Peripatric speciation

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population.:105 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. Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population.:105 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.:106 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. Peripatric speciation was originally proposed by Ernst Mayr in 1954, and fully theoretically modeled in 1982. It is related to the founder effect, where 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,:106 which may play a significant role. Mayr first conceived of the idea by his observations of kingfisher populations in New Guinea and its surrounding islands.:389 Tanysiptera galatea was largely uniform in morphology on the mainland, but the populations on the surrounding islands differed significantly—referring to this pattern as 'peripatric'.:389 This same pattern was observed by many of Mayr's contemporaries at the time such as by E. B. Ford's studies of Maniola jurtina.:522 Around the same time, the botanist Verne Grant developed a model of quantum speciation very similar to Mayr's model in the context of plants. In what has been called Mayr's genetic revolutions, he postulated that genetic drift played the primary role that resulted in this pattern.:389 Seeing that a species cohesion is maintained by conservative forces such as epistasis and the slow pace of the spread of favorable alleles in a large population (based heavily on J. B. S. Haldane's calculations), he reasoned that speciation could only take place in which a population bottleneck occurred.:389 A small, isolated, founder population could be established on an island for example. Containing less genetic variation from the main population, shifts in allele frequencies may occur from different selection pressures.:390 This to further changes in the network of linked loci, driving a cascade of genetic change, or a 'genetic revolution'—a large-scale reorganization of the entire genome of the peripheral population.:391 Mayr did recognize that the chances of success were incredibly low and that extinction was likely; though noting that some examples of successful founder populations existed at the time.:522 Shortly after Mayr, William Louis Brown, Jr. proposed an alternative model of peripatric speciation in 1957 called centrifugal speciation. In 1976 and 1980, the Kaneshiro model of peripatric speciation was developed by Kenneth Y. Kaneshiro which focused on sexual selection as a driver for speciation during population bottlenecks. Peripatric speciation models are identical to models of vicariance (allopatric speciation).:105 Requiring both geographic separation and time, speciation can result as a predictable byproduct. Peripatry can be distinguished from allopatric speciation by three key features::105 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—sometimes rapid—speciation events.:105 Furthermore, two important predictions are invoked, namely that geological or climactic changes cause populations to become locally fragmented (or regionally when considering allopatric speciation), and that an isolated population's reproductive traits evolve enough as to prevent interbreeding upon potential secondary contact. The peripatric model results in, what have been called, progenitor-derivative species pairs, whereby the derivative species (the peripherally isolated population)—geographically and genetically isolated from the progenitor species—diverges. A specific phylogenetic signature results from this mode of speciation: the geographically widespread progenitor species becomes paraphyletic (thereby becoming a paraspecies), with respect to the derivative species (the peripheral isolate).:470 The concept of a paraspecies is therefore a logical consequence of the evolutionary species concept, by which one species gives rise to a daughter species. It is thought that the character traits of the peripherally isolated species become apomorphic, while the central population remains pleisomorphic.