What is parapatric speciation? If this is explained in simple words with a diagram it would of great help,
Parapatric speciation occurs when populations are continuous, but are scattered over large geographical areas and are inactive / motionless (for example, plants or underground rodents).There is no specific outward barrier to gene flow, but individuals are more likely to mate with their geagraphic neighbours than with organisms from distant parts of population area (due to their own features, such as immobility).This reduceses gene flow and promotes divergence, because conditions in the distant parts of population area are different and cause different selection pressure. Due to this, two or more new species are formed in the outer borders of habitat range, where they meet different environmental conditions, with the hybrid zone of mating between them. At first organisms doesn't mate randomly not because they can't phisiologically (reproductive isolation) but because of external influence. Unlike allopatric speciation, when differentiation happens before isolation, in parapatric speciation these processes occur in parallel. For example, plant Anthoxanthum odoratum may be subject of parapatric speciation in response to metal pollution of soils due to the mines. In the areas with high pollution plants evolve towards high level of resistance to metals in the soil. Selection against interbreeding between plants that are resistant to metals in the soil and plants from ancestral population caused a gradual change in the flowering time of plants resistant to metals, which led to the emergence of reproductive isolation in the end.