To illustrate the steps in the allopatric speciation process, we will examine a hypothetical lizard species. Before the process begins, there are populations of this species on both sides of a stream. Each population shows some degree of adaptation for camouflage to the local environment. Lizards occasionally cross the stream and interbreed with individuals on the other side, thus maintaining the populations as members of a single species. The allopatric speciation process begins when a climate change causes the stream to widen into a river. Lizards cannot cross the river and so the two populations become geographically isolated. The river remains a geographic barrier for several thousand years. During that time, the populations become increasingly genetically different due to local adaptation and genetic drift. Now imagine that the climate changes again and the river returns to being a stream. Lizards may once again cross to the other side. If the populations have become genetically rather different, interbreeding between individuals may not occur and the two populations can be considered separate species.