Describe how allopatric speciation occurs.

In a population of a species a barrier forms (for example the formation of a new river, or the division between two islands). This leads to two isolated groups of the original species. In the two isolated groups random mutations occur and build up in the gene pool over time. These mutations lead to different characteristics between the two populations. The second driving factor may be different environments in the two locations which result in differing selection pressures between the two groups. Therefore different alleles are selected for in each group, again resulting in differing characteristics between the two groups. Due to these two processes the populations become so different that if allowed to mix they could no longer breed. This means they are two separate species and speciation has occurred. A classic example of this process is the evolution of Darwin's finches.