Final answer:
Allopatric speciation is when a new species evolves because individuals become geographically separated from the main population, leading to genetic divergence and reproductive incompatibility.
Step-by-step explanation:
Allopatric speciation is the evolution of a new species that occurs when some individuals become geographically isolated from the rest of their species. This isolation can come about through dispersal, where members of a species move to a new area, or through vicariance, where a natural environmental barrier divides organisms of the same species, thus preventing gene flow between the separated populations.
Biologists recognize allopatric speciation as the most common form of speciation event. For this process to occur, one original population must split into two, which then evolve independently to the point of reproductive incompatibility. A notable example of allopatric speciation is provided by the distinct subspecies of spotted owls along the West coast of the United States, where the northern spotted owl shows significant differences from the Mexican spotted owl likely due to past geological barriers like glaciers.