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Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatry 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. One possible consequence of peripatric speciation is that a geographically widespread ancestral species becomes paraphyletic, thereby becoming a paraspecies. The concept of a paraspecies is therefore a logical consequence of the evolutionary species concept, by which one species give rise to a daughter species. An alternative model of peripatric speciation, centrifugal speciation, posits that a species' population experiences periods of geographic range expansion followed by shrinking periods, leaving behind small isolated refugial populations; on the periphery of the main population.
Contents
- Models
- Centrifugal speciation
- Laboratory experiments
- Species patterns on islands and archipelagos
- Species patterns on continents
- References
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.
Peripatric speciation was proposed by Ernst Mayr, and is related to the founder effect, because small living populations may undergo selection bottlenecks. The founder effect is based on models that suggest peripatric speciation can occur by the interaction of selection and genetic drift, which may play a significant role in peripatric speciation.
Models
Peripatric speciation models are identical to models of vicariance (allopatric speciation). Requiring both geographic separation and time, speciation can result as a predictable byproduct. Peripatry can be distinguished from allopatric speciation by three features: 1) the size of the isolated population, 2) strong selection caused by the dispersal and colonization of novel environments, and 3) the effects of genetic drift on small populations.
The size of a 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.
Speciation under the peripatric model invokes two important predictions, namely that geological or climactic changes cause populations to become locally fragmented (or regionally when considering allopatric speciation), and that isolated population's reproductive traits evolve enough as to prevent interbreeding upon potential secondary contact. The phylogenetic signature of this model is that the central population remains pleisomorphic, while the peripheral isolates become apomorphic.
Centrifugal speciation
William Louis Brown, Jr. proposed an alternative model of peripatric speciation in 1957 called centrifugal speciation. This model contrasts with peripatric speciation by virtue of the origin of the genetic novelty that leads to reproductive isolation. A population of a species experiences periods of geographic range expansion followed by periods of contraction. During the contraction phase, fragments of the population become isolated on the periphery of the central population (see figure 2b). Because of the large size and potentially greater genetic variation within the central population, mutations arise more readily. These mutations are left in the isolated peripheral populations, whereby, promoting reproductive isolation. Consequently, Brown suggested that during another expansion phase, the central population would overwhelm the peripheral populations, hindering speciation. However, if the species finds a specialized ecological niche, the two may coexist. The phylogenetic signature of this model is that the central population becomes derived, while the peripheral isolates stay pleisomorphic.
Centrifugal speciation has been largely ignored in the scientific literature, often dominated by the traditional model of peripatric speciation. Despite this, Brown cited a wealth of evidence to support his model, of which has not yet been refuted.
Peromyscus polionotus and P. melanotis (the peripherally isolated species from the central population of P. maniculatus) arose via the centrifugal speciation model. Centrifugal speciation may have taken place in tree kangaroos, South American frogs (Ceratophrys), shrews (Crocidura), and primates (Presbytis melalophos). John C. Briggs associates centrifugal speciation with centers of origin, contending that the centrifugal model is better supported by the data, citing species patterns from the proposed 'center of origin' within the Indo-West Pacific
Laboratory experiments
Peripatric speciation has been researched in both laboratory studies and nature. Coyne and Orr in Speciation suggest that most laboratory studies of allopatric speciation are also examples of peripatric speciation due to their small population sizes and the inevitable divergent selection that they undergo.
Much of the laboratory research concerning peripatry is inextricably linked to founder-effect research. Coyne and Orr conclude that selection's role in speciation is well established, whereas genetic drift's role is unsupported by experimental and field data—suggesting that founder-effect speciation does not occur. Nevertheless, a great deal of research has been conducted on the matter, and one study conducted involving bottleneck populations of Drosophila pseudoobscura found evidence of isolation after a single bottleneck.
Species patterns on islands and archipelagos
Island species provide direct evidence of speciation occurring peripatrically in such that, "the presence of endemic species on oceanic islands whose closest relatives inhabit a nearby continent" must have originated by a colonization event.
Drosophila species on the Hawaiian archipelago have helped researchers understand speciation processes in great detail. It is well established that Drosophila has undergone an adaptive radiation into hundreds of endemic species on the Hawaiian island chain; originating from a single common ancestor (supported from molecular analysis). Studies consistently find that colonization of each island occurred from older to younger islands, speciating peripatrically at least fifty percent of the time. In conjunction with Drosophila, Hawaiian lobeliads (Cyanea) have also undergone an adaptive radiation, with upwards of twenty-seven percent of extant species arising after new island colonization—exemplifying peripatric speciation—once again, occurring in the old-to-young direction.
Several other endemic species in Hawaii also provide evidence of peripatric speciation such as the endemic flightless crickets (Laupala). Shaw estimated that, "17 species out of 36 well-studied cases of speciation were peripatric". Gillespie and Croom also found evidence of peripatry in Hawaiian Tetragnatha spiders.
Species patterns on continents
Coyne and Orr contend that occurrence of peripatry on continents is far more difficult to detect due to the possibility of vicarient explanations being equally likely. However, studies concerning the Californian plant species Clarkia biloba and C. biloba strongly suggest a peripatric origin. In addition, a great deal of research has been conducted on several species of land snails involving chirality that suggests peripatry (with some authors noting other possible interpretations).
A study by Lucinda P. Lawson et al. found evidence for the occurrence of peripatric speciation in the montane spiny throated reed frog species complex (genus: Hyperolius). Lawson maintains that the species' geographic ranges within the Eastern Afromontane Biodiversity Hotspot support a peripatric model that is driving speciation; suggesting that this mode of speciation may play a significant role in "highly fragmented ecosystems".
The chestnut-tailed antbird is located within the Serrania de Huanchaca in Bolivia. Within this region exists an fringe patch of forest (see figure 2c) estimated to have been isolated for 1000-3000 years. Researchers Nathalie Seddon and Joseph A. Tobias found significant song divergence in the population of birds that reside in this isolated patch. They concluded that this measured divergence is evidence of an "early step" in the process of peripatric speciation and "may partly explain the dramatic diversification of suboscines in Amazonia".
In a study of the phylogeny and biogeography of the land snail genus Monacha, the species M. ciscaucasica is thought to have speciated peripatrically from a population of M. roseni. In addition, M. claussi consists of a small population located on the peripheral of the much larger range of M. subcarthusiana suggesting that it also arose by peripatric speciation.