Of the four types of speciation - the one I'm most intrigued by is Parapatric speciation. Thought I'd dive a bit deeper into the topic and see about giving some good examples that really help to define this phenomenon.
Definition of Parapatric Speciation
Bear with me - this is a bit of a rambling and long definition, but should be thorough and complete. Parapatric speciation is where the habitat zones of two diverging populations are not entirely separated, meaning there are no geographic obstacles between the parent species and the diverging species - there is a small, shared habitat area - directly adjacent but usually environmentally different habitats. Quick definition within a definition here: Remember that speciation is the process where new genetically distinct species evolve, which is usually the result of genetic isolation. So you could consider parapatric speciation that interim step to complete speciation. Okay, to continue on with the original definition - these divergent species can continue to come in contact with each other in the shared habitat zone, and can interbreed. This zone however, will cease to exist when the isolating mechanisms for selection or key specific behaviors prevent them from interbreeding. Call that the evolution of reproductive isolating mechanisms.
All is not so clear in Parapatric Speciation land
Much like the division in thinking when it comes to inclusive fitness (see that post on this wiki) there is some controversy as it relates to the idea of speciation. While there has been a common distinction based on geographic area for species, Butlin, Galindo and Grahame (2008) have a different theory. They believe that rather than use a geographic distinction for species, which they say focuses on the extremes, we should look at the forces that are causing the differentiation and determine if there is a genetic basis for the reproductive isolation. They feel that speciation is a long and drawn out process, citing the example of the rough periwinkle (Littorina saxatilis) as their illustration. In the paper posted below, review how they believe it is better to look at the "balance between local adaptation and gene flow, the interaction between components of reproductive isolation and the genetic basis of differentiation." I have listed several papers below explain, or that take some issue or further the debate about using the modes of speciation.
Rova, E. (2010) The role of assortive mating in the initial stages of sympatric and parapatric speciation. Uppsala Universitet. http://uu.diva-portal.org/smash/get/diva2:351048/FULLTEXT01
Butlin, R., Galindo, J., & Grahame, J. (2008) Sympatric, parapatric or allopatric: The most importnat way to classify speciation? Philosophical Transactions of the Royal Society, 363, 2997-3007. doi:10.1098/rstb.2008.0076. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2607313/pdf/rstb20080076.pdf