Natural selection and genetic drift are two different evolutionary mechanisms that can lead to speciation. This is, to the origin of two species from a single species (the common ancestor).
Allopatric speciation is a type of speciation where emerging species arise after an ancestral population is divided by a geographical barrier preventing contact and therefore gene flow between individuals of split populations. The split populations eventually could become new species.
Natural selection occurs when the split populations changes genetically due to environmental pressures. The genetic drift occurs when the genetic changes are due to random sampling of organisms.
