How does geographic isolation attribute to speciation?

this is for a 9th grade review and umm its not in the bookHow does geographic isolation attribute to speciation?
Evolution, which eventually leads to speciation occurs at the population level. When two populations are geographically isolated there can be no gene transfer between the two and the genome of the populations become different through random mutations and other factors (a condition known as genetic drift). Genetic drift over an extended period of time will result in speciation as the organisms continue to adapt to different environments.How does geographic isolation attribute to speciation?
The animals isolated evolve.
Darwin's classic study in evolution was done on finches that had evolved into different species after they migrated to different islands.


The birds developed, over successive generations, different characteristics that best suited their environment. Eventually, the birds on different islands grew so different from one another that they were separate species.