In biology, species is defined as a group of organisms with the ability to interbreed and produce fertile offspring in the natural environment. Other definitions do add the morphological clause, where the group has to look similar to qualify as species, but the ability to interbreed and produce fertile offspring is the most important factor, considering that even closely related species can interbreed, but the resultant offspring is almost always sterile. At times, however, interbreeding in a successfully breeding population comes to a halt as a result of geographic isolation, eventually giving rise to a new species in what is known as allopatric speciation. Geographic isolation refers to the separation of members of a population by a physical barrier, such as a mountain or body of water, which disrupts the gene flow between them and begins the process of speciation. Geographic isolation of a species occurs as a result of physical changes in its natural environment. Right from the formation of mountain ranges and canyons to the formation or destruction of land bridges, all these physical events are considered causes of geographic isolation. Despite the fact that they belong to the same species, the two populations evolve separately after they are isolated. How they will evolve will depend on the environment of their new natural habitat. Different regions, for instance, have different selective pressures. They differ in terms of biotic factors, such as food availability and competition, as well as abiotic factors, like temperature and precipitation. It's the prevalence of these factors that prompts these populations to evolve separately. The process of speciation is not always successful. At times, the separated populations come together and successfully interbreed, merging into a single population once again. It also works the other way round. There are chances that the two separated populations will come together, but won't interbreed. It is in this case that speciation becomes a possibility. Overall, there are many ifs and buts in the process of speciation resulting from geographic isolation. However, the process can take millions of years, and thus, it is not possible to study it in one's lifetime. Luckily, we have quite a few examples of the same to show how the process works. Geographic isolation is important because it instigates the process of speciation. But then, it's worth noting that it only presents the opportunity for the birth of a new species and does not create a new species on its own. The latter can only happen when barriers of reproductive isolation kick in and prevent productive interbreeding.