How does speciation occur?

Reproductive separation occurs, this could be through geographical separation. (1)

Therefore no interbreeding in the population. (2)

This results in separate gene pools. (3)

Variation due to random mutation (4)

The separated populations of the species will be exposed to different environmental/abiotic/biotic conditions/ selection pressures  (5)

The different environments will therefore favour different mutations, depending on what is required for survival. For example, one environment may be colder, therefore individuals with a thicker fur mutation will survive and be selected for. The other population may live in a warmer environment, where a thinner fur is more suited for survival. 

Therefore there will be selection for different/advantageous alleles, depending on the environmental pressures. (6) 

Therefore these selected organisms, with favourable alleles, are able to survive and reproduce. (7) 

Over a long period of time, this will lead to a change in allele frequency. (8)