Sometimes, one species diverges into two by a process called speciation. Speciation can occur when a population becomes divided by an insurmountable physical barrier, such as a mountain range or a body of water. This process is called allopatric speciation, and it is thought to be the way that most new species form. Genetic divergence between the two geographically divided populations can arise for a variety of reasons, often because their environments become different from each other.
Allopatric speciation can also result when a small number of individuals from a population cross an existing barrier and create a new, isolated population. Such a population differs from its parent population because of a phenomenon called the founder effect—the small group of founding individuals has only an incomplete representation of the gene pool of its parent population.
When a few members of a population cross a major geographic barrier, such as a mountain or a body of water, and form a new population on the other side, this is called a "founder event." Due to the small number of founding individuals, the new population may have different allele frequencies than the parent population, which can contribute to genetic drift. If selective pressures in the new environment cause the newly founded population to adapt differently than the parent population, the new population may eventually change so much that the individuals of the new population can no longer interbreed with the original population. This process of new species formation is called allopatric speciation.
We have seen how numerous founder events created different species of fruit fly on the Hawaiian Islands. Studies of the genomes of these flies have revealed that a given species is often more closely related to a species on a neighboring island than it is to other species on its own island. This suggests that species on the same island were created by separate founder events; they are closer genetically to their parent population than to other "founded" populations which may be geographically closer.
Textbook Reference: Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry