were grown from seed at the Berry Botanic Garden in Portland and planted at the original site. Some still survived as of 2001.This species grows atop hills surrounded by flat land. The soils are derived from tuff with a thin top layer of limestone. Associated plants in the habitat include Wyoming big sagebrush, yellow rabbitbrush, basin wildrye, and cheatgrass.Stephanomeria exigua, the small wirelettuce, grows in the same area, and it is thought that the Malheur wirelettuce evolved from it. S. malheurensis is an annual and is susceptible to invasive species of plants, especially cheatgrass.It is thought to have evolved outside the usual model of allopatric speciation, from its parental species Stephanomeria exigua. As such this plant is one of the best plant examples of "quantum speciation," a concept closely aligned with peripatric speciation, parapatric speciation and sympatric speciation.
|Speciation in taxa|
|Tempo and modes|