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.
In other instances, a few members of a population cross an existing physical barrier, forming a new, isolated population. An example of this took place among populations of picture-winged Drosophila in the Hawaiian Islands. When a small number of fruit flies migrated from an older island, Kauai, to a younger island, Oahu, they founded a new population.
The new population differs genetically from the parent population, because a small number of "founder" flies has only an incomplete representation of the genes found in its parent population.
As new islands formed, flies migrated to each one in turn. After arriving on a new island, the founder flies bred with each other rather than returning to the island they came from. The new population became physically isolated from the one on the original island.
Over time, unique selective pressures in the new environment cause the population to evolve new traits. At some point, these flies will no longer be able to interbreed with the parent population—they have become a new species.
More than 800 species of Drosophila exist in the Hawaiian islands, many of them the product of founder events followed by evolution of the populations over time. The closest relative of a species on one island is often a species on a neighboring island, rather than another species on the same island.
Most of these founder events occurred when founder flies traveled from an older island to a younger one, but sometimes flies from a younger island, having evolved into a new species, recolonized an older island.
Chromosomal analysis suggests that at least 45 separate founder events contributed to speciation among flies in the Hawaiian islands. The numbers on the arrows represent the proposed number of founder events between the two islands. The numbers on the islands represent the number of species of picture-winged Drosophila found there.
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.