Describe the steps that occur in allopatric speciation or geographical speciation.

Before speciation occurs we have a population of individuals of the same species that can freely and successfuly mate with each other. In allopatric speciation a geographical barrier forms (a mountain range or river canyon for example) that physically seperates a group of individuals from the main population. See the diagram. The two populations can't interbreed and so there is no gene flow between the populations. The two populations are now in environments that have different climates. These climates will create new selection pressures that mean different traits have a selective advantage in each population, leading to increased genetic differences acumlating and the gene pools becoming isolated. If the reproductive barrier remains for long enough, the genetic differences become so great that if the populations did ever reunite then they would not be able to mate successfully or produce viable offspring. At this point we can consider the two populations to be unique species from each other. Allopatric speciation has occured.