Parapatry is a combination of geographical sympatry and ecological allopatry.
Principle of parapatry
In parapatry two phylogenetically similar species occupy adjacent areas of distribution. They can therefore potentially come into contact and exchange genetic material.
However, ecological (different ecological niches), ethological (different mating rituals), or temporal isolation (different reproductive seasons) can create a reproductive barrier and block this exchange.
Although hybrids exist, the genome of the two species evolves separately and can lead to the emergence of new species.
Diagrammatic representation of two parapatric species (A and C). Their areas of distribution overlap and hybrids (B) emerge although the ecological isolation maintains the genetic difference between A and C. © Alun/Wobble, Wikimedia public domain