Sympatric speciation occurs when reproductive isolation evolves while populations remain in the same geographic region.

Because individuals can still encounter one another, sympatric speciation requires a mechanism that reduces gene exchange within the shared area. Examples include strong habitat specialization, assortative mating and chromosome-number changes.

A particularly clear mechanism is polyploidy, the presence of more than two complete chromosome sets.

If a diploid organism with chromosome number $2n$ produces an unreduced gamete containing $2n$ chromosomes, fusion with another unreduced gamete can produce a $4n$ offspring. That tetraploid may be fertile with other tetraploids but have low fertility when crossed with the original diploid population because meiosis in $3n$ hybrid offspring cannot pair and segregate chromosomes evenly.

Thus a chromosome-number change can create reproductive isolation in very few generations:

$$2n \longrightarrow 4n.$$

In plants, polyploid speciation is especially important. Autopolyploidy duplicates chromosome sets from one lineage; allopolyploidy combines chromosome sets from different lineages, usually through hybridization followed by chromosome doubling.

Sympatric speciation does not mean that geography is irrelevant. It means that a physical geographic barrier is not the initial cause of reproductive isolation.