how does sympatric speciation happen?

Sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that they occur together at least in some places. If these organisms are closely related (e.g. sister species), such a distribution may be the result of sympatric speciation.

Sympatry is one of four theoretical models for the phenomenon of speciation. In contrast to allopatry, populations undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river.

In multicellular eukaryotic organisms, sympatric speciation is thought to be an uncommon but plausible process by which genetic divergence (through reproductive isolation) of various populations from a single parent species and inhabiting the same geographic region leads to the creation of new species. In bacteria, however, the analogous process (defined as "the origin of new bacterial species that occupy definable ecological niches" ) is more common and occurs through horizontal gene transfer.