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Modes of Speciation

Author(s): Tadzia GrandPre, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Peripatric Speciation

Peripatric speciation is a type of allopatric speciation. In both of these geographic modes of speciation, a single population becomes divided into two independent populations that can no longer interbreed because they are separated by some physical barrier. Because of the separation, the two populations evolve independently and eventually become reproductively incompatible. The distinction between allopatric and peripatric speciation is the relative sizes of the populations involved. In allopatric speciation, a population is separated into two relatively large independent populations. In contrast, in peripatric speciation, only a small fraction of the original population becomes geographically isolated. Peripatric speciation originally was known as founder effect speciation because it can occur when a few individuals (the founders) colonize a new habitat, such as an island, thereby establishing a new population away from the parent population.

The same genetic forces drive the evolution of reproductive isolation in both allopatric and peripatric speciation: mutation, natural selection, and genetic drift. However, in peripatric speciation, evolution may occur on a faster time scale because small populations are more susceptible to the random effects of genetic drift. In addition, it is likely that there will be a period of rapid selection and adaptation if the colonists' new habitat is substantially different from their original environment. For example, the founder effect has been used to explain the rapid speciation of Hawaiian fruit flies (Drosophila). Approximately 500 species of Drosophila currently inhabit the Hawaiian archipelago, and all are believed to have descended from a common ancestor that reached the island of Kauai (the oldest of the volcanic islands) over five million years ago. Some believe that as new Hawaiian islands formed as a result of volcanic activity, they were colonized by small groups of Drosophila from the older islands. Because these founders represented only a fraction of the genetic variability present in the parent species, it is likely that genetic drift played a large role in the evolution of genetic distinctiveness between species of Hawaiian Drosophila. In addition, the founders evolved through the process of natural selection to adapt to aspects of the new environment, such as the endemic plant hosts.

The founder effect has been invoked in the speciation events of the Hawaiian Drosophila because so many species were formed in such a relatively short time; the standard allopatric model does not predict that several hundred speciation events should occur in only a few million years. However, the importance of the founder-induced speciation is debated. Some have suggested that it rarely occurs, especially compared to the more standard allopatric model of speciation.