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Modes of Speciation

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

Allopatric Speciation

Allopatric speciation is the evolution of reproductive isolation in populations that are separated by geographic barriers. What constitutes a geographic barrier is not strictly defined, rather, it can be understood as any environmental factor that prevents or dramatically reduces gene flow between two populations. Thus, while geographic barriers most commonly result from large-scale climatic and geological events (mountain formation, glaciation, continental drift), they can also result from strict habitat preferences that "microgeographically" isolate populations.

Geographic isolation prevents gene flow among previously interbreeding populations and allows them to evolve independently. This almost inevitably leads to divergence between the two populations over time as distinct evolutionary changes accumulate: different mutations arise in the different populations, genetic drift fixes different genes in the populations, and the populations undergo different adaptive changes in response to natural selection. Over time, the two populations may become reproductively incompatible or isolated, essentially as a by-product of the genetic divergence of other traits. Eventually, the two (initially identical) species will no longer interbreed, even if they are brought back into contact with one another under natural conditions. Allopatric speciation is considered to be the most common of the known modes of speciation.