Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, 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. Etymologically, sympatry is derived from the Greek roots συν (""together"", ""with"") and πατρίς (""homeland"" or ""fatherland""). The term was invented by Poulton in 1904, who explains the derivation.Sympatric speciation is one of three traditional geographic categories for the phenomenon of speciation. Allopatric speciation is the evolution of geographically isolated populations into distinct species. In this case, divergence is facilitated by the absence of gene flow, which tends to keep populations genetically similar. Parapatric speciation is the evolution of geographically adjacent populations into distinct species. In this case, divergence occurs despite limited interbreeding where the two diverging groups come into contact. In sympatric speciation, there is no geographic constraint to interbreeding. These categories are special cases of a continuum from zero (sympatric) to complete (allopatric) spatial segregation of diverging groups.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"") might be more common because bacteria are less constrained by the homogenizing effects of sexual reproduction and prone to comparatively dramatic and rapid genetic change through horizontal gene transfer.