Describe the process of allopatric speciation. How does this differ from sympatric speciation?

Basic level answer

Description of some form of geographical separation leading to isolated populations

Referencing reduced interbreeding and the development of differences between populations 

Identify that sympatric speciation occurs without geographical separation 

(Important points - must identify allopatric as requiring geographical separation and sympatric not, must reference reduced interbreeding)

Higher level answer 

All of above plus some of the extra details below:

Link together the reduced breeding with reduced gene flow and how this can form barriers to reproduction 

Refer to the mechanisms of barriers forming (different selection pressures in different environments, potential for drift) 

Refer to the populations being unable to interbreed, even ifreturned to a common environment 

Some explanation of how sympatric speciation can occur and the possible reproductive barriers that can produce it