How does speciation occur?

Speciation can be sympatric (both populations in the same place) or allopatric (populations in different places). Speciation can result from natural selection and isolation.
Natural Selection:1) A new selection pressure arises. (eg drought, new predator, etc.)2) The selection pressure acts on existing genetic variation in population.3) Different genotypes are selected for in different groups.4) Natural selection, mutation and genetic drift cause the accumulation of phenotypic differences.5) The groups become reproductively isolated as they can't reproduce to form fertile offspring.6) Speciation has occurred.
Isolation:1) Two populations of the same species become geographically isolated by a feature of the environment. (eg river, mountain range, etc)2) Interbreeding and gene flow between the populations is prevented by the geographic feature.3) Phenotypic differences accumulate between the different groups due to differing selection pressures, genetic drift and mutations.4) The groups become reproductively isolated as they can't reproduce to form fertile offspring.5) Speciation has occurred.