Speciation

Species

• Typically defined as a group of individual organisms that are capable of interbreeding and producing offspring.
• Thus, "species" can also refer to the complete gene pool available to a set of organisms.

Speciation 

• An evolutionary event in which two distinct species arise from a common ancestor, thus creating a branch on a tree of life.
• Also known as "cladogenesis" - 
• Speciation can be natural or artificial
• Natural speciation can be caused by geographical separation or reduced gene flow

Four Modes of Natural Speciation (Classified by Geographical Elements)

1. Allopatric - A population splits into two geographically isolated populations. Each population undergoes evolution independently, to such a degree that when populations come back in contact they are reproductively incompatible and are unable to exchange genetic material.
2. Peripatric - Similar to allopatric, but results from isolated, small peripheral populations becoming reproductively isolated and thus not exchanging genetic material and evolving independently.
3. Parapatric - Occurs when organisms geographic ranges do not typically overlap, but are immediately adjacent to each other. While there is a small continuous overlap area, the contact between groups is minimal, and over time can lead to the development of two distinct species. Speciation caused by reduced gene flow due to varied opportunity for mating throughout population.
4. Sympatric - The formation of two species from the same geographic location. Speciation can result from multiple causes.

• Polyploidy - Events that result in polyploidy can lead to rapid speciation in sympatric situations, particularly in plants

• Hybridization - Hybridization of two different species can create a distinct species reproductively isolated from either parent species. However, it is rare for hybrids to be reproductively incompatible with parent species, and thus it is rare for new species to form through hybridization.

• Gene Transposition - Relocation of genes or chromosome segments can cause speciation. 

Source: Evolution 101: Speciation. http://evolution.berkeley.edu/evosite/evo101/VSpeciation.shtml. Last accessed 8-19-2014.