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