One common mode of speciation is known as geographic, or allopatric speciation. The first stage begins as a result of geographic separation between populations. This may occur when a few colonizers reach a geographically separate habitat. Geographic isolation most often occurs with populations that are completely separated (allopatry) by a physical barrier, such as a mountain range, river, or desert. The separated populations adapt to their own unique environments, becoming so genetically different from one another that members of one population cannot breed with members of the other.