Allopatric Speciation

Biologists categorize the process of allopatric speciation into two categories: dispersal and vicariance.

Dispersal occurs when a few members of the original population relocate to a new geographic area. In contrast, sometimes natural selection favors those organisms that are fit to live and reproduce, while those that cannot adapt are gradually wiped out. This process of allopatric speciation is called vicariance.

Isolation during allopatric speciation can occur due to the formation of a new water body through branching, migration of some populations to a new habitat and inability to return to their original breeding ground (as found in animals), erosion, or seeds transported to an island (as found in plants). The nature of the physical barrier that can cause this type of speciation depends on the biology of the organism and its ability to disperse. For example, if a rodent population gets separated from the original population due to a waterbody, its ability to relocate to its original place of birth would be almost improbable compared to a bird population that can easily move back and forth between the two places. In the second case, gene flow between the two groups is continuous, and thus, they will not form a species, whereas in the former case, the flow of genes will be completely lost, and thus they will form a separate group.    

Steps

These are the common steps through which allopatric speciation occurs:

1. A part of the original population becomes isolated due to geographic barriers, including physical barriers like mountains, rivers, or oceans, that prevent gene flow between the groups.
2. The newly separated group starts experiencing different environmental conditions like climate, food source, and predators, to which they begin to adapt. Over time, this population develops genetic divergence due to natural selection and the accumulation of mutations. 
3. The newly separated group becomes reproductively different (reproductive isolation) and reaches such an extent that they stop interbreeding. At this stage, they are considered new species (founder effect).

Also, it has been found that the further the distance between the newly formed group and the parent population, the greater the chance of speciation. This is because the further the distance between the species, the more probability that the environment will be different, which leads to the development of new traits in the population, which drives speciation.

Here are a few examples of allopatric speciation in nature:

• The present population of Darwin’s Finches in the Galápagos Islands was originally part of the same species, but when they got separated on different islands, they adapted to the conditions of those islands and eventually became separate species with different beak sizes to catch prey according to their need.
• During the formation of Arizona’s Grand Canyon, small mammals, including squirrels that were once part of the original population, could no longer interbreed due to this geographic barrier. Today, the two different varieties of squirrels exist in the north and south of the barrier. At the same time, birds and other animals that could cross the Grand Canyon continued to interbreed and thus were part of the same species.  
• Along the West Coast of the United States, two different species of owls have been found to exist. For example, the northern spotted owl is genetically and phenotypically different from their Mexican cousins.