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, and peripatric speciation may be considered one type or model of allopatric speciation. The primary distinguishing characteristic of peripatric speciation is that one of the populations is much smaller than the other, as opposed to allopatric speciation, in which similarly-sized populations become separated. 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 ; 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.

History

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, 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. 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". 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. 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. 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, he reasoned that speciation could only take place in which a population bottleneck occurred. 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. 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. 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.
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.

Models

Peripatric

Peripatric speciation models are identical to models of vicariance. Requiring both geographic separation and time, speciation can result as a predictable byproduct. Peripatry can be distinguished from allopatric speciation by three key features:
  • The size of the isolated population
  • Strong selection caused by the dispersal and colonization of novel environments,
  • 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—sometimes rapid—speciation events. Furthermore, two important predictions are invoked, namely that geological or climatic changes cause populations to become locally fragmented, 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 —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, with respect to the derivative species. 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 plesiomorphic.
Modern cladistic methods have developed definitions that have incidentally removed derivative species by defining clades in a way that assumes that when a speciation event occurs, the original species no longer exists, while two new species arise; this is not the case in peripatric speciation. Mayr warned against this, as it causes a species to lose their classification status. Loren H. Rieseberg and Luc Brouillet recognized the same dilemma in plant classification.

Quantum and budding speciation

The botanist Verne Grant proposed the term quantum speciation that combined the ideas of J. T. Gulick, Sewall Wright, Mayr, and George Gaylord Simpson. Quantum speciation is a rapid process with large genotypic or phenotypic effects, whereby a new, cross-fertilizing plant species buds off from a larger population as a semi-isolated peripheral population. Inbreeding and genetic drift take place due to the reduced population size, driving changes to the genome that would most likely result in extinction. In rare instances, chromosomal traits with adaptive value may arise, resulting in the origin of a new, derivative species. Evidence for the occurrence of this type of speciation has been found in several plant species pairs: Layia discoidea and L. glandulosa, Clarkia lingulata and C. biloba, and Stephanomeria malheurensis and S. exigua ssp. coronaria.
A closely related model of peripatric speciation is called budding speciation—largely applied in the context of plant speciation. The budding process, where a new species originates at the margins of an ancestral range, is thought to be common in plants—especially in progenitor-derivative species pairs.

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 as small refugial populations on the periphery of the central population. 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, 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 plesiomorphic—the reverse of the general model. In contrast to centrifugal speciation, peripatric speciation has sometimes been referred to as centripetal speciation. 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 arose via the centrifugal speciation model. Centrifugal speciation may have taken place in tree kangaroos, South American frogs, shrews, and primates. 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