How is sympatric speciation similar to allopatric speciations?

allopatric speciation happens when a physical barrier divides two populations of the same species and sympatric speciation happen when no physical barrier divides the member of a population, but methods such as polyploidy (chromosome doubling) do not let the members of the species have fertile offspring, 2 species are formed (the parental "normal" species and the divergent species "polyploids"). Remember that a species is defined as a population that when mated with one another produce fertile offspring. A polyploid and a parental organisms can not produce fertile offspring together so they become two different species and speciation is said to have occurred.