Allopatric speciation is speciation which happesn when biological populations dealing with same species split up. This can happen as a result of population dispersal, or by geolgraphical changes such as mountain formation, island formation or large scale human activites.
The separate populations over time may change distinctly different characteristics. If the geographical barriers are later removed, members of the two populations may be unable to successfully mate with each other, at which point, the genetically isolated groups have emerged as different species. Allopatric isolation is a key factor in speciation and a common process by which new species arise.
Example of Allopatric Speciation
An experiment involving the fruit fly, Drosophila pseudoobscura, conducted by Diane Dodd provides a
possible real-time example of allopatric speciation in action. Essentially Dodd took a single population of fruit flies and “geographically” isolated the group into two separate populations. One population received a diet consisting strictly of starch-based foods; the other received only maltose-based foods (Berkeley, n.d.). After several generations of separation, with each group continuing to receive the specialized diet of either starch or maltose, the two populations were, once again, reunited. Interestingly, Dodd observed that members of the starch-only population exhibited a strong preference to mate with other starch members and vice-versa. This experiment suggests that the two populations of fruit flies were in the process of becoming reproductively isolated, thus providing support for the allopatric model of speciation (Berkeley, n.d.).
University of California, Berkeley (n.d.) Evidence for speciation. Retrieved from: http://evolution.berkeley.edu/evosite/evo101/VC1fEvidenceSpeciation.shtml