When reinforcement occurs, the trait(s) involved in reproductive isolation between the hybridizing parental species (or populations) are more different in sympatry than they are when the two species are in allopatry.
Sympatry is defined as the the presence of two or more related species in the same geographical area at the same time. These species are related because they have been evolved from the same ancestral species but yet are reproductively isolated.
Allopatry is defined as the the presence of related species in areas that are geographically isolated from each other. These species too are evolved from same ancestral species but got isolated in the course of time.
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