Allopatric or geographical speciation is speciation that results when a population is separated by a physical barrier. For example, there was one population of flowers. But as consequence of natural disaster separate environments were formed and therefore there is no gene flow between the population. During the time population had different mutations occurred and diverged in evolutionary plan.
Sympatric speciation is speciation that happens without physical separation of members of the population. Occurs when two species are living in the same environment. Evolution leads to formation of a new species from a surviving ancestral species with reproduction barriers.