Sometimes the change in allele frequency is so different in the new sample of population that they become a different species.

Small group of population called founders left their habitat and goes into new habitat. In new habitat this population (founders) shows different genotype frequency from that of the original population and leads to variation.

The original drifted population becomes founders and the effect is called founder effect.

E.g Evolution of Darwin finches on Galapagos island from some members of birds of main land.