This is a result of natural selection. Natural selection is the result of a barrier, physical such as the ocean or a volanco eruption. This barrier seperates members of a species. One genetic mutation occurs within one organisms DNA which means they are more suited for survival in their surroundings. As a result they are more likely to survie, as they have more adapted DNA for the environment, and so reproduce and pass on the gene via vertical gene transmission. As more and more individuals with this population survives and reproduces to increase the frequency of the gene, the species differentiates from the other population of the species making them different. As a result 2 seperate species exist but derive from the same species.