Explain how allopatric speciation can lead to the formation of 2 species from an ancestral species?

The ancestral species is split into two populations by a geographic barrier, which prevents gene flow. Mutations arise which causes genetic variation between the populations. There are different environmental conditions affecting the populations. For example, there could be different amounts of rainfall affecting each population. Natural selection will cause certain genotypes (and phenotypes) to have a higher chance of survival, whereas other individuals will die. Those that survive can reproduce and pass their genes on to the next generation. After a long time, these populations may meet again but they will not be able to interbreed and produce viable offspring.