@rameen I'm having a crack at it here, working off your answer. I approached it by first outlining some key points of allopatric speciation, then explaining the process of speciation, and then outlining the result.
- Allopatric speciation occurs when a geographical barrier is introduced in the environment, separating a population. Due to this, there is reproductive isolation, where there is no exchange of genetic information between the two populations.
- The different environments introduce different selection pressures to the each population. Given that there is genetic variation within the populations, some individuals will have heritable traits that may be more advantageous than others. These individuals will survive to reproduce, and so will pass on the alleles to their offspring.
- Over many generations, there is a change in the allele frequencies, and speciation occurs, making the two populations unable to interbreed and produce viable, fertile offspring.