Explain in detail ,how does allopatric speciation occur?

Continents can drift apart, migratory birds may be blown off course, or fruit flies can be bred in different labs. Isolation may occur because of great distance or a physical barrier, such as a desert or a river. Various geographical changes can arise such as the movement of continents, and the formation of mountains, islands, bodies of water, or glaciers. Human activities like agriculture or development can change the distribution of species populations. These factors substantially alter a regions geography, resulting in the separation of a species population into isolated subpopulations.

The vicariant populations then undergo genetic changes as they become subjected to different selective pressures, experience genetic drift, and accumulate different mutations in the separated populations gene pools. The barriers prevent the exchange of genetic information between two populations leading to reproductive isolation. If at all the two populations do come into contact, they will be unable to reproduce - effectively speciating.

Allopatric speciation may also result even if a few individuals can cross the barrier to mate with members of the other group. For a speciation to be considered allopatric, gene flow between soon to be species must be greatly reduced - but it does not have to be reduced completely to zero.

Over a period of time individuals of each side of barrier differentiate. This may be due to a founder effect, in which only a part of the genetic variability originally present is represented in a new population; to chance loss of genetic variation through accidents of who happens to reproduce or not; or to adaptations to environmental differences on each side of the barrier.

A special case for f allopatric speciation results from isolating factors such as population dispersal leading to emigration causing speciation. Allopatric speciation is stipulated to be the most common form of speciation.

Geographically separated populations of organisms can undergo allopatric speciation when natural selection acts independently in each population, causing these populations to undergo genetic divergence over time as they adapt to their respective environments. Genetic differences between the populations eventually lead to reproductive isolation, and if the genetic differences become significant enough, the populations may no longer be able to interbreed successfully if they come back into contact, resulting in the formation of distinct species.