Peripatric Speciation

Several factors can lead to peripatric speciation:

• Geographical Isolation: A small group of individuals becomes geographically separated from the main population due to certain factors such as climate changes, migration, or physical barriers like mountains or rivers.
• Small Population Size: Unlike allopatric speciation, where both groups may remain large, peripatric speciation involves a smaller, isolated population. This small size increases the likelihood of genetic drift, where a random change in gene frequency will have a large effect on the population. 
• Natural Selection: Along with genetic drift, natural selection also plays a role in peripatric speciation when the peripheral population has differed from the central population in their habitat. Over time, these changes may become so significant that the peripheral population can no longer interbreed with the central group, forming a new species. 
• Population Bottlenecks: One of the main factors in peripatric speciation, a bottlenecked population experiences faster genetic divergence.
• Reduced Gene Flow: Due to their geographical separation, the peripheral and central populations experience little or no gene flow. It allows the isolated population to evolve independently, with distinct traits suited to its new environment.

Like other speciation event to occur, reproductive isolation is the primary factor that leads to peripatric speciation. 

• Hawaiian Drosophila (Fruit Flies): In the Hawaiian Islands, at least 50% of Drosophila species have evolved through peripatric speciation. Isolated populations adapted to the new environment, and with time, they became reproductively isolated, leading to the formation of new species.
• Hawaiian Lobeliads (Cyanea): Lobeliad plants (Cyanea) in Hawaii have also radiated similarly to drosophila. New species evolved after the colonization of younger islands, with small, isolated populations that started adapting to local environments. This process, driven by genetic drift and natural selection, resulted in the emergence of new species, with around 27% of lobeliads evolving through peripatric speciation.
• Hawaiian Flightless Crickets (Laupala): The flightless crickets of the genus Laupala in Hawaii are another example of peripatric speciation. Of 36 well-studied cases, 17 species are believed to have speciated through this process. Small populations became isolated on different islands, which led to divergence and the formation of new species as they adapted to their local environments.
• Hawaiian Spiders: Numerous spider species, such as Tetragnatha and Orsonwelles, also display peripatric speciation in Hawaii. These spiders colonized these islands in an old-to-young pattern, with isolated populations adapting to specific habitats on each island. With time, this led to the formation of new species.
• Austral Islands Crab Spider (Misumenops rapaensis): In the Austral Islands, Misumenops rapaensis has speciated through peripatric speciation. Genetic studies show that populations on older islands are distinct from those on younger islands, with small, isolated groups evolving unique traits. It has led to the formation of genetically different populations, driving speciation.
• Macaronesian Figworts (Scrophularia lowei): The figwort plant Scrophularia lowei, found in the Macaronesian islands, is believed to have speciated from the mainland species Scrophularia arguta. A small, isolated population of S. lowei adapted to the island environment, eventually diverging from its mainland ancestor and becoming a new species. Similar patterns of speciation are seen in other members of the Scrophularia genus.
• Prairie Dogs (Cynomys species): The Mexican prairie dog (Cynomys mexicanus) is thought to have speciated peripatrically from the black-tailed prairie dog (Cynomys ludovicianus) because of Glacial cycles in Coahuila, Mexico. This population eventually diverged from its ancestral species, leading to the emergence of a new species through peripatric speciation.
• Red Spruce (Picea rubens) and Black Spruce (Picea mariana): Red spruce (Picea rubens) is believed to have speciated from an isolated population of black spruce (Picea mariana) during the Pleistocene glaciation. With the advancement of glaciers, a small population of black spruce became geographically isolated and adapted to new conditions, causing the divergence of red spruce. This pattern of speciation has driven the diversification of the Picea genus in North America.

Several models explain how peripatric speciation occurs. The two most widely studied ones are:

Centrifugal Speciation Model

Proposed by William Louis Brown, Jr. in 1957, this model suggests that new species arise from populations at the periphery of a range as the central population expands and contracts.

Kaneshiro Model

Developed in the 1970s by Kenneth Y. Kaneshiro, this model emphasizes the role of sexual selection during population bottlenecks. It proposes that changes in mating preferences during these bottlenecks drive the formation of new species.