Allopatric Speciation
Allopatric refers to the "different lands" that two species occupy. It is speciation that occurs in geographically isolated populations. It is probably the most common mechanism. Some sort of barrier arises and prevents gene flow. The effectiveness of the barrier varies with species.
A physical barrier cuts off gene flow between two or more populations. The separated populations become reproductively incompatible and cannot interbreed. Whether or not a geographical barrier is effective in preventing gene flow depends on how fast and by what means the organism can travel. Here are several examples of geographical barriers that have been studied in great detail:
Extensive divergence prevents inbreeding. Species separated by geographic barriers will diverge genetically. If divergence is great enough it will prevent inbreeding even if the barrier later disappears.
An archipelago is an island chain some distance away from a continent such as the Galapagos Islands or the Hawaiian Islands. Colonization of islands followed by genetic divergence sets the stage for speciation.
This animation (Audio - Important) describes allopatric speciation.
The Hawaiian Islands are of volcanic origins, and have a variety of habitats. Adaptive radiations have occurred in Hawaiian Honeycreepers . In the absence of other bird species, they radiated to fill numerous niches.
The Galapagos finches are another example of speciation. Apparently, some finches made the trip from the South American coastline to the isolated islands many years ago. Their descendants have spread to other islands and evolved in isolation.
This animation (Audio - Important) discusses differences among finches.