How can natural selection create two different species from one species?

For speciation to occur, two populations of a species must become reproductively isolated. This can result from either the formation of a geographic barrier between two populations of the species, preventing the two populations from mixing, or behavioural or physiological changes which prevent two populations of the species from mating. The behavioural changes could include changes in the timings of breeding seasons or courtship behaviour. Morphological changes can include changes to the morphology of reproductive organs or mechanisms involved in fertilisation.Once they are reproductively isolated, the two populations may experience different selection pressures and will undergo changes in their allele frequencies. Alternatively, the allele frequencies will change at random due to genetic drift. As a result if theses genetic changes, after a period of time the two populations will form two unique species which are unable to breed to produce fertile offspring (which is the biological species definition).