The formation of new biological species is called speciation. Speciation is a result of:
The diagram illustrates what could happen to populations of animals, which become geographically isolated. Once two groups are isolated different mutations occur in each group. If the environments differ, different adaptations are favoured by natural selection.
This leads to different characteristics evolving in each group as time passes. Eventually the groups become so different that if they come together again they are unable to interbreed and are now separate species.
Allopatric speciation is when new species arise due to isolation of a population by geographical barriers. Features such as rivers or mountain ranges isolate populations of animals and plants.
Movement of land-masses by continental drift led to geographical isolation millions of years ago.
Sympatric speciation is when new species arise despite occupying the same geographical area.