Parapatric speciation occurs when populations are separated not by a geographical barrier, such as a body of water, but by an extreme change in habitat. Example- Plants that live on boundaries between very distinct climates may flower at different times in response to their different environments, making them unable to interbreed.
Peripatric speciation occurs when a small group of individuals break off of the main group to form a new species.Similar to allopatric speciation, the two groups are separated by physical barriers such as mountain ranges or waterways, making it almost impossible for the two groups to interbreed. One example is the London Underground Mosquito.
First, a small group from the Culex pipiens species migrated underground into the tunnels of the London Underground. Over time period, this group evolved away from its initial population, developing some characteristics. Now, the London Underground mosquito is found underground in New york and other large cities, and will not interbreed with its initial population. So, we can conclude that speciation has occurred, and Culex molestus has formed reproductive
isolation. When speciation occurs, there is reproductive isolation within a population. That is, for the new species do be defined as different, they must be incapable of interbreeding.
The speciation phenomenon most commonly occurs through polyploidy, in which an offspring or group of offspring will be produced with twice the normal number of chromosomes. So, it is due to the change in chromososmes that lead to speciation.