Speciation, or the origination of new species, begins most often when two populations of the same species become allopatric, the state in which geographic separation of the two populations eliminates effective gene flow between them. Geographic separation can be caused by any physical or ecological feature that prevents a population's movement across it. Speciation is considered complete when the two original populations have evolved enough to have become sympatric, the state in which the two populations have developed over time completely separate gene pools so that the exchange of genetic material would be impossible even if the two populations came into contact again. Three factors aid the divergence of the gene pools of two geographically separated populations. First, there is a high probability that the two populations have different initial gene frequencies. Thus, selection pressure will have different material to work with resulting in increasing differences between the populations. When a small population founds a geographically isolated colony, the probability of this small break-away population having very different gene frequencies than the mother group is extremely high: This is a type of genetic drift called the founder effect. Second, the two populations will experience different mutations, again, providing selection with two different sources of raw material on which to act. Third, the two populations will likely experience different environmental selection pressures.
The divergence of two populations' gene pools caused by an external environmental
