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Dispersal  -  The movement of a population across an existing physical barrier.
Gene flow  -  The movement of genes through a population or between populations through mating.
Hybrid  -  The offspring of parents of two different species. Hybrids are often unable to reproduce, whether because of ill health, deformity, or sterility.
Prezygotic isolation  -  A type of reproductive isolation that occurs before the formation of a zygote can take place. In most cases mating does not even occur. Forms of prezygotic isolation include spatial, behavioral, mechanical and temporal isolation.
Postzygotic isolation  -  A type of reproductive isolation that occurs after members of two different species have mated and produced a hybrid offspring. Such hybrids are usually unable to reproduce.
Reproductive Isolation  -  The inability of the individuals of a species to create offspring with members of any other species. There are two forms: prezygotic and postzygotic.
Vicariance  -  The division of a population by the appearance of a physical barrier.